DIY dual channel power supply idea - Page 1
Forum section: electronics / beginners

author: karoru
author_rank: Regular Contributor
timestamp: September 13, 2015, 10:37:20 pm
content: Hello, I'm a beginner hobby electronics guy (I'm a mathematician that got to study IT to get some bread-earning skills, got into mandatory Computer Systems Architecture where we implemented simple CPU on Altera FPGA, and by some cosmic radiation resonance got an interest in analog stuff) and I'm trying to built some el cheapo dual channel power supply.My goal is to have 2 galvanically isolated channels, let's say 1A, 15V per channel. I really want to avoid winding up a big-ass transformer with isolated secondary windings, but I've got a few leftover 24V, 1.1A switching power supplies (proudly made by Mean Well Hung Lou company, with a pinch of hot-snot and cheap electrolytics). They're not that bad, I can't see any switching glitches on my 25 MHz analog scope.I want to ask for an evaluation of my general plan (in the scale from "just a little stupid" to "very bad"):1. Get 2x24V from these babies and some old-fashioned small transformer&rectifier for let's say 9V DC.2. Each channel has a power darlington driven by opamp, some reference voltage (something along the lines of LM336, I don't really aim for "precise" with my first bigger project), current sense resistor and usual schmick to do the current-limiting and voltage regulation.3. From 9V DC I power volt/ammeters, maybe some relays to turn channels on and off and a micro.4. Rotary encoders to control voltage/current, microcontroller generating PWM to feed optocouplers controlling voltage given to opamps on isolated parts of circuit (these two channels).How wrong I am with such setup?

author: thefamilyman
author_rank: Regular Contributor
timestamp: September 13, 2015, 10:52:46 pm
content: Mean Well is actually a pretty decent brand, we use them all the time for our 24v 10A and 20A DIN rail power supplies with no failure rates yet.Sorry I wasn't helpful, just wanted to clarify that Mean Well isn't a shit random brand.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2015, 05:22:53 pm
content: The concept looks Ok, except that it does not include a read back of voltage or current. The easy way is having the shunt on the low side. Form 24 V raw input one could get up to about 20 V.If the µC should also read and display these values, it's likely better to have the insulation at the external Interface (e.g. USB / RS232 ?), and a separate µC for each channel. In this case one could even get away without the extra 9 V transformer.If not much power is needed, a small royer converter may be an alternative to the extra transformer.PWM may need quite some filtering to get low noise. I would consider a cheap DAC chip (e.g. MCP48xx) if connected diretly to the µc.

author: karoru
author_rank: Regular Contributor
timestamp: September 15, 2015, 11:53:19 am
content: I've built a prototype of power part of the circuit using some junkbin parts (didn't have any Darlington at hand, so just used Toshiba D1548 NPN transistor I've got from my old CRT TV years ago), and by accident it works, doesn't oscillate (I'm a Pole so stability is serious concern) and doesn't smoke on short circuit, so at least that part is in alpha-prototype stage, now I can start groking the digital control stuff.I'd actually had some current readout from the overcurrent protection shunt on high side, but it doesn't account for base current, so low side sensing at least for the readout makes sense.I've took a look on these DACs, and they're wonderful, 10-bit two channel (one for voltage error opamp, one for current protection opamp) MCP4812 costs 3.5$ or so in quantity of one in my local supplier, it has integrated voltage reference, is available in plain DIP8, and controls via SPI. Even including uCs (I'd probably use cheapest AVR with 10-bit ADC I'll find) and rotary encoders it gives me enough spare change (compared to 2 10 turn pots per channel) to buy the knobs hand-made by nude virgins and tweaked by grey beard (tongue at a right angle included)Now to LTSpice, protoboard and Eagle (after I'll bang my head against the wall long enough to discover a nice way to treat separate grounds on schematic).Thank you very much for your response:)