17 Jan
2006
17 Jan
'06
12:10 p.m.
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
That wouldn't make much difference actually. Most of these SMPSUs have 2
capacitors in series. They each charge to around 160V, no matter whether
the mainse is 115V or 230V. For the former, the input rectifier acts as a
voltage doubler, chearging each capacitor to the peak of the AC input on
alternate half cycles. for the latter, the input cirucit is a
conventional bridge rectifier.
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.
Ocuh.
I can understand cheap PC-related PSUs missing out the bleeder ressitors
to xave all of 5p, but I was suprised Tektronix did it. Oh well...
The PSU in my HP9845B is relatively friendly in this respect. It has
bleeder resistors. It also has well-insulated holes in the top cover
through which you can insert voltmeter probes to check the residual
voltage on the smoothing capacitors before taking the PSU module out of
the machine. The procedure given on the top of the supply is to isolate
the mains, wait 5 minutes, insert the discharger tool (which I assume is
just another bleeder resistor, or maybe a shorting link!) and then to
check the thing is discharged with a meter. Then yoy take it out.
-tony