Confused about power supply design

[elliot42]

I have a salvaged 12V transformer and enough spare parts to make a variable power supply (maybe with a fixed 5V line as well).
I'm looking at the schematic for the Silicone Chip Multi-Power Bench Supply and I'm confused about how they get 27V from a 9V AC supply.
I can see how the 13V is the Vp of the 9V RMS after it's been (half wave?) rectified. But I can't figure out how the other lines can get +/- 27V.

Can anyone give me any hints?

[elliot42]

Oh, I think I'm wrong there. If the input is 9V RMS then the Vpp should be ~25V.
Then does the 13V/27V have something to do with being half- and full-wave rectified?

[IanB]

D3 and D4 with the nearby capacitors make a voltage doubler.

[amspire]

9V AC is 9 x 2 x 2^1/2 = 25.2v peak to peak.  D3, D4 and the caps forms a voltage doubler supply arrangement that will give 25.2 minus 2 diode drops or about 24V. I assume they measured 27 as the cheap transformers in the plug packs are probably more like 10-11V AC at no load, so 27V is very possible.

Richard.

[elliot42]

Thanks, that was quick!
Now that I know the type of circuit it looks just like the Greinacher voltage quadrupler described on Wikipedia:

[MikeK]

How do the diodes work to double the voltage?

[ModemHead]

Look at D3 in the original schematic.  Looks like it's the wrong way around, right? But on the negative half-cyle, it conducts and allows the capacitor to charge. Then on the positive half-cycle, you have the transformer winding and the charged cap in series, doubling the voltage.

[Psi]

How do the diodes work to double the voltage?

Simply put.. the diodes allow AC to charge the capacitors in parallel while you discharge them in series.
This means you can double/triple/quadruple/... the voltage by adding extra diode+capacitor sections.

I've seen animations showing how the current flows. should be easy to find if you have a google.

[MikeK]

Thanks, guys.  Now I understand.

[Online Simulations]

In essence, you have diode voltage doubler as the input part of your scheme (diodes D3 and D4 in your scheme)

Actually, this scheme with full explanation is given in:

http://www.cirvirlab.com/index.php/tutorials/92-diode-voltage-doubler-circuit.html

and there also possibility to see waveforms by simulating the circuit in:

http://www.cirvirlab.com/simulation/diode_voltage_doubler_circuit_online.php