Capacitive resistance in an alternating current circuit: basic concepts, formula for calculation

The capacitor has a certain resistance to alternating current and does not conduct direct current at all. This property finds application in various fields of electronics and electrical engineering. Capacitive resistance in an alternating current circuit depends on the frequency of the latter and the capacitance of the capacitor.

Basic concepts

Capacitive resistance is a quantity, which is created by a capacitor included in the circuit. The resistance of the lead wires must be negligible. When an alternating current is supplied, processes occur due to the periodic charging and discharging of the capacitor.

The period is divided into four quarters. During the first quarter, the tension grows. At this moment, a charging current passes through the circuit, the strength of which will decrease, reaching zero, when the electromotive force reaches a positive maximum. The capacitor is fully charged. After that, a voltage drop will begin. The capacitor will be discharged through the load connected to it. A current will flow through the circuit.

By the end of the half-period, the voltage will be zero, and the current will be the highest. Discharging completed. At the beginning of the third quarter, the electromotive force will increase, changing its direction. The charging process will start again. The direction of the charging current in the third quarter will be the same as in the previous one. As the capacitor charges, this value will decrease. By the end of the third quarter, the charging process will be complete.

The electromotive force will reach its highest negative value. And on that plate, on which during the first half-period there was a positive charge, will now be negative. During the fourth quarter, the value of the electromotive force will again tend to zero. The capacitor will discharge. Accordingly, a gradually increasing current will appear in the circuit. The process is repeated. Thus, the phase of the alternating current in the capacitor circuit is ahead of the phase of the voltage by 90 degrees.

Resistance formula

The formula for capacitance is displayed as follows:

• The angular frequency must be calculated first. To do this, the frequency of the current flowing through the circuit (in hertz) must be multiplied by twice the number "pi".
• Then the resulting number should be multiplied by the capacitance of the capacitor in farads.

To get the value of capacitance in ohms, divide one by the number obtained after multiplying the angular frequency by the capacitance. It follows from this formula that the larger the capacitance of the capacitor or the frequency of the alternating current, the lower its resistance.

When the frequency is zero (constant current), the capacitive resistance will become infinitely large. A very large capacitor will conduct current over a wide frequency range.

Application in practice

The capacitor properties are used in the design of various filters. The action of the capacitive resistance in this case depends on the method of connecting the part:

• If it is connected in parallel with the load, you will get a filter that suppresses high frequencies. With their growth, the resistance of the capacitor falls. Accordingly, the load at high frequencies is shunted more than at low frequencies.
• If the part is connected in series with the load, you get a low frequency filter. This circuit also does not allow DC voltage to pass through.

Another area of ​​application is the separation of the variable component from the constant. For example, in the final stages of audio amplifiers. The higher the capacity, the lower the frequency the connected loudspeaker can reproduce.

In the power supply filters, along with the capacitive resistance, the property of accumulation and release of charge is also used. At the moment of increasing the load, the charged capacity of the filter is discharged, giving additional energy. It also suppresses ripple and other parasitic signals by passing them through itself and closing to a common wire. Thus, smoothing and maintaining the voltage across the load within the specified limits, and the elimination of unwanted interstage connections that cause unstable operation are ensured.

Due to their properties, capacitors are used in cases where it is necessary to transmit both direct and alternating current through the same wires. The constant voltage source is connected to the common wire and the second output of the capacitorthrough which the AC voltage source is connected. On the other side, a separation occurs: the variable consumer is connected through a capacitor of the same capacity, and the constant consumer is connected directly, to the part terminals.

A common example of this is an outdoor TV antenna with an amplifier. The TV itself or a cable-connected device called an "injector" supplies the power supply. Signal separation and filtering takes place in the antenna amplifier. Thus, the capacitive resistance of a capacitor is widely used. Filters delay some signals and pass others.

Thanks to this property, it is possible to transfer both alternating and direct voltage at once, which is of no small importance in the construction of some communication lines.