Yikes! I'm glad you haven't been hurt by this. Yeas - its obvious. Here's the deal. Your computer and probably much = of your mains-powered test equipment is grounded = to the AC mains ground. When you connect the = negative rail of your HV supply to ground, you = are essentially shorting out either D7 or D8. Draw out the entire diagram of the input side of = the HV supply. Make sure you include the = neutral-to-ground link in your building's Service Entrance in your diagram. Here's another quick test to show you just how dangerous this whole thing i= s. Connect the AC mains to only the HV bridge = rectifier input. Now grab 60W or so incandescent = light bulb (your desk light will do quite = nicely. Connect one side of the light bulb to = the AC mains ground line, then touch the other = side of the light bulb to your HV ground. Notice = how bright the lamp is (about half brightness). The way I see it, you've got a couple of choices. a) Use a suitably rated AC mains isolation transformer. b) Insert some form of isolation barrier in your = gate drive signal. This can be optical, pulse = transformer, the neat little capacitive couplers from Burr-Brown, etc. Sorry to be the bearer of bad news. dwayne At 02:37 PM 5/17/2007, Crist=F3v=E3o Dalla Costa wrote: >Hello > >The problem is that when I connect the two grounds of the high voltage >and low voltage supplies smoke happens. The low voltage ground is held >steadily at half the high voltage and even if I put a resistor in >there an uncomfortable amount of juice flows and there=B4s no >significant difference in voltage levels. > >I think I'm missing something obvious here. My transformer is >isolated, the ohmmeter doesn't even show a resistance reading >resistance between the coils. Shouldn't the low level supply, when the >grounds are connected, float to meet the high voltage ground >potential? -- = Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist