14 Jan
2006
14 Jan
'06
2:59 p.m.
Tony Duell wrote:
The worst I've seen (as it made me leap) was the
PSU from a Tektronix
terminal (I forget he model number, but it was a colour, raster-scan
one). This was a little SMPSU in a metal can. Unfortunately, there was no
bleeded resistor on the mains smoothing capacitors. If the startup
resistor went open-circuit, the capcitors had no way to discharge. And
the way into the supply was to undo a couple of screws and slide the PCB
out. If you didn't realise the capcitors were likely to still be charged,
you would most likely accidentally touch the mains smoothing capacitors
with painful results.
I've done exactly that with an SMPSU from a SCSI tape unit.
I grabbed the PCB after the unit had failed, been taken out
of service, and left unplugged for a day or so. The main
smoothing capacitor was still charged up (and remember we
have 240V mains here) and discharged across the second and
third fingers of my left hand. I yelled quite a bit. Then,
I found that I had "+" and "-" burned into my fingers from
the capacitor terminals.
--
John Honniball
coredump at gifford.co.uk