Homemade 20V 1A CV-CC PSU - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: October 11, 2021, 03:22:54 pm
content: I lucked out and got a big transformer from a high power stereo ages ago. And I finally got my homemade PSU working with it's rails. It's based off an old Farrnel 20V 1A, CV-CC design that used just zeners for the reference (I added a TL431, I have a bag of them), a differential pair for the CV, and another pair for CC/over-current. So far I only used mostly cheap parts, not even my actual 1%, lower drift resistors.I made an over-temp output shutdown with a thermister. But I added a fan on a switch, and also made a fairly linear fan controller. It's a 24V fan (with a mosfet that acts like 367R), that I run from 5V up to 12V , and that about handles 35W on the heatsink from the 4 BJT's, and holding ~50C. I have to run it higher tho, to overcome the high start-up current. There's lot's of PCB left tho if I mod it some more it over come that.I also added a common 100V 10A V/A meter. The voltage can be set ok, but on mine the the variable resistor needs changing in the op-amp circuit, to read the current correctly.So I finally got it all wired up and in a box and hooked to the transformer after testing the sections from the bench PSU, it's doing very well. Very low ripple so far for a resistive load. But it looks jittery on the order of seconds, the box isn't sealed or shielded and I have wires going everywhere so IDK. There's bound to be a lot of temperature drift. But when I was testing the fan last week, I had the PSU shorted for 1h at max power, and right after wards it went right back to within a few mV of the set voltage before I started.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2021, 04:39:05 pm
content: There is still some (120 Hz) ripple visible. The level is not bad for an old design, but still not great to call home about. The ripple is making the curve look a bit fuzzy.With 3 output transitors this should be OK for the relatively moderate power rating given.

author: voltsandjolts
author_rank: Unknown
timestamp: October 11, 2021, 04:49:32 pm
content: Photos or it didn't happen

author: MathWizard
author_rank: Super Contributor
timestamp: October 11, 2021, 05:15:31 pm
content: The fuss looks this at 5ns. I forget to that blue is the OP, grn was the emitter of the driver for TIP31's. I'll have to find something to power from it, like my stereo, that's around 16V@0.35A. I only have 1100uF on the HV rail, and I never added any amount of caps to the reference network. The best thing is how easy it will be to find a problem if something burns out.The transformer I used, I have not soldered to my PSU, so i can use it for other things too. I want to make an op-amp PSU, or maybe a LM723 PSU, and even add another transformer I have, and then have 2 rails in 1 box.This trans. is probably good for near 30V and 2A.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2021, 06:21:01 pm
content: The high frequency part is a bit strange: could be some radio station or local EMI (e.g. computer or the scope itself ?) or there could be some really high frequency oscillation in the circuit going on. In >100 MHz range the plan alone is of limited help, it is more like having some parasitic oscillation from a poor layout / odd construction.

author: xavier60
author_rank: Super Contributor
timestamp: October 11, 2021, 08:59:30 pm
content: Check if the high frequency is still present when the mains to the PSU is switched off, not unplugged though.

author: MathWizard
author_rank: Super Contributor
timestamp: October 12, 2021, 02:46:10 am
content: Yeah it goes away when the power is off. And I do have a local 99.1MHz station I also see sometimes.It's on the base of the 1st common emitter of the power stage. Looking at the AC spice sim, it seems that it's gain ramps up to 0DB around 10MHz, IDK if thats good or bad for this, as a PSU. I'm no expert on AC analysis or loop gains, or the right way to use the FFT for that matter on my scope.Looking at the base of that 2n5551, chosen for it's high Vce, there's a 206.6MHz oscillation, and I can make it as big as about 400mV w/ 10x probe on 200MHz scope, and I can make it all but go away when play with the current limit 1k pot, and/or the voltage output into my fixed load resistor.I don't have any little cap's across the transformer windings, IDK if there's transformer ringing, so far the AC 207MHz signal looked smaller on the upper side of the 10k on the base of 2n5551I'm making some headway on the AC analysis of a big circuit like this, but so far I know in LTspice, changing the 1.5nF cap on base of the 2n551, lets me move the -3db point left or right, but am I then sacrificing how good it will regulate a similar frequency of changing load ? IDK about phase margins yet eitherway.The main osc. freq does change aswell, I had 84MHz also, here's at shot of it running about 7.8V into 100R, and i turned down the 1k CC pot until the osc was just starting to go, but starting back when 120Hz energy goes through