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From: Jim Yanik on 30 Mar 2010 08:54
"N_Cook" <diverse(a)tcp.co.uk> wrote in
news:hosb3o$r7f$1(a)news.eternal-september.org:

> 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 ?

is there any OVERvoltage shutdown circuit? some protection crowbar you've
missed? some SMPS have crowbars on the 5v logic rail to protect the digital
logic on the load.I've seen where a zener broke too soon and triggered the
SCR too early and caused shutdown.
>
>
> 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.

scope each rail to see how far it climbs.
Maybe one rail is not climbing as much as it should.

> 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.?
>
>
>

use a scope with a current probe to see if you're hitting the current
limit.
Check the PWM IC's housekeeping cap.Maybe it's not holding the IC supply
enough to keep the PWM IC running.(a common failure)

--
Jim Yanik
jyanik
at
localnet
dot com
From: Jim Yanik on 30 Mar 2010 09:04
supervinx <supervinx(a)libero.it> wrote in
news:fVksn.40988$Ua.2752(a)twister2.libero.it:

> 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 ??

scope each rail to see if they rise proportionally.

>>
>> 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.

A DMM will not respond fast enough.
>
> The main capacitors are 200V rated, but I see 215 across them :(
>

By "main caps" do you mean the line supply filter caps,right after the
rectifier? The raw DC supply for the switcher?
If it's a doubler,you should have around 300VDC,if a single-ended
rectifier,about 170VDC. Perhaps you don't have enough input DC.
Or maybe you're seeing some AC along with the DC.

Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch
changes it to single-ended for 220VAC operation.

--
Jim Yanik
jyanik
at
localnet
dot com
From: supervinx on 30 Mar 2010 09:25

> By "main caps" do you mean the line supply filter caps,right after the
> rectifier? The raw DC supply for the switcher? If it's a doubler,you
> should have around 300VDC,if a single-ended rectifier,about 170VDC.
> Perhaps you don't have enough input DC. Or maybe you're seeing some AC
> along with the DC.
>
> Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch
> changes it to single-ended for 220VAC operation.

The PSU is 250Vac: the two capacitors are 680uF/200V (they seem to be
series-connected) and they show, with no load, 215V across each capacitor.
This value is suspect: I would expect 170V, as you stated, so the problem
could lie in the primary side of the PSU.

The 680uF/200V is mandatory ? Can I try to substitute them with, for
example, 1000uF (or more)/400V ? I have them handy ...
From: N_Cook on 30 Mar 2010 11:53


--
--
Jim Yanik <jyanik(a)abuse.gov> wrote in message
news:Xns9D4B5AA62A390jyaniklocalnetcom(a)216.168.3.44...
> "N_Cook" <diverse(a)tcp.co.uk> wrote in
> news:hosb3o$r7f$1(a)news.eternal-september.org:
>
> > 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 ?
>
> is there any OVERvoltage shutdown circuit? some protection crowbar you've
> missed? some SMPS have crowbars on the 5v logic rail to protect the
digital
> logic on the load.I've seen where a zener broke too soon and triggered the
> SCR too early and caused shutdown.
> >
> >
> > 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.
>
> scope each rail to see how far it climbs.
> Maybe one rail is not climbing as much as it should.
>
> > 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.?
> >
> >
> >
>
> use a scope with a current probe to see if you're hitting the current
> limit.
> Check the PWM IC's housekeeping cap.Maybe it's not holding the IC supply
> enough to keep the PWM IC running.(a common failure)
>
> --
> Jim Yanik
> jyanik
> at
> localnet
> dot com


So far confirmed the UC3842 functions properly on a bench 13.5V supply with
original 2nF and also27nF // to it. But gets overriden by a protect sub-cct
on the hot oscillator side. Also that the 2 switching powerfets are
conducting balanced. Problem seems to be a goodness of oscillation monitor
on the hot side of the transformer that rectifies some of the drive and if
not balanced or large enough starts the tick-tick mode. Rectified it must
exceed 15V set by a zener or this mode cuts in , it would seem. Only
reaching 4.9V. But in the process of checking this area a small electo now
has an ESR of 200R wheras checking before any power on it must have been sub
2R or so sitting around idle. 1988 ps with a load of early SM and this cap
of course in an awkward section to replace rather than test, a job for
tomorrow.


From: supervinx on 30 Mar 2010 12:11
Thanks for your help, the fault begins to be more clear ...

Having no oscilloscope, I used spartan methods ;)

1) connected a 12V fan pair: the voltage dropped to 3.5V, but the PSU ran
well
2) added another +12V fan pair, but connected to the +5V line: +12 line
raised to about 9, +5 to 4.80 ... ran with no troubles
3) added another small load on the +5V (47 Ohm) and everything shut
down ... The +5V line connected fans were motionless (too low voltage)
and no motion was noticed adding the 47 Ohm load.

So I must assume there's no voltage peak: the protection scheme could be
a "current sense" and gone wild ?

4) Retained the +12V line connected fans and the 47 Ohm load on the +5
Line (removed the fans on this line): starts ok
5) Tried to move the trimmer (which should adjust the +5V, very slowly)
and power up, with the aforesaid loads.
6) Until 4.59 on the +5 line ... ok
4.60-4.66 starts, lasts few seconds, then shuts off
>4.66 starts-stops immediately

That trimmer is connected to two transistor and some diodes (the small
orange-glass-like ones, not the "usual" black ones.

So, I suspect the fault is in the protection circuit, generating a false
positive.

I think of removing the two transistors and identify them.
If some diode is a Zener, the only way I have to test is to desolder them
and build a small auxiliary circuit (diode-resistor), to check the
breakdown voltage.
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