Switch mode power supply

[Joe]

Hi,

I have a 9V, 0.75A switch mode power supply which has a strange problem. It
does not work (i.e. no voltage output) unless I _slowly_ slide in the IEC
mains lead (so that it sounds like there is some arcing) the first time I
use it for a while. If I just plug in the mains lead quickly or turn on the
socket at the wall it does not work. After it has worked once, and the unit
is slightly warmer perhaps, it operates fine. I can turn it off and on again
at the wall socket with no problems.

Any ideas?
Joe.

 

[Sam Goldwasser]

Hi,

I have a 9V, 0.75A switch mode power supply which has a strange problem. It
does not work (i.e. no voltage output) unless I _slowly_ slide in the IEC
mains lead (so that it sounds like there is some arcing) the first time I
use it for a while. If I just plug in the mains lead quickly or turn on the
socket at the wall it does not work. After it has worked once, and the unit
is slightly warmer perhaps, it operates fine. I can turn it off and on again
at the wall socket with no problems.

Usually, problems that disappear when something warms up are either
bad connections or dried up electrolytic capacitors - the latter being
more likely in this case.

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[Sofie]

Joe:
Very likely the problem you described is caused by faulty or high ESR
electrolytics...... don't continue nursing this problem along..... soon,
you may not be able to get it to work at all and additional components
including the switching transistor or chip may fail. If you do not have an
ESR meter, just replace ALL of the electrolytics.

 

[Joe]

Thank you both for the advice.
Joe

 

[w_tom]

Heat being a significant factor, then you should also know
about the inrush current limiter. When it gets warmer, then
it conducts electricity. Cold - it may limit power so small
that power supply cannot start. However you don't know
anything about that supply until you take some multimeter
readings. This assumes (of course) that you intend to repair
this supply.