In article ,
Ken Scharf wrote:
I always assumed there was voltage across the caps when I worked on the
rig. First pull the plug(s). Wait till the voltmeter drops to zero.
A trick, courtesy of Bob Pease: whenever you build a power supply
circuit which carries line voltage or higher, stick an NE-2 neon bulb
and a suitable dropping resistor across the caps, and place the bulb
where it's clearly visible when the case is opened.
If the bulb is lit (at all) the caps have enough voltage in them to
give you a nasty bite.
THEN put a heavy screwdriver with a well insulated handle from ground
to the HV terminal to be sure!
I've heard cautionary notes about this... if you do this prematurely,
the arc can be scary (and dangerous in some cases - "arc flash" burns
can be severe) and I believe it's possible to damage or wreck some
caps as a side effect of a catastrophic high-current discharge.
A heavy grounding wire, with alligator clips, and with a
current-limiting resistor is another possible solution. You want a
low enough impedance to drain away any charge which may come out of
the cap's dielectric "soakage", but high enough to avoid a dangerous
arc flash when you first hook it up.
Keep on hand in the pocket if you need to
adjust anything when it's hot.
That's one of the best single pieces of advice around! Also, be aware
of what sort of grounded metal is around your workbench. Doesn't help
much to keep one hand in your pocket, if you lean your belly against a
grounded metal bench!
HV isn't the only thing that can get you. High current can give you
a nasty surprise. Like the guy replacing some batteries on a golf cart.
He was using a ratchet wrench to tighten the battery clamps and he
ended on the most positive battery terminal. The handle of the wrench
hit the chassis of the golf cart putting it between 36 volts and ground
of some VERY HEAVY DUTY batteries. Ever see a Sears ratchet wrench
glow WHITE HOT? (and melt?)
Haven't seen that myself, but I've heard of people who have had
screwdrivers, etc. literally vaporized under such circumstances.
--
Dave Platt AE6EO
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