Whenever a capacitor is designed into an electronic circuit, it has been selected to do a specific job - and certain capacitors will perform certain jobs better than some others.
Let us examine these jobs that must be performed by the capacitors and analyze the important factors leading to the proper selection.
Whenever direct current (DC) is applied to a capacitor, the capacitor effectively "blocks" this current and will not allow it to pass through. Utilizing this property of the capacitor, a design engineer can isolate a circuit element from the DC supply. For this job he has a choice to make among many dielectrics, each of which will allow a different amount of tiny "leakage current" to pass through itself. This "leakage current" is a normal result of the random passage of electrons through or around the dielectric (usually in the microampere range), and should not be confused with the normal DC current supply. Thus, the design engineer can match the dielectric with the critical value of "leakage current" that his circuit can tolerate without malfunction.
Just as the capacitor will block out direct current, it will appear to pass alternating current (AC). The capacitor is really charging and discharging in opposite directions each half-cycle as the impressed AC voltage alternates in polarity. To the rest of the circuit this has the same effect as though the capacitor were allowing the passage of the AC signal. By using this ability of the capacitors, the design engineer can "couple" one portion of a circuit to another portion of a circuit or even to a different circuit. Here, the inherent dissipation factor of the capacitor style can be critical if the heating effect of the AC voltage is appreciable.
This job is a good example of the combination of blocking and coupling functions. In this case, the capacitor is used to separate the DC and AC portions of a mixed signal current. Here, the capacitor is placed in parallel with a circuit element. The purpose here is to ensure that the DC portion does appear on the circuit element -but not the AC portion! By properly choosing the capacitor rating, the AC signal sees an apparently low impedance path "through" the capacitor compared to the circuit element and consequently "by-passes" the circuit element. The DC signal sees a low impedance path through the circuit element and travels that way. Obviously, both the leakage current and dissipation factor parameters are to be considered in any application where these factors are critical.
Somewhat related to the coupling function of the capacitor is the characteristics of a capacitor that allows an engineer to discriminate between AC greater the capacitance of a unit, the greater the current will be at any given frequency; and for a given capacitance, the higher the frequency, the more current will pass (up to the resonant frequency of the capacitor - the point of maximum current transfer). By utilizing this characteristic, an engineer can distinguish between AC currents of different frequencies. Again, as in the coupling application, the dissipation factor (heat producer under AC applications) can be critical.
This function is really a control on the "speed of response" of the charging and discharging time for a capacitor and its associated circuitry. The rate at which charge flows in and out of the capacitor is directly controlled by the capacitance (C) and series resistance (R) of the circuit. Therefore, the "timing" or speed of response of a circuit can be controlled by using various RC combinations. This combination of R and C is known as the "TC" or "time constant". Usually, the most critical parameter to be considered in a "timing" application is that of capacitance change with temperature (temperature coefficient). The variance of capacitance with time, and the retrace capabilities of the capacitance after temperature excursions can also be critical in certain circuit applications.
TRANSIENT VOLTAGE SUPPRESSION
This characteristic of a capacitor is widely used to provide "voltage stabilization." Most unregulated power supplies are subject to transient peaks of voltage surges which can cause malfunction in the circuit. By utilizing capacitors, these transient peaks are absorbed or stored by the capacitor and a steady voltage signal is supplied to the circuit. This same principle is used to reduce AC ripple voltages in rectified power supplies. Parameter behavior of the various kinds of capacitors is usually of no critical importance for this application.
By using the capacitor to store energy over a long period of time and then discharging this total energy in a very short time, high currents can be made available to perform welding, photo-flash, heating, and other similar jobs. For the case of fairly high pulsing service, the dissipation factor may be of concern from a heating consideration. The most critical factor here is the current-carrying capabilites of the unit, leads, and connections.
"Make-break" type circuits, whereby mechanical relays or similar devices are used to periodically interrupt a current flow, causes arcing at the contact points. This results in a "noise" signal that can interfere with nearby radio and television reception. The use of capacitor-resistor and/or inductor combinations will reduce or eliminate this interference and prolong the life of the contacts by absorbing the voltage pulse resulting from this current interruption.
POWER FACTOR CORRECTION
Part of the total power generated by, or supplied to a circuit must be used to energize or activate certain components within the circuit. This power is effectively lost within the circuit and therefore not available for other uses. By using capacitors, this percent loss of power can be and is reduced, thus "correcting" or "improving" the power factor of the circuit. The immediate benefit from this "corrected" power factor is the reduction in lost power from the original total power generated, or the ability to increase the total power generated without additional equipment.
Specialized jobs such as voltage and frequency doubling, commutating, etc., are really combinations of the basic functions already described. The selection of the "right capacitor for a specific job" is really a matter of matching the capabilities of the capacitor to the requirements of the job.