Kapasitif Reaktans / Capacitive Reactance

What is Capacitive Reactance?

In RC circuits, when dc voltage is applied to a capacitor, the capacitor itself pulls a charging current from the feed and charges it to a value equal to the applied voltage.

Likewise, when the supply voltage decreases, the charge stored in the capacitor decreases and the capacitor is discharged. However, in an AC circuit where the applied voltage signal is continuously changed from positive to negative polarity at a rate determined by the feeding frequency, for example at a sine wave voltage, the capacitor is constantly charged or discharged at a rate determined by the feeding frequency.

When the capacitor is charging or discharging, a current flows, which is limited to the inner impedance of the capacitor. This inner impedance is usually known as capacitive reassurance and is given the symbol XC in Ohm.

Unlike resistance, which has a constant value, for example, 100Ω, 1kΩ, 10kΩ, etc. (this is due to the fact that the resistance complies with the Ohm law), capacitive reassurance varies according to the applied frequency, so any variation in the feeding frequency will have a major effect on the capacitor's "capacitive reassurance" value.

As the frequency applied to the capacitor increases, it is to reduce the reassurance. Likewise, as the frequency applied to the capacitor decreases, the reassurance value increases. This variation is called the complex impedance of the capacitor.

The complex impedance, electrons in the form of an electric charge on the capacitor plates, seem to pass faster from one plate to another according to the changing frequency.

As the frequency increases, the capacitor passes more load along the plates over a certain period of time, resulting in a larger flow of current than the capacitor, whose internal impedance appears to have decreased. Therefore, it can be said that a capacitor connected to a circuit that changes in a certain frequency range is "frequency dependent".

Capacitive reassurance has the symbol "XC" and units measured in the same Ohm as resistance (R). It is calculated using the following formula:

  • Xc = Capacitive Reactance, (Ω)
  • π (pi) = 3,142
  • ε = Frequency, (Hz)
  • C = Farad, (F)

Capacitive Reassurance Sample

Let's calculate the capacitive reactale value of the 220nf capacitor at a frequency of 1khz and a frequency of 20kHz.

When the frequency is 1 kHz:

When the frequency is 20 kHz:

As the frequency applied to the 220nF capacitor increases, the reassurance value decreases from approximately 723Ω to 36Ω. From here we can tell that the frequency and capacitive reassurance are inversely proportional.For any capacitance value, the reassurance of a capacitor expressed in ohm can be drawn against the frequency, as shown below.

Capacitive Reactance and Frequency Relationship

By rearranging the above reactax formula, we can also find out at what frequency a capacitor will have a certain capacitive reactale (Xc).

Frequency Discovery Example

What should be the frequency applied for a capacitor with a value of 2.2uF to show 200Ω capacitance?

Or we can find the value of the capacitor in Persian terms by knowing the applied frequency and the reactaliate value at that frequency.

Example of Finding a Capacitor in Farad

What is the value in farads of a capacitor with a capacitive reactance of 200Ω and connected to a source of 50 Hz?

From the examples above, we can see that when a capacitor is connected to the variable frequency source, it behaves like "frequency-controlled variable resistance" because its reactance (X) is directly proportional to the frequency. At very low frequencies such as 1Hz, our 220nF capacitor has a high capacitive reacquered value of approximately 723.3KΩ (which gives an open circuit effect).

At very high frequencies such as 1Mhz, the capacitor has a low capacitive reacquer of only 0.72Ω (gives short circuit effect). Therefore, in DC at zero frequency or in a stable state, our 220nF capacitor has infinite reassurance between the plates that looks more like an "open circuit" and prevents the flow of current through it.

Voltage Divider Reminder

Depending on the value of the resistance, different voltages may appear in each resistance and a voltage divider circuit has the ability to divide the feed voltage into R2/(R1+R2). Therefore, when R1 = R2, the output voltage will be half the value of the input voltage. Similarly, any R2 value greater or less than R1 will cause a proportional change in output voltage.

Voltage Divider Circuit

We now know that the recess of a capacitor varies according to the frequency applied to the Xc (complex impedance). Now if we change the above R2 resistance for a capacitor, since the reassurance of the capacitor affects the impedance, the voltage drop between the two components will change as the frequency changes.

Resistance R1's impedance does not change with frequency. Resistors have constant values and are not affected by frequency change. Then the voltage on resistance R1 and therefore the output voltage are determined by the capacitor capacitive reactance at a certain frequency. This then results in an RC voltage dividing circuit connected to the frequency. With this idea in mind, Passive Low Pass Filters and High Pass Filterscan be created by replacing one of the voltage divider resistors with an appropriate capacitor, as shown.

Low Pass Filter

High Pass Filter

Capacitive reassurance makes the capacitor ideal for use in AC filter circuits or DC power supply smoothing (soft-start) circuits, reducing the effects of unwanted surge voltage as the capacitor applies a short-circuit signal path to unwanted frequency signals in the output terminals.

Brief Summary

Therefore, we can summarize the behavior of a capacitor in a variable frequency circuit as a type of frequency-controlled resistance with a high capacitive reacquerence value (open circuit state) at very low frequencies and a low capacitive reacquerence value (short circuit state) at very high frequencies.

It is important to remember these two conditions, and in our next tutorial on passive low-passing filter, we will look at the use of capacitive reactenses to block unwanted high frequency signals and allow only low frequency signals to pass through.