Linear PSU voltage regulation question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 07, 2014, 10:28:05 am
content: I recently restored a none working elderly Farnell 60V , 50 Amp H60/50 linear power supply to working order by making a missing board and fixing a cracked trace to an OpAmp, thanks to help from this great forum.I have been using it for a while and notice that the output voltage drifts high over time, and is not nearly as stable as the machines specification suggests it should be. Can anyone tell me just what components regulate output voltage? I am thinking Z4 on this control board schematic:http://www.chriswilson.tv/schematic.jpgI believe this is a none original part, and perhaps not a temp stable type. If I am right and it's Zener controlled, can I fit a more modern 3 wire IC device and simple circuit to make it as stable as later PSU's? Or should I just change it for a "better" Zener? Suggestions as to a "better" Zener welcomeThe machine's overall layout is here:http://www.chriswilson.tv/schematic-general.jpgThanks for looking.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 07, 2014, 10:49:49 am
content: Z2 is the main voltage reference. Can you hang a DVM across it? When it drifts, is it under load?

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 11:03:16 am
content: Z1,R1,R3,VT1 is a current source for the Z2 main reference voltage zener - it's driven by constant current so it should be stable.as suggested - measure the reference with a DMM - if it holds steady then check the potentiometer for setting the voltage - it might be old and crusty.if the reference is drifting , then you can safely replace it with TL431 + 3 resistors or any other reference.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 07, 2014, 11:10:28 am
content: Sorry, my eyesight is getting worse, I did mean Z2! I even marked it as likely to be the V ref. on the schematic, and still got it wrong. Or maybe it was the drink at the wife's works do on Saturday night.It drifts under no load with a decent digital meter on the output terminals (the machine has analogue meters). It also drifts under light loads. Never really checked it under heavy load as most times a bit of drift at 30 Amps isn't critical with anything I use it for. To be frank it's more a case of I spent ages on restoring it to working order and I would like to go the extra nine yards if it's still not quite right. The original specs are athttp://www.gatesgarth.com/farnell/farnell.pdfSorry it's upside down, I can't fathom out how to save it again as a .pdf once I rotate it in Adobe, and have it keep the correct state when saved...

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 11:28:51 am
content: check the Z2 for drift directly as suggested above

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 07, 2014, 11:51:37 am
content: OK, just by a decent digital meter across Z2?If it's drifting a lot how do I wire in an IC regulator like a TL431 please? I have Googled the IN823 Zener that should be in there (although I suspect it's been changed by someone previously), and there seem to be a range within that family with better temp regulation. However, here in the UK I see no sellers at all, so if it is drfting maybe a TL431 will be as easy to implement? Maybe a IN823 has other more modern type numbers though? Thanks!

author: Andy Watson
author_rank: Super Contributor
timestamp: July 07, 2014, 12:04:02 pm
content: Quote from: Chris Wilson on July 07, 2014, 11:51:37 amIF ...first establish that Z2 is the problem!With no load the output could easily be drifting up due to increased leakage current through the control transistors - in essence, you could be seeing the first stages of thermal runaway. What was the original problem with this power supply?

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 07, 2014, 12:12:49 pm
content: One of the (many) series pass transistors was shorted and a control board (the "Supplies" board in the general schematic), was missing altogether. I renewed them all. I have a sister supply with less output and copied the PCB from that one, which is physically and electrically identical. It also had a couple of blown components due to the pass transistor short, and I found a trace to an OpAmp was cracked. I will measure across Z2 and report back! Thanks. Otherwise it's now am awesomely heavy brute that has worked beautifully since I sorted it, and is unlikely to be nicked unless by a gorilla

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 01:25:42 pm
content: Quote from: Chris Wilson on July 07, 2014, 11:51:37 amactually it's very easy to implement the TL431 reference - see the attached picture. i roughly calculated the current through it to approx 10mA (at the 6,2V ref voltage) - should be similar to the original refrence.with R2 trimpot you can adjust the reference between 2,5V - 7,5V - so just hook your DMM and trim it to 6,2V - as the original one.the values of R2 and R3 are not critical - just the ratio between them (basically what you need is to divide the desired reference voltage to 2,5V at the REF pin by R2 & R3)