Linear power supply failure with load: Some general advice needed! - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2015, 07:07:54 pm
content: Hey.So I recently posted a thread about a mid 1970's dumb terminal I'm restoring - and I'm having some issues with the power supply.I'm making a new thread, since this isn't necessarily specific to that terminal, more of some general lin-psu stuff with that specific application, and I didn't want to bump a thread I had no specific updates on.This terminal has a linear power supply, that delivers some voltages for a CRT monitor; they seem to be working, haven't gotten to them yet, and +-12V, and +5V, for digital/other stuff. Now the 5V rail is the culprit, since, when the main digital boards are disconnected, the output reads a nice 5V (very close),and when i connect the load, the 5V rail read about 1,3V. So it fails under load.Either that or I'm goofing up something: the resistance measured at the 5V terminals on the "load" logic boards are around 180 ohms, with about 150 74 and 74LS IC's. - Is something shorted, that I'm missing?If that isn't the case, is there a typical failure point in linear power supplies that could cause this sort of behavior?-Thanks in advance,

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 21, 2015, 07:18:24 pm
content: No idea, but that's close to two diode drops. Weird.Could there be a current limiter in there kicking in?180 ohms is ... fine, I guess. If you read 1 ohm or less that's when I'd worry. 180 ohms sounds reasonable to a first approximation.Use a resistor as a load instead, isolate the supply from the load and try to get it running with resistors first.Do you have a lab supply to try to power up the logic with a known good supply?Pictures, model #s, part# of power supply chips, etc.. please.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2015, 07:26:35 pm
content: Well I'll have to test with a load, +try out the logic boards on a good psu, good idea! - The model is a Cifer PS502 serial computer terminal, no info on it whatsoever. Damn, had hoped it was a typical symptom of some specific problem.Thanks again.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 21, 2015, 08:02:33 pm
content: When you measure 180 ohms, are you just using a multimeter? If so, that's probably incorrect. Most DMMs use a low voltage ohms circuit that's designed so it won't turn on a semiconductor junction. All you're reading is miscellaneous resistors and capacitor leakage. A board with 150 TTL ICs could easily draw over an amp of current. A quick flip through an old 74LS data book shows power dissipation in the 30 to > 100 mw range for each chip (6 to 20 ma). Does the power supply have maximum output ratings on it?Your power supply may be smart enough to detect an overcurrent situation and shut down to protect itself. As Alex said, test the power supply and the board seperately to see what's going on.Ed

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2015, 10:33:25 pm
content: So; The psu's 5V rail has no problem delivering 5V through a 100 ohm resistor, aand the logic board maxes out the current (1.5A) on my lab psu.Guess the PSU wasn't the issue after all! Thanks a bunch!Now, whether the problem with the logic board is an IC or some discrete component: by measuring the rail with a multimeter while it's off; could one deduce that it's an IC? Since the multimeter, as stated in a previous post, wouldn't do anything to power the chips?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 21, 2015, 11:18:47 pm
content: Quote from: ChristofferB on March 21, 2015, 10:33:25 pmWell hold on there, all you've shown is that your power supply can drive 5V/100R=50mA, not exactly a stress test. You should put a 1R 5W resistor on there, and see if the supply can deliver 1A, try to suck more and more current to see. Or keep stacking 100R resistors, how many do you have?Does the lab PSU also drop its voltage when it reaches 1.5A?

author: edpalmer42
author_rank: Super Contributor
timestamp: March 21, 2015, 11:31:17 pm
content: Quote from: ChristofferB on March 21, 2015, 10:33:25 pmNo, you haven't tested the power supply properly yet. A 100 ohm resistor only draws 50 ma. You need a 5 ohm, 5 watt resistor to draw one amp. Although you don't know what the drain of the circuit is supposed to be, a one amp drain will show that the power supply is likely good.Alex: Check your math. A one ohm resistor would draw 5 amps!Quote from: ChristofferB on March 21, 2015, 10:33:25 pmYes, you've right. That suggests that the problem isn't any of the bypass capacitors. If there are any electrolytics or tantalums on the board, a resistance measurement might show an initial low reading that slowly rises to the 180 ohm level as the capacitors charge and the current drops.When you power the logic board from your lab supply, what voltage do you get at the 1.5 amp limit? If it's a volt or two, that means that something could be dissipating a watt or two. Do you have a digital thermometer or, much better, a thermal camera to figure out what's getting warm? I don't think that a working TTL device would draw much current at a volt or two, but I could be wrong. From the pictures, it looks like all the chips are in sockets. You could even try unplugging them and figure it out that way. Make sure you make notes of what IC was in what socket! Of course, don't plug or unplug ICs while the board is powered! Maybe unplug a row at a time, power the circuit, and see if the current has dropped.Ed

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 21, 2015, 11:33:45 pm
content: Yeah, 5 ohms...

author: SeanB
author_rank: Super Contributor
timestamp: March 22, 2015, 08:28:21 am
content: TTL chips draw around 30-50mA just sitting around, so 150 chips will be drawing a minimum of 4A from ythe supply, so it will probably be capable of delivering around 8A to 10A. The 1R 10W resistor in a glass of cold water as a test will at least get the PSU into the right load range, though the resistor will boil the water in a minute or two. Very likely the PSU has the problem, either failed power supply capacitors ( those are typically 10 000uF 16V caps in a large can) or failed smaller capacitors around the LM723 regulator, or an open circuit resistor.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2015, 09:45:25 pm
content: Guess I was a bit tired yesturday! Didn't really concider what i was doing! In any event, have been doing as suggested, and sure enough, under a 5 ohm load it fails and drops the voltage rail to about a volt, slowly falling. So the psu is the problem after all. Ive gone ahead and changed most of the electrolytes, but haven't tried it afterwards yet. Heres an image of the board:the charred areas seems to be old damage: they're definitely fixed by previous owner, and they aren't in the 5V rail area of the board.The grey wires from the clamp is to a large capacitor that's clamped to the chassis - looks like a botch.In a linear power supply, is it safe to assume that large electrolytes can be replaced with different values? - like changing a 680 uF with a 1000uF one?Again, thanks for all the input.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 23, 2015, 04:16:18 am
content: So there are big transistors mounted on a heatsink somewhere else and wired back to this board? I see contacts labelled e,b,c in a few places. Maybe big rectifiers on the same heatsink?Yes, you can replace an electrolytic with a larger value as long as the voltage rating is equal or greater. But don't take that too far. For example, it would be a bad idea to blindly install a new capacitor that's 10 times larger than the original.Ed

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2015, 11:58:21 am
content: Yes, there is, 2n3055, i think (don't have it in front of me) - First i thought that was the issue, and i had some lying around anyway, so i changed the 5V rail one. Shouldn't be the issue. Rectifiers are on the other side of the board, clamping down onto a large heatsink.After changing most capacitors; same 5V rail issue, but the humming of the transformer stopped. Almost gave me a heart attack, thought for a minute it had died completely!

author: edpalmer42
author_rank: Super Contributor
timestamp: March 23, 2015, 07:06:32 pm
content: HUMMING?? You didn't say anything about humming! How loud was it? If it was happening when the power supply wasn't connected to the load there might have been a short somewhere in the supply. Maybe that fixed your problem.The annoying thing about a linear power supply is that everything is DC-connected to everything else. If one component blows, you typically see cascade failures that take out multiple other active devices. The same thing often happens in high-quality audio power amps. You might have to replace that 723 regulator or the round metal driver transistor (?) with the wavy heatsink. You're almost to the point where you should trace out the schematic so you can start tracing through the circuit with a DMM. The original symptom (proper output voltage with almost no current capability) suggests that the regulator is working, but the main power path has a high resistance, but not an open, either in the main power path or it's control path.Ed

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2015, 09:03:34 pm
content: I didn't really think the hum was much of an issue. Lots of older gear has that transformer hum, but yeah, loud enough to hear it. - Anyway that solved itself.Yeah, should trace out the circuitry i know. But really, there's so few components associated with the 5V part, might just replace them systematically. The transistor with the wavy heatsink is a BFY 50. I have some BFY 51's lying around i might try. they're a bit lower in voltage rating, but since it's used on the 5V stuff it probably won't matter.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 24, 2015, 07:29:28 am
content: Yes, a systematic replacement should work. Watch out for the situation where one dead part kills the new part you just installed.If your multimeter has a diode test function you can check the transistor without unsoldering it. Nearby components can confuse the reading, but shorts and opens are pretty obvious.Make sure you come back and tell us how things turn out. I hate it when I don't hear the end of the story!Ed