Switching PSU shutting off under every load
in [Repair]
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From: supervinx on 30 Mar 2010 02:55 Hi to everyone, since it's the first time I write in this NG ;) I've little experience in switching PSUs ... I've one of them (coming from an Amiga 2000) which shuts off at the first load. With no load at all, it delivered 5.9V out of 5 nominal. I reduced it (with the trimmer) to about 5 with no load. Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but with more serios loads (e.g. an HD), it shuts again. I checked in-circuit capacitors for shorts and ESR increase, tested diodes and transistors. It seems an aging problem (the trimmers were still sealed, so no tampering), but who's bad ?
From: N_Cook on 30 Mar 2010 04:01 supervinx <supervinx(a)libero.it> wrote in message news:ffhsn.152240$9f6.195196(a)twister1.libero.it... > Hi to everyone, since it's the first time I write in this NG ;) > > I've little experience in switching PSUs ... > I've one of them (coming from an Amiga 2000) which shuts off at the first > load. > With no load at all, it delivered 5.9V out of 5 nominal. > I reduced it (with the trimmer) to about 5 with no load. > Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but > with more serios loads (e.g. an HD), it shuts again. > I checked in-circuit capacitors for shorts and ESR increase, tested > diodes and transistors. > It seems an aging problem (the trimmers were still sealed, so no > tampering), but who's bad ? As it happens I will try fathoming a dedicated SMPS today that as it stands is in tick-tick mode, 1 ultrasonic cycle per second. Putting a dropped LED on each output shows life on all rails. Plan of campaign will be Run up on variac to find lowest point and will still run of sorts, tick-tick Put a x10 larger timing capacitor over the one on the 555 that feeds the UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated, LED of the optoisolator, over-riding the error feed. This will be a monostable of variable period from 1mS to a second or so with push switch, hopefully to elicit which area is playing up by gradually ramping up . All active, big Rs seem ok and caps ESR-check ok. I suspect false error signal. Anyone else any ideas/advice on a generic plan of action for these sorts of situations.?
From: jeanyves on 30 Mar 2010 05:54 On 2010-03-30 10:01:16 +0200, N_Cook said: > supervinx <supervinx(a)libero.it> wrote in message > news:ffhsn.152240$9f6.195196(a)twister1.libero.it... >> Hi to everyone, since it's the first time I write in this NG ;) >> >> I've little experience in switching PSUs ... >> I've one of them (coming from an Amiga 2000) which shuts off at the first >> load. >> With no load at all, it delivered 5.9V out of 5 nominal. >> I reduced it (with the trimmer) to about 5 with no load. >> Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but >> with more serios loads (e.g. an HD), it shuts again. >> I checked in-circuit capacitors for shorts and ESR increase, tested >> diodes and transistors. >> It seems an aging problem (the trimmers were still sealed, so no >> tampering), but who's bad ? > > > As it happens I will try fathoming a dedicated SMPS today that as it stands > is in tick-tick mode, 1 ultrasonic cycle per second. > Putting a dropped LED on each output shows life on all rails. > Plan of campaign will be > Run up on variac to find lowest point and will still run of sorts, tick-tick > Put a x10 larger timing capacitor over the one on the 555 that feeds the > UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated, > LED of the optoisolator, over-riding the error feed. This will be a > monostable of variable period from 1mS to a second or so with push switch, > hopefully to elicit which area is playing up by gradually ramping up . > All active, big Rs seem ok and caps ESR-check ok. I suspect false error > signal. > Anyone else any ideas/advice on a generic plan of action for these sorts of > situations.? you checked the esr and caps but I would desoldier the main 400v cap and test its capacity and esr out of the box. compare with a new one. also as you have a uc3842, I would do the same with the small (10uF or so) cap that is on the + and - pins of this uc. often this is the problem. regards, -- Jean-Yves.
From: Phil Allison on 30 Mar 2010 06:14 "supervinx" > > I've little experience in switching PSUs ... > I've one of them (coming from an Amiga 2000) which shuts off at the first > load. > With no load at all, it delivered 5.9V out of 5 nominal. ** Many PC type switching PSUs need a load to be applied on ALL the outputs on order to work properly. Why not apply load resistances ( not incandescent lamps either) to each DC rail and see if it holds up then ?? Thing is, when you load just the 5 volt rail, the other DC rails go UP in voltage - and that may trigger the out of range voltage detector circuit and stop the whole damn show. ..... Phil
From: supervinx on 30 Mar 2010 07:04
Il Tue, 30 Mar 2010 21:14:04 +1100, Phil Allison ha scritto: > "supervinx" > > >> I've little experience in switching PSUs ... I've one of them (coming >> from an Amiga 2000) which shuts off at the first load. >> With no load at all, it delivered 5.9V out of 5 nominal. > > ** Many PC type switching PSUs need a load to be applied on ALL the > outputs on order to work properly. > > Why not apply load resistances ( not incandescent lamps either) to each > DC rail and see if it holds up then ?? > > Thing is, when you load just the 5 volt rail, the other DC rails go UP > in voltage - and that may trigger the out of range voltage detector > circuit and stop the whole damn show. > > > > .... Phil The PSU shuts off under normal operations while connected to his MB. I connected it to an external HD (load on both +5 and +12): I see the voltage raising and the voltage detector shutting the PSU down. I have no oscilloscope handy ... with a digital multimeter I can see the +5 rising about 4.8, the +12 about 9 and then stop. The main capacitors are 200V rated, but I see 215 across them :( |