Vacuum Pumps

                           PRACTICAL ASPECTS OF VACUUM TECHNOLOGY

                                                       Vacuum Pumps

The operation of most vacuum systems can be divided into two regimes. The first involves pumping the system from atmosphere to a pressure at which a high-vacuum pump can be brought into operation. This is traditionally known as the rough vacuum regime and the pumps used are commonly referred to as roughing pumps. Clearly, a system that operates at an ultimate pressure within the capability of the roughing pump will require no additional pumps. Once the system has been roughed down, a high vacuum pump must be used to achieve lower pressures.
If the high-vacuum pump is the type known as a transfer pump, such as a diffusion or turbomolecular pump, it will require the continuous support of the roughing pump in order to maintain the pressure at the exit of the highvacuum pump at a tolerable level (in this phase of the pumping operation the function of the roughing pump has changed, and it is frequently referred to as a backing or forepump). Transfer pumps have the advantage that their capacity for continuous pumping of gas, within their operating pressure range, is limited only by their reliability.
They do not accumulate gas, an important consideration where hazardous gases are involved. Note that the
reliability of transfer pumping systems depends upon the satisfactory performance of two separate pumps. A second class of pumps, known collectively as capture pumps, require no further support from a roughing pump once they have started to pump. Examples of this class are cryogenic pumps and sputter-ion pumps. These types of pump have the advantage that the vacuum system is isolated from the atmosphere, so that system operation depends upon the reliability of only one pump. Their disadvantage is that they can provide only limited storage of pumped gas, and as that limit is reached, pumping will deteriorate.
The effect of such a limitation is quite different for the two examples cited. A cryogenic pump can be totally regenerated by a brief purging at ambient temperature, but a sputter-ion pump requires replacement of its internal components.

One aspect of the cryopump that should not be overlooked is that hazardous gases are stored, unchanged,
within the pump, so that an unexpected failure of the pump can release these accumulated gases, requiring provision for their automatic safe dispersal in such an emergency.

         Roughing Pumps

Two classes of roughing pumps are in use. The first type, the oil-sealed mechanical pump, is by far the most common, but because of the enormous concern in the semiconductor industry about oil contamination, a second type, the so-called ‘‘dry’’ pump, is now frequently used. In this context, ‘‘dry’’ implies the absence of volatile organics in the part of the pump that communicates with the vacuum system.
Oil-Sealed Pumps The earliest roughing pumps used either a piston or liquid to displace the gas. The first production methods for incandescent lamps used such pumps, and the development of the oil-sealed mechanical pump by Gaede, around 1907, was driven by the need to accelerate the pumping process.

Applications :
The modern versions of this pump are the most economic and convenient for achieving pressures as
low as the 10-4 torr range. The pumps are widely used as a backing pump for both diffusion and turbomolecular pumps; in this application the backstreaming of mechanical pump oil is intercepted by the high vacuum pump, and a foreline trap is not required.

Operating Principles:
 The oil-sealed pump is a positive displacement pump, of either the vane or piston type, with a compression ratio of the order of 105:1 (Dobrowolski,1979). It is available as a single or two-stage pump, capable
of reaching base pressures in the 10-2 and 10-4 torr range, respectively. The pump uses oil to maintain sealing, and to provide lubrication and heat transfer, particularly at the contact between the sliding vanes and the pump wall.Oil also serves to fill the significant dead space leading to the exhaust valve, essentially functioning as a hydraulic valve lifter and permitting the very high compression ratio.
The speed of such pumps is often quoted as the ‘‘free-air displacement,’’ which is simply the volume swept by the pump rotor. In a typical two-stage pump this speed is sustained down to ~ 1 x 10-1 torr; below this pressure the speed decreases, reaching zero in the 10-5 torr range. If a pump is to sustain pressures near the bottom of its range, the required pump size must be determined from published pumping-speed performance data. It should be noted that mechanical pumps have relatively small pumping
speed, at least when compared with typical highvacuum pumps. A typical laboratory-sized pump, powered
by a 1/3 hp motor, may have a speed of ~3.5 cubic feet per minute (cfm), or rather less than 2 L/s, as compared to the smallest turbomolecular pump, which has a rated speed of 50 L/s.

Avoiding Oil Contamination from an Oil-Sealed Mechanical Pump:

 The versatility and reliability of the oil-sealed mechanical pump carries with it a serious penalty. When
used improperly, contamination of the vacuum system is inevitable. These pumps are probably the most prevalent source of oil contamination in vacuum systems. The problem arises when thay are untrapped and pump a system down to its ultimate pressure, often in the free molecular flow regime. In this regime, oil molecules flow freely into the vacuum chamber. The problem can readily be avoided by careful control of the pumping procedures, but possible system or operator malfunction, leading to contamination, must be considered. For many years, it was common practice to leave a system in the standby condition evacuated
only by an untrapped mechanical pump, making contamination inevitable.

Mechanical pump oil has a vapor pressure, at room temperature,in the low 10-5 torr range when first installed,
but this rapidly deteriorates up to two orders of magnitude as the pump is operated (Holland, 1971). A pump operates at temperatures of 60C, or higher, so the oil vapor pressure far exceeds 10-3 torr, and evaporation results in a substantial flux of oil into the roughing line.
When a system at atmospheric pressure is connected to the mechanical pump, the initial gas flow from the vacuum chamber is in the viscous flow regime, and oil molecules are driven back to the pump by collisions with the gas being exhausted (Holland, 1971; Lewin, 1985). Provided the roughing process is terminated while the gas flow is still in the viscous flow regime, no significant contamination of the vacuum chamber will occur.

The condition for viscous flow is given by the equation
PD >/ 0.5
where P is the pressure in torr and D is the internal diameter of the roughing line in centimeters.

Termination of the roughing process in the viscous flow region is entirely practical when the high-vacuum pump is either a turbomolecular or modern diffusion pump (see precautions discussed under Diffusion Pumps and Turbomolecular Pumps, below). Once these pumps are in operation, they function as an effective barrier against oil migration into the system from the forepump. Hoffman (1979) has described the use of a continuous gas purge on the foreline of a diffusion-pumped system as a means of avoiding backstreaming from the forepump.

Foreline Traps.

 A foreline trap is a second approach to preventing oil backstreaming. If a liquid nitrogen-cooled trap is always in place between a forepump and the vacuum chamber, cleanliness is assured. But the operative word is ‘‘always.’’ If the trap warms to ambient temperature, oil from the trap will migrate upstream, and this is
much more serious if it occurs while the line is evacuated. A different class of trap uses an adsorbent for oil. Typical adsorbents are activated alumina, molecular sieve (a synthetic zeolite), a proprietary ceramic (Micromaze foreline traps; Kurt J. Lesker Co.), and metal wool. The metal wool traps have much less capacity than the other types, and unless there is evidence of their efficacy, they are best avoided. Published data show that activated alumina can trap 99% of the backstreaming oil molecules (Fulker, 1968). However, one must know when such traps should be reactivated. Unequivocal determination requires insertion of an oil-detection device, such as a mass spectrometer, on the foreline. The saturation time of a trap depends upon the rate of oil influx, which in turn depends upon the vapor pressure of oil in the pump and the conductance of the line between pump and trap. The only safe procedure is frequent reactivation of traps on a conservative
schedule. Reactivation may be done by venting the system, replacing the adsorbent with a new charge, or by baking the adsorbent in a stream of dry air or inert gas to a temperature of -300C for several hours. Some traps can be regenerated by heating in situ, but only using a stream of inert gas, at a pressure in the viscous flow region, flowing from the system side of the trap to the pump (D.J. Santeler, pers. comm.). The foreline is isolated from the rest of the system and the gas flow is continued throughout the heating cycle, until the trap has cooled back to ambient temperature. An adsorbent foreline trap must be optically dense, so the oil molecules have no path past the adsorbent; commercial traps do not always fulfill this basic requirement. Where regeneration of the foreline trap has been totally neglected, acceptable performance may still be achieved simply because a diffusion pump or turbomolecular pump serves as the true ‘‘trap,’’ intercepting the oil from the forepump.

Oil contamination can also result from improperly turning a pump off. If it is stopped and left under vacuum, oil
frequently leaks slowly across the exhaust valve into the pump. When it is partially filled with oil, a hydraulic
lock may prevent the pump from starting. Continued leakage will drive oil into the vacuum system itself; an interesting procedure for recovery from such a catastrophe has been described (Hoffman, 1979).
Whenever the pump is stopped, either deliberately or by power failure or other failure, automatic controls that first isolate it from the vacuum system, and then vent it to atmospheric pressure, should be used.
Most gases exhausted from a system, including oxygen and nitrogen, are readily removed from the pump oil, but some can liquify under maximum compression just before the exhaust valve opens. Such liquids mix with the oil and are more difficult to remove. They include water and solvents frequently used to clean system components. When pumping large volumes of air from a vacuum chamber, particularly during periods of high humidity (or whenever solvent residues are present), it is advantageous to use a gas-ballast feature commonly fitted to two-stage and also to some single-stage pumps. This feature admits air during the final stage of compression, raising the pressure and forcing the exhaust valve to open before the partial pressure of water has reached saturation. The ballast feature minimizes pump contamination and reduces pumpdown
time for a chamber exposed to humid air, although at the cost of about ten-times-poorer base pressure.

GENERAL VACUUM TECHNIQUES

                       
                          GENERAL VACUUM TECHNIQUES

INTRODUCTION

In this unit we discuss the procedures and equipment used to maintain a vacuum system at pressures in the range from 10-3 to 10-11 torr. Total and partial pressure gauges used in this range are also described.
Because there is a wide variety of equipment, we describe each of the various components, including details
of their principles and technique of operation, as well as their recommended uses. SI units are not used in this unit. The American Vacuum Society attempted their introduction many years ago, but the more traditional units continue to dominate inthis field in North America. Our usage will be consistent with that generally found in the current literature. The following units will be used.

Pressure is given in torr. 1 torr is equivalent to 133.32 pascal (Pa).
Volume is given in liters (L), and time in seconds (s). The flow of gas through a system, i.e., the ‘‘throughput’’
(Q), is given in torr-L/s. Pumping speed (S) and conductance (C) are given in L/s.

PRINCIPLES OF VACUUM TECHNOLOGY

The most difficult step in designing and building a vacuum system is defining precisely the conditions required to fulfill the purpose at hand. Important factors to consider include:

1. The required system operating pressure and the gaseous impurities that must be avoided;
2. The frequency with which the system must be vented to the atmosphere, and the required recycling time;
3. The kind of access to the vacuum system needed for the insertion or removal of samples.

For systems operating at pressures of 10-6 to 10-7 torr,venting the system is the simplest way to gain access, but for ultrahigh vacuum (UHV), e.g., below 10-8 torr, the pumpdown time can be very long, and system bakeout would usually be required. A vacuum load-lock antechamber for the introduction and removal of samples may be essential in such applications. Because it is difficult to address all of the above questions,
a viable specification of system performance is often neglected, and it is all too easy to assemble a more sophisticated and expensive system than necessary, or, if budgets are low, to compromise on an inadequate system that cannot easily be upgraded. Before any discussion of the specific components of a vacuum system, it is instructive to consider the factors that govern the ultimate, or base, pressure. The pressure can be calculated from
P =Q/S  ------(1)

where P is the pressure in torr, Q is the total flow, or throughput of gas, in torr-L/s, and S is the pumping speed in L/s.
The influx of gas, Q, can be a combination of a deliberate influx of process gas from an exterior source and gas originating in the system itself. With no external source, the base pressure achieved is frequently used as the principle indicator of system performance. The most important internal sources of gas are outgassing from the walls and permeation from the atmosphere, most frequently through elastomer O-rings. There may also be leaks, but these can readily be reduced to negligible levels by proper system design and construction. Vacuum pumps also contribute to background pressure, and here again careful selection  and operation will minimize such problems.

The Problem of Outgassing Of the sources of gas described above, outgassing is often the most important. With a new system, the origin of outgassing may be in the manufacture of the materials used in construction, in handling during construction, and in exposure of the system to the atmosphere. In general these
sources scale with the area of the system walls, so that it is wise to minimize the surface area and to avoid porous materials in construction. For example, aluminum is an excellent choice for use in vacuum systems, but anodized aluminum has a porous oxide layer that provides an internal surface for gas adsorption many times greater than the apparent surface, making it much less suitable for use in vacuum.

The rate of outgassing in a new, unbaked system, fabricated from materials such as aluminum and stainless
steel, is initially very high, on the order of 10-6 to 10-7 torr-L/s - cm2 of surface area after one hour of exposure to vacuum (O’Hanlon, 1989). With continued pumping, the rate falls by one or two orders of magnitude during the first 24 hr, but thereafter drops very slowly over many months. Typically the main residual gas is water vapor. In a clean vacuum system, operating at ambient temperature and containing only a moderate number of O-rings, the lowest achievable pressure is usually 10-7 to mid-10-8 torr. The limiting factor is generally residual outgassing, not the capability of the high-vacuum pump.

The outgassing load is highest when a new system is put into service, but with steady use the sins of construction are slowly erased, and on each subsequent evacuation, the system will reach its typical base pressure more rapidly. However, water will persist as the major outgassing load. Every time a system is vented to air, the walls are exposed to moisture and one or more layers of water will adsorb virtually instantaneously. The amount adsorbed will be greatest when the relative humidity is high, increasing the time needed to reach base pressure.
Water is bound by physical adsorption, a reversible process,but the binding energy of adsorption is so great
that the rate of desorption is slow at ambient temperature.Physical adsorption involves van der Waal’s forces, which are relatively weak. Physical adsorption should be distinguished from chemisorption, which typically involves the formation of chemical-type bonding of a gas to an atomically clean surface—for example, oxygen on a stainless steel surface. Chemisorption of gas is irreversible under all conditions normally encountered in a vacuum system.
After the first few minutes of pumping, pressures are almost always in the free molecular flow regime, and
when a water molecule is desorbed, it experiences only collisions with the walls, rather than with other molecules.
Consequently, as it leaves the system, it is readsorbed many times, and on each occasion desorption is a slow process.One way of accelerating the removal of adsorbed water is by purging at a pressure in the viscous flow region, using a dry gas such as nitrogen or argon. Under viscous flow conditions, the desorbed water molecules rarely reach the system walls, and readsorption is greatly reduced. A second method is to heat the system above its normal operating temperature.

Any process that reduces the adsorption of water in a vacuum system will improve the rate of pumpdown. The
simplest procedure is to vent a vacuum system with a dry gas rather than with atmospheric air, and to minimize the time the system remains open following such a procedure.

Dry air will work well, but it is usually more convenient to substitute nitrogen or argon. From Equation 1, it is evident that there are two approaches to achieving a lower ultimate pressure, and hence a low impurity level, in a system. The first is to increase the effective pumping speed, and the second is to reduce the outgassing rate. There are severe limitations to the first approach. In a typical system, most of one wall of the chamber will be occupied by the connection to the high-vacuum pump; this limits the size of pump that can be used, imposing an upper limit on the achievable pumping speed. As already noted, the ultimate pressure achieved in an unbaked system having this configuration will rarely reach the mid-10-8 torr range. Even if one could
mount a similar-sized pump on every side, the best to be expected would be a 6-fold improvement, achieving a base pressure barely into the 10-9 torr range, even after very long exhaust times.

It is evident that, to routinely reach pressures in the 10-10 torr range in a realistic period of time, a reduction
in the rate of outgassing is necessary—e.g., by heating the vacuum system. Baking an entire system to 400C
for 16 hr can produce outgassing rates of 10-15 torr-L/ s-cm2 (Alpert, 1959), a reduction of 108 from those found after 1 hr of pumping at ambient temperature. The magnitude of this reduction shows that as large a portion as possible of a system should be heated to obtain maximum advantage

Thin Film Solar Cell

Thin film solar cell




A Thin-Film Solar Cell (TFSC), also called a Thin-Film Photovoltaic Cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers.

Many different photovoltaic materials are deposited with various deposition methods on a variety of substrates. Thin Film Solar Cells are usually categorized according to the photovoltaic material used. The following catgories exist:

• Cadmium Telluride (CdTe)

• Copper indium gallium selenide (CIS or CIGS)

• Dye-sensitized solar cell (DSC)

• Organic solar cell

• Thin-film silicon (TF-Si)

The TFSC types that appear in bold letters are currently in mass production while the rest are still in the development or pilot-plant phase. As you can see, in most cases the TFSC are named after the photovoltaic material that they use, although there seems to be some confusion regarding some TFSC types because manufacturers and researchers use different names to describe the same technologies.

Contents

• 1 Time Award

• 2 Thin-film silicon

o 2.1 Design / Fabrication

o 2.2 Micromorphous silicon

o 2.3 Efficiency

o 2.4 BIPV

• 3 Organic solar cells

• 4 Efficiency

• 5 Cost and Market

• 6 Installations

• 7 See also

• 8 References

o 8.1 Sources

• 9 External link




Thin-film silicon

A silicon thin-film cell is a thin-film cell that uses amorphous (a-Si or a-Si:H), protocrystalline, nanocrystalline (nc-Si or nc-Si:H) or black silicon

Thin-film silicon is opposed to wafer silicon (also called bulky or crystalline silicon).

Design / Fabrication

The silicon is mainly deposited by chemical vapor deposition, typically plasma-enhanced (PE-CVD), from silane gas and hydrogen gas. Other deposition techniques being investigated include sputtering and hot wire techniques.

The silicon is deposited on glass, plastic or metal which has been coated with a layer of transparent conducting oxide (TCO).

A p-i-n structure is usually used, as opposed to an n-i-p structure. This is because the mobility of electrons in a-Si:H is roughly 1 or 2 orders of magnitude larger than that of holes, and thus the collection rate of electrons moving from the p- to n-type contact is better than holes moving from p- to n-type contact. Therefore, the p-type layer should be placed at the top where the light intensity is stronger, so that the majority of the charge carriers crossing the junction would be electrons[2].

Micromorphous silicon

Micromorphous silicon module technology combines two different types of silicon, amorphous and microcrystalline, in a top and a bottom photovoltaic cell. Use of protocrystalline silicon for the intrinsic layer has shown to optimize the open circuit voltage of an a-Si photovoltaic cell. [3]

Efficiency

These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon (also called crystalline or wafer silicon), but are also less expensive to produce. The quantum efficiency of thin-film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.

Recently, solutions to overcome the limitations of thin-film silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects.[citation needed]

BIPV

A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semitransparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

Organic solar cells

The Organic solar cell is another alternative to the more conventional materials used to make photovoltaics. Although a very novel technology it is promising since it offers a very low cost solution.

Efficiency

Efficiency is anticipated to rise from a current 6%–12% to 10%–15% in the coming years, with a potential of more than 20% in the longer term. [4]

Cost and Market

Main articles: List of photovoltaics companies and Low-cost photovoltaic cell

Scaling factors, efficiency gains and the new production technologies are expected to reduce thin-film module manufacturing costs to €1/Wp (and below) in the near future. The thin-film PV market, showing a spectacular annual growth rate of 126% in 2007[5]

In recent years, the manufacturers of thin-film solar modules are bringing costs down and gaining in competitive strength through advanced thin film technology. However, the traditional crystalline silicon technologies will not give up their market positions until the next few years later because they still hold considerable development potential in terms of the cost. Efficiency of thin film solar is considerably lower and thin film solar manufacturing equipment suppliers intend to score costs of below USD 1/MW, and claimed by Anwell Technologies Limited that they intend to bring it down further to USD 0.5/MW. [6] Those equipment suppliers have been doing R&D for micro-morphous silicon modules since 2008. This technology represents a development based on the thin-film panels made of ordinary amorphous silicon marketed at present that brings higher cell efficiency by depositing an additional absorber layer made of micro crystalline silicon on the amorphous layer. Some equipment suppliers even claim that there will be machinery in market to manufacture these new modules at USD 0.70. [7] With such potential of further development of thin film solar technology, the European Photovoltaic Industry Association (EPIA) expects that manufacturing capacities for these technologies will double to over 4GW by 2010 representing a market share of around 20%. [8]

Installations

First Solar, the CdTe thin-film manufacturer stated that "at the end of 2007, over 300 MW of First Solar PV modules had been installed worldwide." [9] Below is a list of several recent installations:[10]

1. Since 16 October 2008, Germany's largest thin-film pitched roof system, constructed by Riedel Recycling, has been in operation and producing solar power in Moers near Duisburg. Over eleven thousand cadmium telluride modules, from First Solar, deliver a total of 837 kW [11].

2. First Solar recently completed a 2.4 MW rooftop installation as part of Southern California Edison program to install 250 MW of rooftop solar panels throughout Southern California over by 2013. [12]

3. First Solar announced a 7.5 MW system to be installed in Blythe, CA, where the California Public Utilities Commission has accepted a 12 ¢/kWh power purchase agreement with First Solar (after the application of all incentives). [13]

4. Construction of a 10 MW plant in the Nevada desert began in July 2008. [2] [3] First Solar is partnering with Sempra Generation, which will own and operate the PV power-plant, being built next to their natural gas plant.

5. Riedel Recycling (837 kW and will deliver around 750 MWh per year)

6. Stadtwerke Trier (SWT) in Trier, Germany (the plant is expected to produce over 9 GWh annually)

7. A 40 MW system is being installed by juwi group in Waldpolenz Solar Park, Germany. At the time of its announcement, it was both the largest planned and lowest cost PV system in the world. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2 per installed watt. [14]

On the other hand, the solar energy experts at Denver-based Conergy Americas and officials at California's South San Joaquin Irrigation District (SSJID)[15] have installed what is believed to be the world's first single-axis solar tracking system featuring thin-film photovoltaic cells. [16]

Flexible CdTe cells with a record efficiency of 12.4%

EMPA, the Swiss Federal Laboratories for Materials Testing and Research in Dubendorf, Switzerland, has improved the efficiency of flexible CdTe thin film solar cells to 12.4%.



To improve the stability of CdTe/CdS solar cells, a new back contacting process was developed. According to the laboratory, new materials for the buffer layer and the metallisation yield stable cells. Accelerated testing under aggravated conditions (1 sun irradiation @ 80°C cell temperature) show the stability of these cells corresponding to 70 years in the field, it added.



The Laboratory for Thin Films and Photovoltaics, which is working on both CdTe and CuIn1-xGaxSe2, attached flexible CdTe thin-film solar cells on a lightweight polyimide film by using low temperatures (lower than 450°C) vacuum evaporation process to grow CdS/CdTe layers. The researchers used zinc oxide doped on aluminium (ZnO:Al) as a transparent electric contact instead of the expensive indium tin oxide (ITO) layer used in earlier 11.4% solar cells. In addition to being cheaper, the ZnO/ZnO:Al bi-layer improved process yield and reproducibility of high efficiency solar cells, according to EMPA.



The 12.4% efficiency of the flexible thin-film solar cells was measured under standard AMI.5 illumination condition. The parameters were Voc = 823 mV, Jsc = 19.6 mA.cm-2, FF = 76.5%.

References

1. ^ http://www.time.com/time/specials/packages/article/0,28804,1852747_1854195_1854153,00.html

2. ^ Amorphes Silizium für Solarzellen[1]

3. ^ J. M. Pearce, N. Podraza, R. W. Collins, M.M. Al-Jassim, K.M. Jones, J. Deng, and C. R. Wronski "Optimization of Open-Circuit Voltage in Amorphous Silicon Solar Cells with Mixed Phase (Amorphous + Nanocrystalline) p-Type Contacts of Low Nanocrystalline Content", Journal of Applied Physics, 101(11), 114301, 2007.http://me.queensu.ca/people/pearce/publications/documents/t14.pdf

4. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

5. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

6. ^ "ANWELL produces its first solar panel" (html). NextInsight. 2008-09-02. http://www.nextinsight.com.sg/content/view/1480/60/.

7. ^ "Photovoltaics: Thin-film technology about to make its breakthrough" (html). Solar server. 2008-08-07. http://www.solarserver.de/solarmagazin/index-e.html.

8. ^ "EPIA projects a rosy picture for the thin film industry" (html). Thin film today. http://social.thinfilmtoday.com/news/epia-projects-rosy-picture-thin-film-industry.

9. ^ http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=324202

10. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

11. ^ http://www.pv-tech.org/news/_a/germanys_largest_thin_film_pitched_roof_system_begins_production/

12. ^ "California Utility to Install 250MW of Roof-Top Solar". SustainableBusiness.com. 2008-03-27. http://www.sustainablebusiness.com/index.cfm/go/news.display/id/15670.

13. ^ "First Solar announces two solar projects with Southern California Edison". Semiconductor-Today.com. 2008-07-17. http://www.semiconductor-today.com/news_items/2008/JULY/FIRSTSOLAR_170708.htm.

14. ^ Report at juwi.dePDF (401 KB)

15. ^ http://www.ssjid.com

16. ^ http://www.renewableenergyworld.com/rea/partner/conergy-inc-1210/news/article/2009/04/conergy-brings-worlds-first-known-thin-film-solar-energy-tracking-system-and-400000-in-annual-utility-bill-savings-to-californias-south-san-joaquin-irrigation-district?cmpid=WNL-Wednesday-April22-2009

Sources

• Grama, S. “A Survey of Thin-Film Solar Photovoltaic Industry & Technologies.” Massachusetts Institute of Technology, 2008.

• Green, Martin A. “Consolidation of thin-film photovoltaic technology: the coming decade of opportunity.” Progress in Photovoltaics: Research and Applications 14, no. 5 (2006): 383-392.

• Green, M. A. “Recent developments in photovoltaics.” Solar Energy 76, no. 1-3 (2004): 3-8.

• Beaucarne, Guy. “Silicon Thin-Film Solar Cells.” Advances in OptoElectronics 2007 (August 2007): 12.

• Ullal, H. S., and B. von Roedern. “Thin Film CIGS and CdTe Photovoltaic Technologies: Commercialization, Critical Issues, and Applications; Preprint” (2007).

• Hegedus, S. “Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers.” Progress in Photovoltaics: Research and Applications 14, no. 5 (2006): 393-411.

• Poortmans, J., and V. Arkhipov. Thin Film Solar Cells: Fabrication, Characterization and Applications. Wiley, 2006.

• Wronski, C.R., B. Von Roedern, and A. Kolodziej. “Thin-film Si:H-based solar cells.” Vacuum 82, no. 10 (June 3, 2008): 1145-1150.

• Chopra, K. L., P. D. Paulson, and V. Dutta. “Thin-film solar cells: an overview.” Progress in Photovoltaics: Research and Applications 12, no. 2-3 (2004): 69-92.

• Hamakawa, Y. Thin-Film Solar Cells: Next Generation Photovoltaics and Its Applications. Springer, 2004.

• Green, Martin. “Thin-film solar cells: review of materials, technologies and commercial status.” Journal of Materials Science: Materials in Electronics 18, no. 0 (October 1, 2007): 15-19.







Flexible electronics





An Olympus Stylus camera without the case, showing the flex circuit assembly.

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide and PEEK Film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use. Flexible substrates have several advantages in many application:

• Tightly assembled electronic packages, where electrical connections are required in 3 axes, such as cameras (static application).

• Electrical connections where the assembly is required to flex during its normal use, such as folding cell phones (dynamic application).

• Electrical connections between sub-assemblies to replace wire harnesses, which are heavier and bulkier, such as in rockets and satellites.

• Electrical connections where board thickness or space constraints are driving factors.

Contents

• 1 Applications

o 1.1 Photovoltaic cells

• 2 See also

• 3 References


Applications

Flex circuits are often used as connectors in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring. In addition to cameras, a common application of flex circuits is in computer keyboard manufacturing; most keyboards made today use flex circuits for the switch matrix.

In LCD fabrication, glass is used as a substrate. If thin flexible plastic or metal foil is used as the substrate instead, the entire system can be flexible, as the film deposited on top of the substrate is usually very thin, on the order of a few micrometres.

OLEDs are normally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display.

Photovoltaic cells

Flexible solar cells have been developed for powering satellites. These cells are lightweight, can be rolled up for launch, and are easily deployable, making them a good match for the application.

On the other hand, Copper indium gallium diselenide (CIGS) solar cells are lightweight, flexible, and durable, which make them ideal for portable power (including solar jackets). CIGS is 1.5 to 2x greater in performance than comparable thin film flexible solar materials.

Hartley oscillator

Schematic diagram

Original Patent Drawing.

The Hartley oscillator is an LC electronic oscillator that derives its feedback from a tapped coil in parallel with a capacitor (the tank circuit). Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented as such. A Hartley oscillator is essentially any configuration that uses a pair of series-connected coils and a single capacitor (see Colpitts oscillator for the equivalent oscillator using two capacitors and one coil). It was invented by Ralph Hartley, who filed for a patent on June 1, 1915 and was awarded patent number [3] 1,356,763 on October 26, 1920.

Contents [hide] 1 Operation 2 Applications 3 History 4 See also 5 References 6 External links

 Operation

A Hartley oscillator is made up of the following:

• Two inductors in series, which need not be mutual
• One tuning capacitor

Advantages of the Hartley oscillator include:

• The frequency may be varied using a variable capacitor
• The output amplitude remains constant over the frequency range
• Either a tapped coil or two fixed inductors are needed

Disadvantages include:

• Harmonic-rich content if taken from the amplifier and not directly from the LC circuit.

Note that, if the inductance of the two partial coils L₁ and L₂ is given (e.g. in a simulator), the total effective inductance that determines the frequency of the oscillation is (coupling factor k):

(see ^[1])

[edit] Applications

Part of Scott 310E circuit diagram

The Hartley oscillator was extensively used on all broadcast bands including the FM 88-108MHz band. An example is given of the Scott 310E RF oscillator for its FM section.

 History

The Hartley oscillator was invented by Ralph V.L. Hartley while he was working for the Research Laboratory of the Western Electric Company. Hartley invented and patented the design in 1915 while overseeing Bell System's transatlantic radiotelephone tests. In 1946 Hartley was awarded the IRE medal of honor "For his early work on oscillating circuits employing triode tubes and likewise for his early recognition and clear exposition of the fundamental relationship between the total amount of information which may be transmitted over a transmission system of limited band-width and the time required."^[2](The second half of the citation refers to Hartley's work in information theory which largely paralleled Harry Nyquist.)

[edit] See also

• Armstrong oscillator
• Colpitts oscillator
• Clapp oscillator
• Vačkář oscillator
• Opto-Electronic Oscillator

 References

1. ^ Jim McLucas, Hartley oscillator requires no coupled inductors, EDN October 26, 2006 [1]
2. ^ Ralph V. L. Hartley, Legacies, IEEE History Center, updated January 23 2003, [2]

• Radiotron Designer's Handbook, 4th edition

• Ulrich L. Rohde, Ajay K. Poddar, Georg Böck "The Design of Modern Microwave Oscillators for Wireless Applications ", John Wiley & Sons, New York, NY, May, 2005, ISBN 0-471-72342-8.

• George Vendelin, Anthony M. Pavio, Ulrich L. Rohde " Microwave Circuit Design Using Linear and Nonlinear Techniques ", John Wiley & Sons, New York, NY, May, 2005, ISBN 0-471-41479-4.

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Types of Capacitors

Types of capacitor

Practical capacitors are often classified according to the material used as the dielectric, with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films (so-called electrolytic capacitors).

Contents 1 Capacitor construction 2 Types of dielectric 3 Fixed capacitor comparisons 4 Variable capacitors 5 Non-ideal properties of practical capacitors 5.1 Breakdown voltage 5.2 Q factor, dissipation and tan-delta 5.3 Equivalent series resistance (ESR) 5.4 Equivalent series inductance (ESL) 5.5 Maximum voltage and current 5.6 Temperature dependence 5.7 Aging 5.8 Dielectric absorption (soakage) 5.9 Voltage non-linearities 5.10 Leakage 6 Component values and identification 6.1 Capacitor markings 6.2 Standard values 6.3 Colour coding 7 See also 8 Notes 9 References 10 External links

[edit] Capacitor construction

Structure of a surface mount (SMT) film capacitor.

Capacitors have thin conducting plates (usually made of metal), separated by a layer of dielectric, then stacked or rolled to form a compact device.

Many types of capacitors are available commercially, with capacitances ranging from the picofarad,microfarad range to more than a farad, and voltage ratings up to hundreds of kilovolts. In general, the higher the capacitance and voltage rating, the larger the physical size of the capacitor and the higher the cost. Tolerances in capacitance value for discrete capacitors are usually specified as a percentage of the nominal value. Tolerances ranging from 50% (electrolytic types) to less than 1% are commonly available.

Another figure of merit for capacitors is stability with respect to time and temperature, sometimes called drift. Variable capacitors are generally less stable than fixed types.

The electrodes need round edges to avoid field electron emission. Air has a low breakdown voltage, so any air inside a capacitor - especially at plate edges - will reduce the voltage rating. Even closed air bubbles in the insulator or between the insulator and the electrode lead to gas discharge, particularly in AC or High frequency applications. Groups of identically constructed capacitor elements are often connected in series for operation at higher voltage. High voltage capacitors need large, smooth, and round terminals to prevent corona discharge.

 Types of dielectric

Capacitor	Polarized Capacitor	Variable Capacitor

Capacitor symbols

• Air-gap: An air-gap capacitor has a low dielectric loss. Large-valued, tunable capacitors that can be used for resonating HF antennas can be made this way.
• Ceramic: The main differences between ceramic dielectric types are the temperature coefficient of capacitance, and the dielectric loss. C0G and NP0 (negative-positive-zero, i.e. ±0) dielectrics have the lowest losses, and are used in filters, as timing elements, and for balancing crystal oscillators. Ceramic capacitors tend to have low inductance because of their small size. NP0 refers to the shape of the capacitor's temperature coefficient graph (how much the capacitance changes with temperature). NP0 means that the graph is flat and the device is not affected by temperature changes.
  • C0G or NP0 — Typically 1 pF to 0.1 µF, 5%. High tolerance and good temperature performance. Larger and more expensive.
  • X7R — Typical 100 pF to 22 µF, 10%. Good for non-critical coupling, timing applications. Subject to microphonics.
  • Z5U or 2E6 — Typical 0.001 µF to 10 µF, 20%. Good for bypass, coupling applications. Low price and small size. Subject to microphonics.
  • Ceramic chip: 1% accurate, values up to about 1 µF, typically made from Lead zirconate titanate (PZT) ferroelectric ceramic
• Gimmick: These type of capacitors are made by twisting 2 piece of insulated wires. Values usually ranges from 3pF to 15pF.Usually used is homemade Hem circuit of VHF circuits as a oscillation feedback.
• Trimmer: These type of Capacitors have a rotating plate (which can be rotated to change the capacitance) separated from a fixed plate by a dielectric medium. Typically values from 5-60 pF.
• Glass — used to form extremely stable, reliable capacitors.
• Paper — common in antique radio equipment, paper dielectric and aluminum foil layers rolled into a cylinder and sealed with wax. Low values up to a few μF, working voltage up to several hundred volts, oil-impregnated bathtub types to 5,000 V used for motor starting and high-voltage power supplies, and up to 25,000 V for large oil-impregnated energy discharge types.
• Polycarbonate good for filters, low tempco, good aging, expensive
• Polyester, (PET film): (from about 1 nF to 10 μF) signal capacitors, integrators.
• Polystyrene: (usually in the picofarad range) stable signal capacitors.
• Polypropylene: low-loss, high voltage, resistant to breakdown, signal capacitors.
• PTFE or Teflon : higher performing and more expensive than other plastic dielectrics.
• Silvered mica: These are fast and stable for HF and low VHF RF circuits, but expensive.
• Electrolytic capacitors have a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits, e.g. in power-supply filters or as coupling capacitors in audio amplifiers. High-capacity electrolytics, also known as supercapacitors or ultracapacitors, have applications similar to those of rechargeable batteries, e.g. in electrically powered vehicles;
• Printed circuit board: Metal conductive areas in different layers of a multi-layer printed circuit board can act as a highly stable capacitor. It is common industry practice to fill unused areas of one PCB layer with the ground conductor and another layer with the power conductor, forming a large distributed capacitor between the layers, or to make power traces broader than signal traces.
• In integrated circuits, small capacitors can be formed through appropriate patterns of metallization on an isolating substrate.
• Vacuum: vacuum variable capacitors are generally expensive, housed in glass or ceramic body, typically rated for 5kV - 30kV. Typically used in high power RF transmitters because the dielectric has virtually no loss and is self-healing. May be fixed or adjustable.

Fixed capacitor comparisons

Capacitor type	Dielectric used	Features/applications	Disadvantages
Paper Capacitors	Paper or oil-impregnated paper	Impregnated paper was extensively used for older capacitors, using wax, oil, or epoxy as an impregnant. Oil-Kraft paper capacitors are still used in certain high voltage applications. Has mostly been replaced by plastic film capacitors.	Large size. Also, paper is highly hygroscopic, absorbing moisture from the atmosphere despite plastic enclosures and impregnates. Absorbed moisture degrades performance by increasing dielectric losses (power factor) and decreasing insulation resistance.
Metalized Paper Capacitors	Paper	Comparatively smaller in size than paper-foil capacitors	Suitable only for lower current applications. Has been largely superseded by metalized film capacitors
PET film Capacitor	Polyester film	Smaller in size when compared to paper or polypropylene capacitors of comparable specifications. May use plates of foil, metalized film, or a combination. PET film capacitors have almost completely replaced paper capacitors for most DC electronic applications. Operating voltages up to 60,000VDC and operating temperatures up to 125°C. Low moisture absorption.	Temperature stability is poorer than paper capacitors. Usable at low (AC power) frequencies, but inappropriate for RF applications due to excessive dielectric heating.
Kapton Capacitor	Kapton polyimide film	Similar to PET film, but significantly higher operating temperature (up to 250°C).	Higher cost than PET. Temperature stability is poorer than paper capacitors. Usable at low (AC power) frequencies, but inappropriate for RF applications due to excessive dielectric heating.
Polystyrene Capacitor	Polystyrene	Excellent general purpose plastic film capacitor. Excellent stability, low moisture pick-up and a slightly negative temperature coefficient that can be used to match the positive temperature co-efficient of other components. Ideal for low power RF and precision analog applications	Maximum operating temperature is limited to about +85°C. Comparatively bigger in size.
Polycarbonate Plastic Film Capacitor	Polycarbonate	Superior insulation resistance, dissipation factor, and dielectric absorption versus polystyrene capacitors. Moisture pick-up is less, with about +/- 80 ppm temperature co-efficient. Can use full operating voltage across entire temperature range (-55°C to 125°C)	Maximum operating temperature limited to about 125°C.
Polypropylene Plastic Film Capacitors	Polypropylene	Has become the most popular capacitor dielectric[citation needed]. Extremely low dissipation factor, higher dielectric strength than polycarbonate and polyester films, low moisture absorption, and high insulation resistance. May use plates of foil, metalized film, or a combination. Film is compatible with self-healing technology to improve reliability. Usable in high frequency applications due to very low dielectric losses. Larger value and higher voltage types from 1 to 100μF at up to 440V AC are used as run capacitors in some types of single phase electric motors.	More susceptible to damage from transient over-voltages or voltage reversals than oil-impregnated Kraft paper for pulsed power energy discharge applications.
Polysulphone Plastic Film Capacitors	Polysulfone	Similar to polycarbonate. Can withstand full voltage at comparatively higher temperatures. Moisture pick-up is typically 0.2%, limiting its stability.	Very limited availability and higher cost
PTFE Fluorocarbon (TEFLON) Film Capacitors	Polytetra- fluoroethylene	Lowest loss solid dielectric. Operating temperatures up to 250°C, extremely high insulation resistance, and good stability. Used in stringent, mission-critical applications	Large size (due to low dielectric constant), and higher cost than other film capacitors.
Polyamide Plastic Film Capacitors	Polyamide	Operating temperatures of up to 200°C. High insulation resistance, good stability and low dissipation factor.	Large size and high cost.
Metalized Plastic Film Capacitors	Polyester or Polycarbonate	Reliable and significantly smaller in size. Thin metalization can be used to advantage by making capacitors "self healing".	Thin plates limit maximum current carrying capability.
Stacked Plate Mica Capacitors	Mica	Advantages of mica capacitors arise from the fact that the dielectric material (mica) is inert. It does not change physically or chemically with age and it has good temperature stability. Very resistant to corona damage	Unless properly sealed, susceptible to moisture pick-up which will increase the power factor and decrease insulation resistance. Higher cost due to scarcity of high grade dielectric material and manually-intensive assembly.
Metalized Mica or Silver Mica Capacitors	Mica	Silver mica capacitors have the above mentioned advantages. In addition, they have much reduced moisture infiltration.	Higher cost
Glass Capacitors	Glass	Similar to Mica Capacitors. Stability and frequency characteristics are better than silver mica capacitors. Ultra-reliable, ultra-stable, and resistant to nuclear radiation.	High cost.
Class-I Temperature Compensating Type Ceramic Capacitors	Mixture of complex Titanate compounds	Low cost and small size, excellent high frequency characteristics and good reliability. Predictable linear capacitance change with operating temperature. Available in voltages up to 15,000 volts	Capacitance changes with change in applied voltage, with frequency and with aging effects.
Class-II High dielectric strength Type Ceramic Capacitors	Barium titanate based dielectrics	Smaller than Class-I type due to higher dielectric strength of ceramics used. Available in voltages up to 50,000 volts.	Not as stable as Class-I type with respect to temperature, and capacitance changes significantly with applied voltage.
Aluminum Electrolytic Capacitors	Aluminum oxide	Very large capacitance to volume ratio, inexpensive, polarized. Primary applications are as smoothing and reservoir capacitors in power supplies.	Dielectric leakage is high, large internal resistance and inductance limits high frequency performance, poor low temperature stability and loose tolerances. May vent or burst open when overloaded and/or overheated. Limited to about 500 volts.
Lithium Ion Capacitors	Lithium ion	The Lithium Ion Capacitors have a higher Power Density as compared to batteries and LIC’s are safer in use than LIB’s in which thermal runaway reactions may occur. Compared to Electric Double Layer Capacitor (EDLC), the LIC has a higher output voltage. They both have similar Power Densities, but Energy Density of an LIC is much higher.	New technology.
Tantalum Electrolytic Capacitors	Tantalum oxide	Large capacitance to volume ratio, smaller size, good stability, wide operating temperature range, long reliable operating life. Extensively used in miniaturized equipment and computers. Available in both polarized and unpolarized varieties. Solid tantalum capacitors have much better characteristics than their wet counterparts.	Higher cost than aluminum electrolytic capacitors. Voltage limited to about 50 volts. Explodes quite violently when voltage rating, current rating, or slew rates are exceeded, or when a polarized version is subjected to reverse voltage.
Electrolytic double-layer capacitors (EDLC) Supercapacitors	Thin Electrolyte layer and Activated Carbon	Extremely large capacitance to volume ratio, small size, low ESR. Available in hundreds, or thousands, of farads. A relatively new capacitor technology. Often used to temporarily provide power to equipment during battery replacement. Can rapidly absorb and deliver larger currents than batteries during charging and discharging, making them valuable for hybrid vehicles. Polarized, low operating voltage (volts per capacitor cell). Groups of cells are stacked to provide higher overall operating voltage.	Relatively high cost.
Alternating current oil-filled Capacitors	Oil-impregnated paper	Usually PET or polypropylene film dielectric. Primarily designed to provide very large capacitance for industrial AC applications to withstand large currents and high peak voltages at power line frequencies. The applications include AC motor starting and running, phase splitting, power factor correction, voltage regulation, control equipment, etc..	Limited to low frequency applications due to high dielectric losses at higher frequencies.
Direct current oil-filled capacitors	Paper or Paper-polyester film combination	Primarily designed for DC applications such as filtering, bypassing, coupling, arc suppression, voltage doubling, etc...	Operating voltage rating must be derated as per the curve supplied by the manufacturer if the DC contains ripple. Physically larger than polymer dielectric counterparts.
Energy Storage Capacitors	Kraft capacitor paper impregnated with electrical grade castor oil or similar high dielectric constant fluid, with extended foil plates	Designed specifically for intermittent duty, high current discharge applications. More tolerant of voltage reversal than many polymer dielectrics. Typical applications include pulsed power, electromagnetic forming, pulsed lasers, Marx generators, and pulsed welders.	Physically large and heavy. Significantly lower energy density than polymer dielectric systems. Not self-healing. Device may fail catastrophically due to high stored energy.
Vacuum Capacitors	Vacuum capacitors use highly evacuated glass or ceramic chamber with concentric cylindrical electrodes.	Extremely low loss. Used for high voltage high power RF applications, such as transmitters and induction heating where even a small amount of dielectric loss would cause excessive heating. Can be self-healing if arc-over current is limited.	Very high cost, fragile, physically large, and relatively low capacitance.

A 12 pF, 20 kV fixed vacuum capacitor

Two 8 μF, 525 V paper electrolytic capacitors in a 1930s radio.^[1]

Variable capacitors

Main article: Variable capacitor

Variable capacitors may have their capacitance intentionally and repeatedly changed over the life of the device. They include capacitors that use a mechanical construction to change the distance between the plates, or the amount of plate surface area which overlaps, and variable capacitance diodes that change their capacitance as a function of the applied reverse bias voltage.

Variable capacitance is also used in sensors for physical quantities, including microphones, pressure and hygro sensors.

 Non-ideal properties of practical capacitors

Breakdown voltage

The breakdown voltage of the dielectric limits the power density of capacitors. For a particular dielectric, the breakdown voltage is proportional to the thickness of the dielectric.

If a manufacturer makes a new capacitor with the same dielectric as some old capacitor, but with half the thickness of the dielectric, the new capacitor has half the breakdown voltage of the old capacitor.

Because the plates are closer together, the manufacturer can put twice the parallel-plate area inside the new capacitor and still fit it in the same volume (capacitor size) as the old capacitor. Since the capacitance of a parallel-plate capacitor is given by:

this new capacitor has 4 times the capacitance as the old capacitor.

Since the energy stored in a capacitor is given by:

this new capacitor has the same maximum energy density as the old capacitor.

The energy density depends only on the dielectric. Making a few thick layers of dielectric (which can support a high voltage, but results in a low capacitance), or making many very thin layers of dielectric (which results in a low breakdown voltage, but a higher capacitance) has no effect on the energy density.

 Q factor, dissipation and tan-delta

Capacitors have "Q" (quality) factor (and the inverse, dissipation factor , D or tan-delta) which relates capacitance at a certain frequency to the combined losses due to dielectric leakage and series internal resistance ( also known as ESR) dissipation factor (dielectric loss). The lower the 'Q', the lossier the capacitor. Aluminum Electrolytic types have typically low Q factors. High Q capacitors tend to exhibit low DC leakage currents. Tan-delta is the tangent of the phase angle between voltage and current in the capacitor. This angle is sometimes called the loss angle. It is related to the power factor which is zero for an ideal capacitor.

 Equivalent series resistance (ESR)

This is an effective resistance that is used to describe the resistive parts of the impedance of certain electronic components. The theoretical treatment of devices such as capacitors and inductors tends to assume they are ideal or "perfect" devices, contributing only capacitance or inductance to the circuit. However, all physical devices are constructed of materials with finite electrical resistance, which means that all real-world components contain some resistance in addition to their other properties. A low ESR capacitor typically has an ESR of 0.01 Ω. Low values are preferred for high-current, pulse applications. Low ESR capacitors have the capability to deliver huge currents into short circuits, which can be dangerous.

For capacitors, ESR takes into account the internal lead and plate resistances and other factors. An easy way to deal with these inherent resistances in circuit analysis is to express each real capacitor as a combination of an ideal component and a small resistor in series, the resistor having a value equal to the resistance of the physical device.

 Equivalent series inductance (ESL)

ESL in signal capacitors is mainly caused by the leads used to connect the plates to the outside world and the series interconnects used to join sets of plates together internally. For any real-world capacitor, there is a frequency above DC at which it ceases to behave as a pure capacitance. This is called the (first) resonant frequency. This is critically important with decoupling high-speed logic circuits from the power supply. The decoupling capacitor supplies transient current to the chip. Without decouplers, the IC demands current faster than the connection to the power supply can supply it, as parts of the circuit rapidly switch on and off. Large capacitors tend to have much higher ESL than small ones. As a result, electronics will frequently use multiple bypass capacitors — a small 0.1 µF rated for high frequencies and a large electrolytic rated for lower frequencies, and occasionally, an intermediate value capacitor.

Maximum voltage and current

Important properties of capacitors are the maximum working voltage (potential, measured in volts) and the amount of energy lost in the dielectric. For high-power or high-speed capacitors, the maximum ripple current, peak current, fault current, and percent voltage reversal are further considerations. Typically the voltage is 66% of the rated voltage. A voltage higher than that, usually reduces the life expectancy depending on manufacturer. The time for a voltage to discharge is 6 time constants.

Temperature dependence

Another major non-ideality is temperature coefficient (change in capacitance with temperature) which is usually quoted in parts per million (ppm) per degree Celsius.

Aging

When refurbishing old (especially audio) equipment, it is a good idea to replace all of the electrolyte-based capacitors. After long storage, the electrolyte and dielectric layer within electrolytic capacitors may deteriorate; before powering up equipment with old electrolytics, it may be useful to apply low voltage to allow the capacitors to reform before applying full voltage. Deteriorating capacitors are a frequent cause of hum in aging audio equipment.

Non polarised capacitors also suffer from aging, changing their values slightly over long periods of time.

In high voltage DC applications, accumulated capacitor stress due to in-rush currents at circuit power-up can be minimized with a pre-charge circuit.

Dielectric absorption (soakage)

Some types of dielectrics, when they have been holding a high voltage for a long time, maintain a "memory" of that voltage. After they have been quickly discharged to zero volts, if they are then left disconnected, the voltage across the capacitor will slowly recover some fixed percentage -- up to 10% -- of the "remembered" voltage. This percentage is a measure of the dielectric absorption, and depends on the type of dielectric.

In the construction of long-time-constant integrators, it is important that the capacitor will not retain a residual charge when shorted. This phenomenon of unwanted charge storage is called dielectric absorption or soakage, and it effectively creates a memory effect in the capacitor. This is a non-linear phenomenon, and is also important when building very low distortion filters. This is also why, for safety, high voltage capacitors are stored with their terminals short circuited.

For long-time-constant integrators and sample-and-hold systems, good designers pick capacitors that have almost no dielectric absorption hysteresis -- capacitors such as those employing polystyrene, polypropylene, NPO ceramic, and Teflon dielectrics. ^{[2] [3]}

Voltage non-linearities

Capacitors may also change capacitance with applied voltage. This effect is more prevalent in high 'k' ceramic and some high voltage capacitors. This can be another small source of non-linearity when building low distortion filters.

Leakage

Capacitors also have some level of parasitic resistance across the terminals which is called 'leakage'. This fundamentally limits how long capacitors can store charge. Historically, this was a major source of problems in some types of applications (long RC timers, sample-and-holds, etc.)

Component values and identification

Capacitor markings

Most capacitors have numbers printed on their bodies to indicate their electrical characteristics.

Some are indicated with XYZ K/M VOLTS V where XYZ stands for the capacitance, the letters K or M indicate the tolerance of ±10% or ±20% respectively and VOLTS V represents the working voltage.

Example:

A capacitor that has written on its body:

105 K 330 V

is a capacitor 10 x 10^5 pF ± 10% with a working voltage of 330 V

A capacitor

103 M 100 V

is a capacitor 10 x 10³ pF ± 20% with a work voltage of 100 V, or 10,000 x 10^-12 farad, or 0.01 µF (microfarad).

The rule is: x 10^Z pF (picofarads) tolerance work voltage

 Standard values

In the early days of electronics, components were often made to fit a specific need, the values of early capacitors were of arbitrary (usually integer) base numbers. The more common values included 1.0, 1.5, 2.0, 3.0, 5.0, 6.0, and 8.0 as base numbers, but they were not necessarily limited to these values. Values were generally in microfarads (µF) and could be multiplied by any power of ten; picofarads were often called micro-microfarads (µµF) then.

In the late 1960s, a standardized set of geometrically increasing base values was introduced. According to the number of values per decade, these were called the E3, E6 or E12 series:

Series	Values
E3	1.0				2.2				4.7
E6	1.0		1.5		2.2		3.3		4.7		6.8
E12	1.0	1.2	1.5	1.8	2.2	2.7	3.3	3.9	4.7	5.6	6.8	8.2

The same series are used for resistors, where E24/E48/E96 series are additionally used for even lower-tolerance components. These number series are known as preferred values.

Since most electrolytic capacitors have a tolerance range of ±20%, meaning that the manufacturer is stating that the actual value of the capacitor lies within ±20% of its nominal value, they are normally available in E6 (or even just E3) series values only (e.g. 2200 µF, 3300 µF, 4700 µF)  – the tolerance ranges overlap the intermediate values from the next higher series anyway.

Other types of capacitors, e.g. ceramic, can be manufactured to tighter tolerances and are available in E12 values (e.g. 47 pF, 56 pF, 68 pF).

Capacitors were once specified by their values in either microfarads or picofarads, which meant that both very small (such as 0.01 µF) and very large (such as 10,000 pF) numbers were in common use. Nowadays, it is considered preferable to use the nanofarad as well, and specify all values in the numeric range 1 - 999 only. Above 999 µF, the practice is not yet in common use; capacitors are not usually specified in millifarads (mF), probably because it would be too easily confused with microfarads (for which mF was once an acceptable abbreviation).

A table giving translations of previous commonly used multiples is as follows:

preferred	in pF	in nF	in µF
1pF	1	0.001	0.000,001
10pF	10	0.01	0.000,01
100pF	100	0.1	0.000,1
1nF	1000	1	0.001
10nF	10,000	10	0.01
100nF	100,000	100	0.1
1µF	1,000,000	1000	1

[edit] Colour coding

Color		Significant digits	Multiplier	Capacitance tolerance	Characteristic	DC working voltage	Operating temperature	EIA/vibration
	Black	0	1	±20%	—	—	−55 °C to +70 °C	10 to 55 Hz
	Brown	1	10	±1%	B	100	—	—
	Red	2	100	±2%	C	—	−55 °C to +85°C	—
	Orange	3	1,000	—	D	300	—	—
	Yellow	4	10,000	—	E	—	−55 °C to +125°C	10 to 2000 Hz
	Green	5	—	±5%	F	500	—	—
	Blue	6	—	—	—	—	−55 °C to +150 °C	—
	Violet	7	—	—	—	—	—	—
	Grey	8	—	—	—	—	—	—
	White	9	—	—	—	—	—	EIA
	Gold	—	—	±0.5%*	—	1000	—	—
	Silver	—	—	±10%	—	—	—	—

*Or ±0.5 pF, whichever is greater.

• Capacitor plague (premature failure of some electrolytic capacitors)
• Supercapacitor
• Electronic devices and circuits
• Electronic color code
• Inductor
• Flux capacitor (fictional component from the Back to the Future film series)

 Notes

1. ^ The abbreviation "MF" was used to indicate micro-Farads at the time; "MMF" was common for micro-microfarad = 10^-12F or picofarads.
2. ^ "Understand Capacitor Soakage to Optimize Analog Systems" by Bob Pease 1982 http://www.national.com/rap/Application/0,1570,28,00.html
3. ^ "Modeling Dielectric Absorption in Capacitors" by Ken Kundert http://www.designers-guide.org/Modeling/da.pdf

 References

• Tre Clifford Super Charged: A Tiny South Korean Company is Out to Make Capacitors Powerful enough to Propel the Next Generation of Hybrid-Electric Cars, IEEE Spectrum, January, 2005 Vol 42, No. 1, North American Edition.
• The ARRL Handbook for Radio Amateurs, 68th ed, The Amateur Radio Relay League, Newington CT USA, 1991
• Basic Circuit Theory with Digital Computations, Lawrence P. Huelsman, Prentice-Hall, 1972
• Philosophical Transactions of the Royal Society LXXII, Appendix 8, 1782 (Volta coins the word condenser)
• A. K. Maini Electronic Projects for Beginners, "Pustak Mahal", 2nd Edition: March, 1998 (INDIA)

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