Patrick Schaefer wrote:
Am 29.03.2010 17:43 schrieb rich12345:I've seen plenty of windows PC's assume room temperature after their power supplies decided to be a bit too energetic. No POST beeps, no video output. DEAD.Inside the PSU is a crowbar circuit that shuts down the regulator in case of overvoltage.The +12V line is compared to a reference voltage, and when it exceeds the limit, a thyristor fires and shorts +12V to Gnd. As all other voltages are tied to +12V by the transformer winding ration, they are shut down, too.For additional protection you may add a second crowbar circuit to the +5V line.
Clearly, my bad power supply had both failed regulation and a failed crowbar--a relatively unlikely situation, one would hope. I've only seen it in one failed power supply of dozens, which is good. The bad news is that the crowbar can fail silently at any time, giving no indication that it has failed. Then a subsequent failure of the regulation loop could be catastrophic. Perhaps the best solution would be to add a switch to force the crowbar, proving it works. (Switching in a parallel resistor to cause it to trip at 11v would be perfect.) -michael NadaNet and AppleCrate II: parallel computing for Apple II computers! Home page: http://home.comcast.net/~mjmahon "The wastebasket is our most important design tool--and it's seriously underused."