2016-01-22 21:48 GMT+01:00 j...@cimmeri.com <j...@cimmeri.com>: > On 1/22/2016 2:54 PM, Mattis Lind wrote: > >> Short: R27 in my VT100 PSU is hot and smelling. Why? >> >> Long: I think it has been 20 years since I powered up this VT100 so I did >> it carefully. Used a Variac and a bench supply. It switched just fine and >> delivered the steady 5V out when the input was at approx 50V (115V input). >> All the other voltages looked fine at full AC input. But there was this >> little smell from R27. >> >> > You don't use Variacs with switch mode power supplies... not unless you > want to burn them up. I'll sometimes use one, but only to provide a very > fast soft start. Even that's risky.
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. > > - J. > >