Capacitor blocks DC but passes AC

Let us see how to prove that capacitor blocks DC but passes AC using a practical circuit. For that we shall perform an experiment as explained below:

Aim: To study the behavior of capacitor to AC/DC current through it.

Apparatus: Battery – 6/12V, C – 1000mF, 25V rating, DMM – high internal resistance DMM like MASTECH/HP/APLAB, etc.; Switch (Sw) – SPDT switch, small piano or toggle type, Connecting wires – 5A current rating wires with crocodile clips at both ends.

Connection Diagrams:

Testing capacitor on DC & AC voltage source with a speaker

Testing the same capacitor on AC & DC voltage source with a DMM/AMM

• Capacitor blocks DC current but passes AC current. It opposes AC current also.
• This opposition is INVERSELY PROPORTIONAL to the frequency of AC current. It is given as: F= 1/(2.Pi.f.C)
• As the frequency of DC = 0, this opposition is infinite. Hence, DC current cannot flow through a capacitor.
• Due to this property of capacitor, it is widely used to couple AC signal and decouple DC signal in amplifier circuits.

Procedure:

Follow the procedure step by step and write down your observations –

1. Connect the apparatus as shown in above connection diagram.
2. Put the switch (sw) in position-1.
3. Try to listen to any sound from the speaker, but you should hear nothing. This is because we are applying DC supply to the speaker.
4. So it proves that DC current cannot flow through the capacitor.
5. Now put the switch (sw) in position-2.
6. Again listen to any sound produced from the speaker.
7. You will hear some constant vibrating sound from the speaker.
8. The frequency of this sound will be equal to the frequency of AC voltage.
9. Now repeat the same procedure by replacing speaker with a multimeter as shown in the next connection diagram.
10. Try to measure the AC current as well as DC current using multimeter in both positions of the switch (sw).
11. Note down your observations.

By checking the readings and noted down observations, you will come to know that capacitor blocks DC but passes AC.

In our lab we tested the circuits practically also standard measuring instruments and found that capaictor blocks DC and passes AC. While testing on AC we recommend to use high value tantalum capacitor if the frequency of your AC voltage source is low.
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