Capacitors are widely used in many electrical and electronic applications. If you pick up any circuit board, open any power supply, or look inside any piece of electronic equipment, chances are you will find capacitors of one type or another. These components are used for a variety of reasons in both dc and ac applications.
Applications of capacitors
The main applications of capacitors are the following:
One of the most basic applications of a capacitor is as a backup voltage source for low-power circuits such as certain types of semiconductor memories in computers. This particular application requires a very high capacitance value and negligible leakage.
The storage capacitor is connected between the dc power supply input to the circuit and ground. When the circuit is operating from its normal power supply, the capacitor remains fully charged to the dc power supply voltage. If the normal power source is disrupted, effectively removing the power supply from the circuit, the storage capacitor temporarily becomes the power source for the circuit.
A capacitor provides voltage and current to a circuit as long as its charge remains sufficient. As current is drawn by the circuit, the charge is removed from the capacitor, and the voltage decreases. For this reason, the storage capacitor can only be used as a temporary power source. The length of time that a capacitor can provide sufficient power to the circuit depends on the capacitance and the amount of current drawn by the circuit. The smaller the current and the higher the capacitance, the longer the time a capacitor can provide power to a circuit.
Power supply filtering
A basic dc power supply consists of a circuit known as a rectifier followed by a filter. The rectifier converts the 120 V, 60 Hz sinusoidal voltage available at a standard outlet to a pulsating dc voltage that can be either a half-wave rectified voltage or a full-wave rectified voltage, depending on the type of rectifier circuit. A half-wave rectifier removes each negative half-cycle of the sinusoidal voltage. A full-wave rectifier actually reverses the polarity of the negative portion of each cycle. Both half-wave and full-wave rectified voltages are dc because, even though they are changing, they do not alternate polarity.
To be useful for powering electronic circuits, the rectified voltage must be changed to constant dc voltage because all circuits require constant power. The filter nearly eliminates the fluctuations in the rectified voltage and ideally provides a smooth constant-value dc voltage to the load that is the electronic circuit.
DC blocking and AC coupling
Capacitors are commonly used to block the constant dc voltage in one part of a circuit from getting to another part. As an example of this, a capacitor is connected between two stages of an amplifier to prevent the dc voltage at the output of stage 1 from affecting the dc voltage at the input of stage 2.
Assume that, for proper operation, the output of stage 1 has a zero-dc voltage and the input to stage 2 has a 3 V dc voltage. The capacitor prevents the 3 V dc at stage 2 from getting to the stage 1 output and affecting its zero value, and vice versa.
If a sinusoidal signal voltage is applied to the input to stage 1, the signal voltage is increased (amplified) and appears on the output of stage 1. The amplified signal voltage is then coupled through the capacitor to the input of stage 2 where it is superimposed on the 3 V dc level and then again amplified by stage 2. In order for the signal voltage to be passed through the capacitor without being reduced, the capacitor must be large enough so that its reactance at the frequency of the signal voltage is negligible. In this type of application, the capacitor is known as a coupling capacitor, which ideally appears as an open to dc and as a short to ac. As the signal frequency is reduced, the capacitive reactance increases, and, at some point, the capacitive reactance becomes large enough to cause a significant reduction in ac voltage between stage 1 and stage 2.
Power line decoupling
Capacitors connected from the dc supply voltage line to the ground are used on circuit boards to decouple unwanted voltage transients or spikes that occur on the dc supply voltage because of fast switching digital circuits. A voltage transient contains high frequencies that may affect the operation of the circuits. These transients are shorted to the ground through the very low reactance of the decoupling capacitors. Several decoupling capacitors are often used at various points along the supply voltage line on a circuit board.
Another capacitor application is to bypass an ac voltage around a resistor in a circuit without affecting the dc voltage across the resistor. In amplifier circuits, for example, dc voltages called bias voltages are required at various points. For the amplifier to operate properly, certain bias voltages must remain constant and, therefore, any ac voltages must be removed. A sufficiently large capacitor connected from a bias point to the ground provides a low reactance path to the ground for ac voltages, leaving the constant dc bias voltage at the given point. At lower frequencies, the bypass capacitor becomes less effective because of its increased reactance.
Capacitors are essential to the operation of a class of circuits called filters that are used for selecting one ac signal with a certain specified frequency from a wide range of signals with many different frequencies or for selecting a certain band of frequencies and eliminating all others. A common example of this application is in radio and television receivers where it is necessary to select the signal transmitted from a given station and eliminate or filter out the signals transmitted from all the other stations in the area.
When you tune your radio or TV, you are actually changing the capacitance in the tuner circuit (which is a type of filter) so that only the signal from the station or channel you want passes through to the receiver circuitry. Capacitors are used in conjunction with resistors, inductors, and other components in these types of filters. The main characteristic of a filter is its frequency selectivity, which is based on the fact that the reactance of a capacitor depends on frequency (XC = 1/2pfC).
Another important area in which capacitors are used is in timing circuits that generate specified time delays or produce waveforms with specific characteristics. Recall that the time constant of a circuit with resistance and capacitance can be controlled by selecting appropriate values for R and C. The charging time of a capacitor can be used as a basic time delay in various types of circuits; however, capacitors tend to have more tolerance variation than other components, so RC timing circuits are not used when the timing is critical. An example application is a circuit that controls the turn indicators on your car where the light flashes on and off at regular intervals.
Power factor correction
Power factor is the ratio of the real current or voltage received by a load to the root mean square (RMS) value of the current or voltage that was supposed to be acquired by the same load. The fact that the two become different is due to the presence of reactive power in the circuit which gets dissipated. Improving the power factor means reducing the phase difference between voltage and current. Since the majority of the loads are of inductive nature, they require some amount of reactive power for them to function. Therefore, for the better use of electrical appliances with a minimum amount of electrical consumption, the power factor should necessarily be increased and should be brought near to 1. This can be easily done with the help of automatic power factor correction capacitors.
Noise suppression capacitors are most widely applied as countermeasures to noise occurring in inverters, switching power units, brush motors, and to the full range of office automation equipment.
Single and three-phase electrical motors need, for their starting, a capacitor that generates a displaced current creating a rotating magnetic field. The capacitor can be used also for permanent operation, it maintains the required magnetic field and it compensates for the motor’s inductive load. There are two types of capacitors used for those applications.
Motor starting capacitors are electrolytic capacitors with high capacitance value (μF), able to provide a high starting torque to the motor. They are disconnected at the end of the starting in order to avoid overload to the motor winding.
Motor running capacitors are used to improve the value of the cosϕ when the motor is working at rated load conditions, they are permanently connected to the motor.
The capacitors for lighting applications offer the ideal solution to compensate fluorescent and discharge lamps. All fluorescent and discharge lamps need a reactor to switch and keep on the electric arc. This kind of load is very inductive (cosϕ ≤0.5) and it generates a very high value of absorbed current. In order to decrease the absorbed current (optimizing the cable section of the supply cables) and to improve the value of the cosϕ; it is necessary to add a capacitor in the circuit. The national regulation of many countries imposes the use of compensation capacitors in lighting installations.
Dynamic memories in computers use very tiny capacitors as the basic storage element for binary information, which consists of two binary digits, 1 and 0. A charged capacitor can represent a stored 1 and a discharged capacitor can represent a stored 0. Patterns of 1s and 0s that make up binary data are stored in a memory that consists of an array of capacitors with associated circuitry. You will study this topic in a computer or digital fundamentals course.
Pulsed power and weapons
The improvement in the performance of high energy density capacitors used in pulsed power has accelerated over the past few years. This has resulted from increased research sponsored by the US Army Research Laboratory, in support of the US Military’s needs. Capacitors for use in pulsed discharge circuits can be divided into two broad categories. The first category is capacitors that use thin (5.5µm) aluminum foil electrodes to conduct current through the capacitors. The second category is capacitors that have metalized electrodes where the electrode is vapor-deposited on a dielectric. The electrodes are typically aluminum or zinc with a thickness of around 300 Å (.0003 µm) and are deposited on the capacitor dielectric prior to winding the capacitor.