Thread: Faulty SMPSU.
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Old May 23rd 21, 08:20 PM posted to sci.electronics.repair
T i m T i m is offline
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Default Faulty SMPSU.

On Sat, 22 May 2021 18:07:17 -0700 (PDT), whit3rd
wrote:

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).


Agreed. I think I read .2xxV on my DMM diode test in the forward (x2)
and as you say nothing in the reverse (x2 + 2 (between the two
outers)).

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?


I believe they we

"Panasonic 1000uf 16v Electrolytic Capacitor 105 LOW ESR FR Series
EEUFR1C102L" ?

https://industrial.panasonic.com/ww/...ls/EEUFR1C102L

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.


(Interesting, thanks) From that spec the only value I can see related
to ripple is:

"Rated ripple current-1 (mA) 1560.0"

So the 4 would give 6.24A but would that still impact the off load
voltage?

I might see if I can rig up my USB scope and actually 'see' what the
output is doing.

Cheers, T i m

p.s. Because I need the 3D printer running I've since bought 2 x Mean
Well (SP-200-12) 12V x 16.5A PSUs and have tacked them in to be able
to get going (and need to design and print some new mounting brackets
g). However, whilst it looks like I could end up spending ~10 GBP
(more if you count to total cost when it's more sensible to buy 10 off
something when I only need one) to get this old PSU running (assuming
I ever do etc) and could buy another cheap one new for just a few
pounds more, I am interested to see if I can get it going (with the
guidance of some good folk here etc g), firstly, because I would
find it satisfying, second because I then have a spare PSU and third I
wouldn't them have to throw the rest away. ;-)