Using a Variac to revive power supplies
ard at p850ug1.demon.co.uk
Thu Dec 23 14:31:26 CST 2010
> I've recently acquired a Variac (220V 10A) and was wondering what the
> collectives views are on using them to revive long dormant power supplies.
Some people swear by the Variac for such things, I am not sure why...
> Previously I have removed the electrolytics and reformed them with a bench
> power supply setup, slowly ramping up the voltage (as per
> http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm). Am I right in
> thinking I can use the Variac to effectively reform the capacitors in
Maybe, maybe not. You could ramp up the votlage across the smoothing
capacitors (straight after the transformer/rectifier), but don't you want
to limit the currnet (if they need reforming) or at least check the
leakage current. In which case you will ahve to disconenct one end of the
Anyyhing downstream of the rgualtor can't really be reformed using a
variaco n the mains input either..
> I'm currently rebuilding one of my PDP-8/E's which has a linear PSU which I
> believe is ok, but will it also work with later DEC switchmode PSUs?
I would never use a variac on an SMPSU. SMPSUs are one of the few things
that approciomate a constant _power_ load which emans they actually draw
more current as the input voltage decreases. It's possible to damage some
tupes of SMPUS by giving them too little input voltage.
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