Old Philips PSU - Page 1
Forum section: electronics / beginners

author: ady.price
author_rank: Contributor
timestamp: March 05, 2016, 01:51:40 am
content: Hey. I found this old Philips power supply. And when I say old, I mean really old (at least 20 years by the looks of it), with analog V and A meters. The old dinosaur still works great. The problem is that after I shut it down, even after I pull the plug, it has Negative 0.2 V on the output. So I thought maybe I can discharge whatever residual charge from wherever, so I shorted the outputs with a 1Mega Ohm resistor, and I kept measuring the V across it, and 5 minutes later it still had negative 0.05V. I even shorted them without the resistor after that and the bastards still had a voltage across them. I stopped trying at about -.02V. So.. anyone know where that might have come from and if I could fix that? It's not a huge V but I figure this kind of thing might be a symptom of a bigger problem, and I know it's not my multimeter.Sent from my Nexus 6 using Tapatalk

author: MK14
author_rank: Super Contributor
timestamp: March 05, 2016, 02:14:02 am
content: I'm slightly guessing (as I have NOT seen the schematics for your PSU), and partly basing it on what I remember about other (probably) similar power supplies.To get the 0V to 30V output range (or whatever it is), especially in the "old days". Analogue meters = probably rather old. They needed to actually have a (very approximately) -4V supply.I.e. internally it goes from something like 40V (with ripple) down to -4V.This is because having a control which adjusts down to 0V is actually NOT that easy, especially with old, electronics.I can't remember the exact details, but I think there are multiple transformer taps, to help achieve the above. (Probably helps give somewhere to fix the 0V ground terminal to, as well).tl;drIt is really something like 44V (peek, needed as the ripple voltage drops, until the next power cycle) internally.Hence the PSU really thinks it is going from +4V up to +34V. But the ground pin is connected up in such a way, that it is at 0V. (i.e. +4V, making it at 4V - 4V = 0V, using transformer taps or something).Hence you can sometimes get slight negative voltages. Especially when it is ancient, and probably well out of calibration.My recall of the above is NOT 100%. So if anyone knows more about this, please feel free to correct me.I don't have the schematics for the PSU I am talking about, so could be a bit wrong.I have at least one (similar, ancient) PSU. I can remember a similar phenomenon (slight negative voltages).The voltage drops, and voltage overhead requirements of transistors, tend to impose this requirement. Modern day rail to rail op-amps, etc etc. Considerably improve PSU designers options, in alternative ways round these issues.

author: ady.price
author_rank: Contributor
timestamp: March 05, 2016, 02:39:34 am
content: my problem is not with the - V, but with --the fact that -- , --how long-- and --where-- it keeps the charge after I disconnect it from the grid, and short the outputs. I would think that the capacitors would discharge pretty fast.

author: MK14
author_rank: Super Contributor
timestamp: March 05, 2016, 02:46:25 am
content: Quote from: ady.price on March 05, 2016, 02:39:34 amThe old (especially) capacitors, were quite bad at being able to keep some of their charge, even when SHORT-CIRCUITED.There are technical explanations of the process I have read previously, and there is a technical name for it, which I CAN'T remember.But it is something to do with the charge (partly) moving to other parts of the capacitor.Here is an article about it:http://www.keith-snook.info/capacitor-soakage.htmlExtract:Quote

author: ady.price
author_rank: Contributor
timestamp: March 05, 2016, 02:57:32 am
content: Ha ha. I think it might be because the current is set at 0 A, so that's why it took so long to discharge. I'm still confused about what the current setting in PSU's does. I still don't know if that's min or max current (I read something, i don't remember where, that a perfect power supply will increase and decrease V as needed to keep the current constant, so I always assumed it's not max). I only used batteries and my phone charger to power my projects until now, because I didn't know enough about power supplies to choose one (still don't as you can see).

author: MK14
author_rank: Super Contributor
timestamp: March 05, 2016, 03:02:08 am
content: Quote from: ady.price on March 05, 2016, 02:57:32 amIt is usually setting the MAXIMUM allowed current output.So if you set the voltage to 12 volts, and the current limit to 1 amp.As long as you use (no more than), 1 amp. The output remains perfectly steady, at 12V.But if you try to exceed 1 amp. The voltage immediately drops, until EXACTLY 1 amp is flowing. Even if you short circuit the outputs.I.e. It has gone from CV (constant Voltage), into Constant Current (CI) mode.As soon as the current consumption drops BELOW 1 amp, it immediately reverts back to constant voltage. (12V in my example).

author: ady.price
author_rank: Contributor
timestamp: March 05, 2016, 03:23:36 am
content: Quote from: MK14 on March 05, 2016, 03:02:08 amThanks for taking the time to help me. That cleared both the problems I mentioned.