Thread: Faulty SMPSU.
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Old May 23rd 21, 03:07 AM posted to sci.electronics.repair
whit3rd whit3rd is offline
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Default Faulty SMPSU.

On Saturday, May 22, 2021 at 2:31:00 AM UTC-7, T i m wrote:
On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:


... the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.


Coming back to this before buying / replacing the switcher, I tested
it out of circuit on my DMM (diode test, all ways etc) and it seemed
to check out ok?


They almost always fail with a dead short, couldn't miss that with
a diode check ( 0.2 to 0.8V is a typical good-rectifier reading forward,
and no conduction in reverse).

FWIW I measure about 10V DC on the output (no load) and only about .5V
on the AC range


Since the capacitors on output were replaced, are you sure they're low-ESR types?
The ripple current ratings have to add up to about the output (20A) of the unit.
Correct capacitance but low ripple current ratings (aka high ESR) on the replacements
could explain bad output with good rectifier.