[sdiy] OT: Whining Transformer

[Batz Goodfortune]

Y-ellow Harry, Steve, 'n' other mutants.

At 10:48 PM 10/20/04 -0700, harrybissell wrote:
>It is very hard to repair switching power supplies unless you have the
>schematics, parts
>sources, and a good deal of skill (luck)

I find voodoo helps. Before attempting any SMPS repair work I always bathe 
in the blood of a chicken. Depending on the scale of the problem I may opt 
to bite the testicals off a male goat and/or slaughter it then drink the 
blood. If you're not prepared to do this, I suggest leaving SMPS repair 
work alone.

>Probably it is NOT the transformer.  Usually switching power supplies
>operate well above the
>audio range. Sometimes, because of load problems (too much / too little)
>they go into non-continuous
>oscillation... and these bursts can produce audio noise.  This WILL be
>heard in the transformer
>as the core changes with the magnetic fields.

Curiously, because these things are cheap-assed, the heat sinks for the 
switching transistors and the FREDs (Thanks to Eric Barbour for that one. 
Look it up in your funk and wagnals.) are made of steel not aluminium. If 
they're placed either side of the pulse transformer and not terribly 
ridged, they can sometimes sing. This would be especially true of push-pull 
designs. Where the flux is pushing and pulling of course. As the actress 
said to the bishop.

>I'd look CAREFULLY at the electrolyic caps first.  If you have a scope
>with a true differential probe
>(yeah, right... most folks don't... but it is needed to safely look at
>line powered switchers...)
>you can examine the output from the line capacitor (usually runs at
>about 300VDC) and look for
>excess ripple, and look at the actual outputs for other signs of
>trouble.

CRO killers aside. Another source could be the input filters. 
Caps/inductors etc. In many linear systems there's a small input line 
filter on the mains to protect the unit from mains spikes. In the case of 
an SMPS it's there mainly to protect the mains from the spikes generated by 
the SMPS. Nasty business. You could try shoving a surge buster on the end 
and see what happens.

>I'd consider a new supply if one is available.  If you try and
>troubleshoot the supply without a load...
>it will not work, or blow up !  If you try and make a load for it... try
>getting rid of the possibly hundreds
>of watts of heat.   Usually people are just not set up for this type of
>repair.  I usually do not bother
>myself.

Probably the best advise is just buy/acquire a new one. Even 400watt ones 
are cheap-as these days. It's not worth the hassle. But out of interest, 
the load resistor required doesn't have to be as big as popular belief 
would suggest. In fact as little as a few hundred ohms at 1/2 watt can 
suffice in most cases. You only need to load up the 5 volt supply. It only 
needs to prevent over-shoot on the output capacitors. An RC time constant 
of about half a second is good enough. For most SMPS designs, the SMPS 
controller chip is powered by the 12 volt output rail. Turning the PSU on 
causes a line spike which while not enough current to run anything major, 
is enough to charge the capacitors on the 5 volt rail to their max. The 
SMPS chip powers up but instantly senses over voltage and shuts the pulsing 
down. Periodically it will sense the voltage has dropped on the 5 volt line 
and give a pulse or two. This once again charges the cap up and the chips 
shuts it down again.

Meanwhile, on the 12 volt rail, because there's only a small amount of 
drive getting to the 5 volt rail, and this is the only form of regulation 
feedback, there is sod all voltage generated across it's caps. So you get 
this really weak voltage and current on all the other rails. All you have 
to do is put a resistor across the 5 volt rail that will knock the peaks 
off the initial pulses and keep the capacitor in a discharging direction 
and the supply should fire up as happy as Harry. (and we all know how happy 
Harry can be.)

Just a bit of SMPS trivia for you. One day I'll forget all this crap and 
the adventure will start again.

But down here we couldn't hear the sound of a whining SMPS over the sound 
of whining Australians after they realized how much they were duped at the 
last election. SUCKERS!

Be absolutely Icebox.

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