bridge-rectifierpower supply
I'm looking at different bridge rectifier circuits and found this one. Capacitors C5-C8 are smoothing capacitors. Both positive DC and negative DC have capacitors C1-C4 linking to both AC lines.
What is the function of C1-C4?
simulate this circuit – Schematic created using CircuitLab
it's not surprising you are seeing close to 20Vdc at the output. The nominal voltage of your transformer secondary is 12Vac (which could be even higher ~14Vac). Now, the ac voltages you have are RMS. So the capacitor is going to charge up to the peak value of the sine wave, which is 12Vac*sqrt(2) = ~17V for a secondary output of 12Vac. If this were more like 14Vac at the secondary, it would make sense why you are seeing 20Vdc at the output.
If you want to do approach #2, you need two fully isolated windings. The way you show it has two diode drops in one direction shorting out the windings as @WhatRoughBeast points out. You cannot use a center-tapped winding- the common must be made on the other side of the bridge rectifiers.
The advantage of the second method (with two secondary windings isolated from each other) is that it uses the transformer more efficiently if the loads on the positive and negative rails are greatly imbalanced. You can get as much as 24% more DC current out of the transformer by using the two bridges.
Best Answer
Those extra small value capacitors are to avoid the rectifier generating RF interference.
When a diode has been conducting and then current reverses it can stop conducting very suddenly when the charge carriers are depleted. This can occur extremely rapidly and generate interference to many MHz.
These sudden changes in current can cause interference to other parts of the circuit or even other equipment. It is modulated at 60Hz or 120Hz (US power frequency) so would be heard as a buzz in audio equipment.
The usual avoidance technique is as shown in the circuit. Ferrite beads may also be required in stubborn cases.
The technique can be used intentionally to generate high frequencies or narrow pulses, especially when high frequency devices are not easily available.
Step Recovery Diode