Confused about power supply design - Page 1
Forum section: electronics / beginners

author: elliot42
author_rank: Contributor
timestamp: May 04, 2012, 05:09:20 am
content: 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?

author: elliot42
author_rank: Contributor
timestamp: May 04, 2012, 05:14:35 am
content: 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?

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2012, 05:15:33 am
content: D3 and D4 with the nearby capacitors make a voltage doubler.

author: amspire
author_rank: Super Contributor
timestamp: May 04, 2012, 05:15:46 am
content: 9V AC is 9 x 2 x 21/2= 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.

author: elliot42
author_rank: Contributor
timestamp: May 04, 2012, 05:29:09 am
content: Thanks, that was quick!Now that I know the type of circuit it looks just like the Greinacher voltage quadrupler described on Wikipedia:

author: MikeK
author_rank: Super Contributor
timestamp: May 04, 2012, 11:53:13 am
content: How do the diodes work to double the voltage?

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 04, 2012, 12:16:47 pm
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2012, 12:33:01 pm
content: Quote from: MikeK on May 04, 2012, 11:53:13 amSimply 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.

author: MikeK
author_rank: Super Contributor
timestamp: May 04, 2012, 01:37:34 pm
content: Thanks, guys. Now I understand.

author: Online Simulations
author_rank: Newbie
timestamp: January 31, 2013, 12:34:09 pm
content: 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.htmland there also possibility to see waveforms by simulating the circuit in:http://www.cirvirlab.com/simulation/diode_voltage_doubler_circuit_online.php