AN INTRODUCTION TO CAPACITORS by David Johnson, Los Banos, CA. (This article originally appeared in the Canadian LF/VLF newsletter _Northern Observer_ [issue #23, mid-December 1990]. It is posted here by permission of the author.) Inexperienced techs, and engineers fresh out of school, frequently assume that once you know the value of a capacitor you know almost everything there is to know about it that's important. After all, this rule of thumb works with resistors most of the time. But there are very important differences between the various types of capacitors-- which is the reason that so many different types are marketed. I will discuss the important properties of the more common types of capacitors that you are likely to encounter. This discussion will not cover specialty capacitors. ALUMINUM ELECTROLYTICS pack a lot of capacitance into a relatively small volume, and are cheap. They are used primarily for power-supply filtering, low-frequency AC coupling, and DC blocking. In general they are not suitable for other applications because they have very sloppy tolerances, exhibit marked temperature drift, have low Q, require DC biasing of known polarity (except special nonpolarized units), and have high leakage currents, especially if the capacitor has not been used for a long period of time. If biased with the wrong polarity of DC voltage, they usually explode within several seconds or minutes, creating a substantial hazard. Aluminum electrolytic capacitors gradually deteriorate with age, though this problem is not a serious one with modern units. Never use old or surplus electrolytics-- buy new; they're cheap. Aluminum electrolytics give best life and performance when used at close to their rated voltage. Don't use a 100 volt unit in a 10 volt circuit. A 16 volt unit would be much better. Aluminum electrolytics are widely available in ratings from 1 to 10,000 microfarads (uF), with voltage ratings up to several hundred volts in the low to medium capacitance ranges. SOLID TANTALUMS. These are solid semiconductor electrolytics with properties that are generally better than aluminum units. They are widely used for power-supply decoupling and low-frequency AC coupling. They are much more stable with time and temperature than are aluminum electrolytics, and have higher Q as well. It's OK to use them at voltages much lower than their rating, but they _are_ polarized and should never be allowed to become reverse-biased. A reverse-biased solid tantalum may work for seconds to several days, but it _will_ fail, usually with a horrible stench and possibly fire. Solid tantalums are sometimes used in non-critical timing and frequency filtering circuits. Solid tantalum capacitors are widely available in ratings from 0.47 to 47 microfarads at up to 35 volts in the lower capacitance values. POLYESTER (Mylar (tm)) FILM CAPS. These popular capacitors are widely available in ratings from .001 to 10 uF, at voltages up to 630 volts in the medium and low capacitance values. They receive use in a wide variety of applications, particularly in audio-frequency circuits. Their temperature curve is fairly flat at room temperature, providing good stability when used in equipment that will not be used outdoors. Aging is generally less than 1% over the life of the product. Q factor is several hundred at audio frequencies, but plummets at higher frequencies, making them generally unsuited for RF work. Dielectric absorption and DC leakage preclude their use in high-performance sample-and-hold circuits. POLYCARBONATE FILM CAPS are rather similar to polyester caps, but most of their electrical properties are better. The temperature curve is relatively flat over a much wider range and is generally within the range of +25 to -50 ppm per degree C at room temperature. Q is about 1000 in the audio range, dropping to about 100 at 1 MHz. Dielectric absorption and leakage are low enough to allow use in medium- performance sample-and-hold circuits. Polycarbs are bulkier than polyesters for a given capacitance and voltage rating, and they cost more. They are available in tighter tolerances (1%) than polyesters, and they generally age less than 0.2% over the life of the product. Polycarbs are manufactured in the same capacitances and voltage ranges as polyesters, but are not as widely available as polyester since they are more costly. Probably the most common uses for polycarbs are high-Q audio filters, tight-tolerance RC timers, and precision instrumentation circuits. POLYPROPYLENE FILM CAPS. Somewhat similar to polycarb, but reserved for even ore critical applications. Aging is generally less than 0.1% over the life of the product, dielectric absorption and DC leakage specs are even better, and Q is several thousand in the audio- frequency range. The temperature coefficient is fairly steep-- about -220 parts per million per deg. C.-- but the curve is flat, making it possible to use other components to compensate for temperature drift over a wide range if that is necessary. Polypropylene film caps are used to some extent in lower frequency RF work, where positive tempco inductors can be used to balance the tempco of the capacitor to produce a very stable tuned circuit. Polypropylene caps are generally available in the range of 470 pF to 1 uF, up to 630 volts, are harder to find than polyesters and polycarbs, and are more expensive and bulkier than polycarbs. Digi-Key sells 2% tolerance units and most manufacturers have 1% tolerance units available. Most usage of polypropylene caps is in high-performance tuned circuits, timers, and sample-and-hold circuits; however, they are starting to see use in switching power-supplies because their high Q allows them to handle large amounts of power without overheating or reducing efficiency. POLYSTYRENE FILM CAPS. Extremely low dielectric absorption and DC leakage make this a popular capacitor for high-performance S/H circuits and integrating A/D converters. Its superb stability (ageing typically less than .05% over life) and predictably low temperature coefficient (-150 to -190 ppm per deg. C) over a very wide temperature range make this a common capacitor in high-performance instrumentation. Its extremely high Q (over 10000 in the audio range, and several hundred at 1 MHz) make it popular for lower-frequency RF tuned circuits. Readily available in values from 100 pF to .01 uF-- Higher values are difficult to find, and they are very bulky. Polystyrene caps are very easily damaged by heat from soldering, and by solvents. Because of this, polystyrene capacitors are losing popularity (primarily to polypropylenes and to NPO ceramic capacitors). EXOTIC PLASTIC-FILM CAPS include Teflon (tm), polysulfone, parylene and other dielectrics. These are generally available only on special order from the factory, and they aren't used except when nothing else will do. The exotics generally exhibit some combination of high stability and/or operation at high temperatures. CERAMIC CAPACITORS. These are even ore diverse a group than the plastic film capacitors, and are even less interchangeable. When you buy surplus or used ceramic caps, you usually don't know what kind you are getting. If you don't know what kind they are, don't buy 'em. NPO (COG) CERAMICS. These are very stable with time and temperature, and have high Q at RF. Temperature coefficient is les than +-50 ppm/deg.C and ageing is extremely low. Widely available in values form 5 pF to 1000 pF (and less widely available 1 to 10000 pF) in two popular styles: disc and multilayer. The multilayers are more compact in the larger capacitance values. A tubular style used to be popular but is rarely seen anymore. When handling and soldering ceramic capacitors of any type, use care-- the leads on some manufacturers' units are poorly bonded to the capacitor plates, and detach easily. On some, the attachment solder melts at a temperature lower than that of ordinary solder, so if in doubt, use pliers as a heat-sink on the leads. If the wire _does_ detach, the protective coating will often hold it in place, leaving an open or intermittent capacitor that looks physically perfect. NPO capacitors are widely used in RF circuitry. They are gradually displacing micas in most applications because they have higher Q and better stability than most micas. In addition, the multilayer NPO's are much smaller than micas. X7R CERAMICS. These are designed for general-purpose use where stability is not critical. A rather wobbly temperature curve keeps capacitance variation due to temperature change within +-5% of room temperature value from -40 to +90C. Capacitance can decrease up to several percent with aging. Initial tolerance is generally 10 or 20%- - tighter tolerances are not available because aging effects would make tighter tolerances meaningless. Three styles are popular-- disc, multilayer, and military CK05 box style. Popular values range from .001 to 0.1 uF; lower and higher values are available. Polyester film caps are gradually displacing X7R ceramics in many applications; however, where space is critical, multilayer X7R's can still beat polyester film caps for size. Z5U CERAMICS. These are used almost exclusively for power-supply bypassing. Their capacitance changes markedly with temperature, and there is an appreciable capacitance loss with aging. Both disc and multilayer styles are popular, and values from .01 to 2 uF are readily available. The multilayer type can be smaller than an aluminum electrolytic of the same capacitance. Z5U's have lower impedance at high frequencies than do electrolytics, and do not require a DC bias voltage since they aren't polarized. Because of these reasons, they are more popular than aluminum electrolytics in values below 1 uF. TEMPERATURE-COMPENSATING CERAMICS are generally similar to NPO ceramics, but are designed to have a specified negative temperature coefficient. They are hard to find, and are used primarily for temperature compensation of other components such as inductors or quartz crystals at RF. REDUCED TITANATE (BARRIER TYPE) CERAMICS. Not used much anymore. This group comprises a wide variety of temperature coefficients and capacitance values. They require DC polarization, have low voltage ratings (as low as 3 volts), have extremely high leakage. They met a need for small capacitors in the early days of the transistor, before modern multilayer ceramics were developed. OTHER CERAMICS. There are other ceramic types available, the most popular of which are probably formulations designed to get more capacitance per unit volume for power-supply bypassing applications. These formulations are generally less stable than Z5U and should be used only for power-supply bypassing. Values up to about 10 uF are available. MICA CAPACITORS have a low temperature coefficient, good stability, and high Q at radio frequencies. However, the performance of NPO ceramics is in general slightly superior, and the NPO's are usually cheaper and smaller as well. Mica capacitors are readily available in values from about 3 pF to several thousand pF. They are still a bit easier to obtain in tight tolerances (to 1%) and high voltage ratings than are NPO's. PAPER CAPACITORS used to be _the_ general-purpose capacitor, but have been almost entirely superseded by polyester and ceramic capacitors. They are still used to some extend in applications involving AC line power, for instance, motor-starting capacitors. The paper is normally impregnated with an oil or some other material that is the actual dielectric. FILM VARIABLE CAPACITORS. These are the little plastic variable capacitors in modern portable radios. The dielectric is a plastic film-- polyethylene, I think, but possibly something else. The temperature coefficient is probably about -200 ppm/deg C, which compensates a tuning inductor having a tempco of +200 ppm. Installing a proper knob on one of these (or even installing it, period) can take some ingenuity or access to rare hardware. Sometimes good hardware can be cannibalized from a junked receiver. AIR VARIABLE CAPACITORS. More stable than the film variables, and available in a much wider range of voltages, capacitances and styles. Unfortunately, there are few manufacturers still in business, and prices of new units are quite high. Experimenters usually obtain air variable caps through surplus houses or by cannibalizing old equipment. The high voltages encountered in most transmitters preclude the use of film variable caps. Many air variable caps come with a 1/4 inch diameter shaft, making it easy to install a knob or other hardware. Maximum capacitance available is about .002 uF. VACUUM VARIABLE CAPACITORS withstand very high voltages, and are used in the power and antenna circuits of radio transmitters. They are very expensive, generally over $200 if purchased new. Experimenters usually obtain them from surplus houses or by cannibalizing old equipment. TRIMMER CAPACITORS. These are designed for set-and-forget, not to be frequently adjusted like the other variable capacitors discussed. Popular types are rotary )air, film or ceramic dielectric), and mica compression types. Air types are most stable, and generally most expensive. Film and ceramic types often have rather high temperature coefficients. Mica compression type has low temp. coefficient, but is prone to come out of adjustment by itself. The mica compression type is available in maximum values up to 2000 pF, ceramic and film types up to about 150 pF, and air type to about 60 pF. DOUBLE LAYER "MEMORY BACKUP" CAPACITORS. These devices have electrical properties intermediate between an electrolytic capacitor and a rechargeable battery. Nominal capacitance values range from about .02 to several Farads. They must be used at about their rated voltages (which is usually 5.5 volts). They are used to provide power backup to CMOS memory for up to several hours in the event of a power outage. I am not aware of any other use for these "capacitors." -- posted with author's permission by Frank Reid W9MKV reid@ucs.indiana.edu e