Tell me about this mad power supply design... - Page 1
Forum section: electronics / projects

author: mc172
author_rank: Frequent Contributor
timestamp: November 02, 2017, 01:20:07 pm
content: A colleague has been given the attached schematic in a college assignment. He has to suggest in his assignment what the expected inputs and outputs are. He knows what it'ssupposedto be but clearly that's not what you're going to get with this thing!It's taken from here, among other places:http://powersupply33.com/50v-dc-3a-stabilized-and-regulated-power-supply.htmlThis contains all sorts of bonkers, but can anyone tell me what they were thinking when they designed it? It doesn't even work if you flip the rails and put all of the diodes the right way around.There's even a PCB layout:http://www.circuitosimpresos.org/2012/03/03/circuito-impreso-para-power-supply-50v-3a/

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2017, 04:25:59 pm
content: At a quick look, it should work if Q1 and Q2 are replaced with PNP types.

author: orolo
author_rank: Frequent Contributor
timestamp: November 02, 2017, 04:37:56 pm
content: At first sight, I think this would work better if Q1 and Q2 had their emitters and collectors flipped. Also, C4 should be quite bigger than 100nF. Who knows, this might work even without the E-C flipping, but I wouldn't bet on it.Edit: Oh, I see Benta suggests PNPs. I think it should be NPNs, but correctly placed.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2017, 04:43:19 pm
content: Orolo, gotta agree. Flipping the power stage transistors is right.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 02, 2017, 04:49:57 pm
content: The circuit with values from the linked web page won't work as advertised, period, even if you turn transistors Q1 and Q2 right way around. Withcorrectresistor values and emitter-collector swapped for Q1/Q2 it may work.CheersAlex

author: eblc1388
author_rank: Frequent Contributor
timestamp: November 02, 2017, 06:32:21 pm
content: Looks how close the transformer secondary PCB trace is to the AC mains.Good luck to those who actually built it.

author: nctnico
author_rank: Super Contributor
timestamp: November 02, 2017, 06:35:58 pm
content: I agree.That PCB layout is outright dangerous!

author: StillTrying
author_rank: Super Contributor
timestamp: November 02, 2017, 07:05:33 pm
content: I'm surprised nobody's noticed there's nothing to turn Q1 on.I think if Q1 and Q2 were made into a darlington pair it would work sort of.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2017, 07:22:11 pm
content: Quote from: StillTrying on November 02, 2017, 07:05:33 pmIf they're flipped as orolo said, theyarea darlington.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 08:12:44 pm
content: But hey, at least it's not MC34063?Tim

author: StillTrying
author_rank: Super Contributor
timestamp: November 02, 2017, 08:36:36 pm
content: Well erm, it works as drawn but only up to 500mA output and regulation is rubbish.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 08:41:08 pm
content: Heh...Trap for young players, SPICE doesn't model BJT avalanche breakdown, so you can put 50V E-B on any transistor, no problem.Inverted hFE is also hit and miss (the BR parameter), sometimes being correct, or close; sometimes terribly far off.Tim

author: StillTrying
author_rank: Super Contributor
timestamp: November 02, 2017, 08:59:11 pm
content: There's only -22 volts across each B-E, no wonder the regulation isn't very good.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2017, 09:22:32 pm
content: Quote from: T3sl4co1l on November 02, 2017, 08:12:44 pmWell, it could have been a 555

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2017, 09:24:09 pm
content: Q1 and Q2 should be wired as a PNP Darlington pair.On second thoughts. I give up.After a bit more thinking, it will work with Q2 and Q1 changed to PNP with the Emitter or Q2 to the Base of Q1. Both Collectors to the unregulated supply. R3 might go across B-E of Q1.

author: mc172
author_rank: Frequent Contributor
timestamp: November 02, 2017, 09:34:29 pm
content: Quote from: xavier60 on November 02, 2017, 09:24:09 pmI gave up as well. I messed around in the simulation with all flavours of NPN and PNP, for all three transistors, with the C-E junctions both ways. It was all crap.

author: mc172
author_rank: Frequent Contributor
timestamp: November 02, 2017, 09:35:50 pm
content: Quote from: StillTrying on November 02, 2017, 08:36:36 pmTop marks for doing the simulation and even supplying the file!I came to similar conclusions, mainly the last word.

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2017, 10:12:47 pm
content: I see another problem. With Q2 and Q1 changed to a PNP Darlington, Q3 would have control of it, but in the wrong direction. The output will either stay low latch up at full voltage.

author: mikerj
author_rank: Super Contributor
timestamp: November 02, 2017, 10:23:55 pm
content: It kinda works if you correct the obvious mistakes. The output drops about 36mV over a 0-3Amp load, not horribly bad for a simple regulator.

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2017, 10:27:30 pm
content: It would work better with Q2 as NPN and Q1 as PNP. Collector of Q2 to the Base of Q1. Emitter of Q2 and Collector or Q1 connected to the unregulated supply.Extra- or simply an all NPN Darlington pair with the Emitters connected to the unregulated supply.