Thread: Faulty SMPSU.
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T i m T i m is offline
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Default Faulty SMPSU.

On Tue, 18 May 2021 16:03:31 -0400, legg wrote:

snip

They're shorted on the printed wiring, so just simple 2wires in
parallel.


Ok, so just to increase the current carrying capacity or something
more subtle? I'm more of a DC / LV / serial datastream sorta guy and
know 'frequencies' can do weird stuff sometimes (skin effect etc). ;-)

On closer examination of choke, I see it's been impregnated, so
there's little likelihood of fault due to enamel failure or
vibration.


Good news, thanks.

snip

Ok, I just pulled these had found:

The 100uF, 25V:
VLoss= 1.8%
Capacity= 90uF
ESR= initially 2.1 ohm then dropped to .69 when repeating the test.


These are often part of feedback network for simplest integrated
switcher ICs. Probably not the culprit, but low side of tolerance
on an electrolytic is indication of end-of-life.


Noted.

Changing esr can
indicated corroded/intermitent inner contact.


Ok, and cheap / easy enough to replace so ...


The 4.7uF, 50V:
VLoss= 3.0%
Capacity 4755nF
ESR= 5.5 Ohm and stayed around that after several repeats of the test.

So, is that our 'bad' cap?

snip
Small 4u7 can have esr in the mid-ohms range. Probably not an issue
on housekeeping supply, but parts with 1R esr will allow start up
always in cold weather.


Ok.

Check rectifier feeding it for typical
forward voltage. If open or shorted, the unit would only run
intermittently (short would affect Lp measurement, but only if
in-circuit)


D7 checks out as a std rectifier on my DMM (ohms Diode test).

Main transformer primary inductance of a forward converter
running off mains will be more than 400uH - typically 2mH.


Ok, I'll try that now ('strike whilst the iron is hot'). ;-)

https://ibb.co/N33cKSf

Lower values suggest shorting of windings or terminal
connections/traces.


Understood.

What I think is the primary (marked in red) was:
.4 ohms, 17.4mH.


That's ok as long as measuring units are correct (common mistake).


My tester actually displays the values as quoted so I'm happy there.
;-)

Fairly high value for this power level and topology, but only
low side limit is important.


Ok.

Another winding on the primary side (marked yellow and going to the
small caps etc) showed:
.3 ohms, .08mH

FWIW, the output seems to be made of 3 windings in parallel (marked in
green and tested wired that way) measured:
.02 ohm, .25mH.


No surprises.


Good news. The txfmr also 'looks' ok FWIW, no burn marks etc.

Integrated switchers can be finicky when it comes to self-generated
noise.


So that's the 5 pin TOxxx case device?

This makes snubbers and clamps pretty important.


Do you mean the small electrolytic's, diode and other caps (is that
what they are / doing)?

As a last
resort, check primary parts wired across main primary winding and
secondary side RC before replacing the big integrated switch.


So are we still on for replacing the opto device RL? I don't think my
tester can do that, other than the opto side as a diode possibly and
if it wasn't so cheap and already fairly old, I'd set up a test for it
myself on some breadboard. ;-)

Just to give me a better visualisation on all this, could you point me
to a schematic of something similar perchance please?

Cheers, T i m