Blog posts of '2014' 'December'

Capacitors value

Q. I was looking at the schematic diagram of an amplifier and I noticed that there was a 0.01 µF capacitor connected in parallel with the filter capacitor, which was 5,000µF.

The 0.01 µF capacitors value is so much smaller than the filter capacitors that it doesn't make any difference in the capacitance there.

Do you think this is a mistake?

A. No. The smaller value capacitor is there for a good reason. The 5,000µF capacitor is big, physically. It is basically two foil plates and an insulator rolled up into a tube. As a result, the two plates have some inductance.

At higher audio frequencies, and above the audible range, this inductance has a significant amount of reactance. This counteracts the capacitive reactance of the filter capacitor at these frequencies.

This would lead to oscillations in the amplifier, as some of the output signal would get coupled back to the input stages by way of the common power supply line.

The 0.01 pµF capacitor is much smaller, physically, than the 5,000µF one. It has far less inductance, so it is still effective at the higher frequencies. So it has a low capacitive reactance at the higher frequencies, and short-circuits the filter capacitor for them. This prevents oscillations.

Q. How does AC flow through capacitors? I thought electrons couldn’t flow through the dielectric in a capacitor.

A. You’re right, Electrons can’t flow through the dielectric in a capacitor because it is an insulator. But the electric fields from the charged plates on either side of the dielectric do pass through the dielectric.

On one half-cycle of the AC, the AC source puts a positive voltage on one plate and a negative voltage on the other plate. Electrons are drawn off the positive plate and an equal number are pushed onto the negative plate. No electrons pass through the dielectric -only the electric fields.

Electrons move through the rest of the circuit, though, as the capacitor starts to charge. But before it does, the polarity of the AC reverses. Now electrons are moved in the opposite direction. They are pushed onto the plate that had been positive and pulled off the plate that had been negative. Again, current flows through the rest of the circuit, but not between the plates.

In circuits like RC coupled amplifiers there are resistors in the current path. As a result, most of the AC voltage appears across these resistors. The voltage across the capacitor doesn’t have time to change because the AC changes polarity too fast. This is another way of saying that the reactance of the capacitor is low at the frequency ofthe AC.

DC, on the other hand, can’t get through a capacitor. This is because the dielectric is an insulator. Current will flow when the capacitor is first charged by the DC, with electrons flowing onto the negative plate and an equal number flowing off the positive plate. But once the capacitor has charged, no further current will flow.