> > Well. It depends if you read the schematics before you do so. In this case > I powered the startup voltage from a 12 V bench supply. Normally the > startup voltage comes from a small mains transformer and a 7812. Then I > supplied the primary side voltage for the main switch transistor using a > variac. Works perfectly well if you know what you are doing. Have done this > practice with many SMPS supplies.
Remember an SMPSU approximates to a constant _power_ load. It will draw more current as the input voltage decreases. As a result, if the supply is fairly well loaded on the output side, the primary side current can be high enough to damage the chopper transistor if you run it at low input voltage. -tony
