Basics of Capacitors

Capacitors are passive electronic components that store and release electrical energy. They hold the charge by creating a potential difference between two conductors. The stored charge can then be released as a current or voltage. They’re used in many areas of electrical engineering, for example, for smoothing power supply, filtering noise from a signal, changing alternating currents to direct currents, or acting as a timing element in digital electronics. In this blog, we will explain the basics of capacitors.

What is a capacitor?

A capacitor can be thought of as a switch that allows the flow of electrical energy to be regulated by creating a voltage. You can think of a capacitor like an electrical outlet that allows the flow of electric current from one point to another. Capacitors are widely used in electronic circuits for this purpose because they can be charged and discharged many times in a short period. When a capacitor is fully charged, it can store a considerable amount of energy; when released, its voltage drops to zero, similar to disconnecting the battery. However, capacitors are different from batteries because they are cheaper and smaller. Still, they don’t last for as long, whereas a battery can last for hundreds or even thousands of charge-discharge cycles.

How does a capacitor store charge?

The capacitor consists of two plates. A potential difference and an electric field are created when a voltage is applied between the two plates. Electrons move from the positive electrode plate of the capacitor to the negative electrode plate. Positive charges accumulate on one side, and negative charges get on the other side. The electric field contains potential energy. The simplest structure of a capacitor is two parallel metal plates with a space between them. Moreover, the charge stored in a capacitor is proportional to the voltage of a particular capacitance. Discharging the capacitor reduces the charge on the capacitor, which reduces the voltage.

Q = V C

Q Charge measured in Coulombs
C Capacitance measured in Farad.
V Applied voltage measured in Volts.

Voltage rating of capacitors:

Capacitors have a limit on the amount of voltage that can be applied between the plates. Technicians need to know the voltage rating, which is the maximum DC voltage that can be used without the risk of damaging the equipment. This rated voltage is commonly referred to as the breakdown voltage, operating voltage, or rated voltage. If the voltage applied to the plates is too high, the dielectric will be destroyed, and an arc discharge will occur between the plates. The capacitor can then be short-circuited, and the direct current flowing through the capacitor can damage other parts of the device. The nominal voltage of a capacitor is a factor in determining the actual capacitance; as the capacitance decreases, the dielectric thickness increases. High-voltage capacitors with thin dielectrics need a large plate area to have the same capacitance as low-voltage ones with thin dielectrics.

Capacitors in DC:

When the capacitor is connected to a DC power supply and the switch is closed, the capacitor’s plates will be negatively and positively charged. When an electron moves from one plate to another, a current flows through the external circuit. The current through the circuit peaks when the switch closes but then declines until it reaches zero. As soon as the voltage difference between the two plates equals the battery’s voltage, the current goes to zero. When the switch is opened, as shown in the second figure, the plate remains loaded. However, if the source is short-circuited, it will discharge rapidly, as shown in the third figure. It should be understood that while the capacitor is charging or discharging, current will flow through the circuit even though the circuit is passing through the gap between the capacitor plates. Electricity is only available during charging and discharging, and this period is usually short.

Capacitors in AC:

When the battery is replaced with an AC power supply, the behavior of the capacitor is very different from that of DC. When alternating current is applied to the circuit, the charge on the plate is constantly changing. This means that the current must first flow repeatedly from A to B clockwise, then from B to A counterclockwise. However, no current flows through the insulator between the capacitor plates, and it always flows through the rest of the circuit. In circuits where only capacitance is present, the current leads the applied voltage instead of circuits with inductance where the current lags behind the voltage.

Capacitors in Parallel and Series:

By connecting capacitors in parallel, you can calculate the total capacitance by summing all the individual capacitors.

When capacitors are connected in series, the total capacitance of the circuit is the reciprocal of all added capacitance.

Xc Capacitive Reactance:

The effectiveness of a capacitor in passing alternating current depends on the capacitance of the circuit and the applied frequency. The extent to which an AC can flow depends mainly on the capacitance value of the capacitor (Farad). The higher the capacitance of a capacitor, the more electrons it needs to be measured by Coulomb to charge the capacitor fully. As soon as the capacitor reaches a fully charged state, the capacitor’s polarity becomes opposite to the polarity of the applied voltage, essentially acting as an open circuit.

To further explain this characteristic and how it appears in AC circuits, consider the following:

If the capacitance value of the capacitor is significant, that is, if a relatively large number of electrons is needed to keep the capacitor fully charged, a considerable amount of high-frequency current will pass through the capacitor without being fully charged. In this case, the frequency is high enough, and the capacitance is high enough that the capacitor may provide little or no resistance to current if there is not enough time for the capacitor to charge fully. However, the smaller the capacitance, the fewer electrons are needed to charge fully, and the capacitor is sufficient to provide significant resistance to current acting like an open circuit.

The current in an AC circuit can be controlled by changing the capacitance of the circuit in the same way that a resistor controls the current. The actual AC reactance Xc, like the resistor, is measured in ohms (Ω). The following factors determine capacitive reactance Xc:

Capacitive Reactance in Series and Parallel:

If the capacitors are connected in series, the total reactance is equal to the sum of the individual reactances.

The total reactance of a capacitor connected in parallel is determined in the same way that the total resistance is calculated in a parallel connection.

RC Time Constant:

The time it takes for a capacitor to charge fully is proportional to the capacitance and resistance of the circuit. The resistance of the circuit introduces a time factor in the charging and discharging of the capacitor. When a capacitor charges or discharges through a resistor, it takes a certain amount of time to fully charge or discharge. The voltage of the capacitor does not change immediately. The rate of charging or discharging is determined by the time constant of the circuit. The time constant of the series circuit RC is the time interval corresponding to the product of the resistance in ohms and the capacitance in farads and is represented by the Greek letter tau (τ).

The formula time is the time it takes to recharge to 63% of the power supply voltage. The time required to bring the charge to about 99% of the source voltage is about 5τ.

As you can see from the time constant plot, there is no continuous movement of direct current through the capacitor. A suitable capacitor cuts off direct current and allows the effects of pulsating direct current or alternating current to pass through.

Current and Voltage Phase in Reactive Circuits:

In contrast to pure ohm circuits, capacitive and inductive reactance significantly affects the phase relationship between the applied AC voltage and the corresponding current in the circuit.

When the current and voltage pass zero and reach the maximum simultaneously, the current and voltage are said to be in phase. When the current and voltage give zero and get their maximum values at different times, the current and voltage are out of phase. In circuits with only one inductor, the current reaches its maximum slower than the voltage, delaying the voltage by 90 °.

In a circuit containing only capacitance, the current reaches its peak before the voltage, and the current leads the voltage by 90 °. The amount of current in a circuit that lags or leads over the voltage depends on the relative resistance, inductance, and capacitance.