Pete Edwards wrote:
On 06/02/06, Tony Duell <ard at
p850ug1.demon.co.uk> wrote:
It is a very bad idea to 'run up' a swtich mode PSU (which this is) on a
Variac. SMPSUs appear as conatant power loads, as the input voltage
decreases, the current increases. Some supplies will blow their chopper
transistors under such conditions.
Having witnessed the effect of a sustained brownout (I think it went down
to 50V for about 15 minutes) on a UPS-less machine room full of kit with
240V switched mode PSUs, I know that's a no-no. I think the survival rate
was about 20%.
While working doing tech support for retail EPOS equipment, I had a
batch of calls all for the same supermarket, all for tills that wouldn't
power on. About six, all logged at the same time.
So - phone the customer. Asked the customer services manager (for it is
them who deal with these things) if she could think of something that
might have caused this.
"Ah," she said, "yes. A JCB dug through a cable and blew up the
substation that we run off."
"Ok, then. Did they come up at all when power was restored?"
"Yes - for a while anyway, maybe about two hours"
"Right," I said, "What happened after two hours?"
"Well the standby genny ran out of diesel..."
That'll be six new power supplies, then. And billable, at that.
Gordon.