Re: hot plugs

[Dalvenjah FoxFire <dal.dalvenjah@net>]

On Fri, Feb 05, 1999 at 12:30:44PM -0800, Jim Serio put this into my mailbox:
> 
> Do MB's have some sort of circuit breaker? On many
> a occasion, while working on mine, I've pulled cards
> or dropepd screwes and hit something that caused the 
> power to shut off. Immediatly trying to switch it back
> on results in nothing, but waiting a minute or so
> seems to resolve the problem. I assume either the 
> PS or MB have some sort of circuit protection and
> some sort of timed breaker?

I think most PC-type power supplies have some sort of cut-out that
does this; and yeah, they 'reset' after a minute or two.

> On a related note, while talking with a co-worker
> who was studying for his A++ cert, one of the
> questions is soemthing like:
> 
> To discharge the capacitor in a monitor, you need to:
> A. do nothing. There is no power when unplugged
> B. Wait 24 hours
> C. use a screw driver to short it
> D. ?
> 
> Anyway, the correct answer is/was C! Has nayone tried
> this? I imagine Carl or some other CE/EE could confirm,
> ut I'd think you'd fry your ass off, or at least get a jolt.
> On a related note, I've heard that power supplies, while
> they do retain some juice even when unplugged, only
> offer a good jolt if you open them and touch somethign you
> shoudln't (before discharging it). But it's apparently non
> life-threatening. Confirm?

Depends. Don't try it. }:>

The first thing you do when opening anything with a high-voltage
picture tube is indeed to discharge the high voltage supply. There
are special tools for this; the most common is a thin metal spike
with a wire that looks like a spark plug lead leading to a ground
clamp; the spike has a 2-inch-diameter rubber handle around it.
You connect the ground clamp to a good ground, then wedge the spike
under the high-voltage connector's rubber gasket, touching the
metal connector post, to discharge the supply.

Note: keep ALL your body parts away from any type of ground or
metal thing, including static ground straps, while doing this; I once
did this operation on a Mac Plus while holding my hand next to a metal
part of the case. Despite the two-inch-thick rubber handle, I
managed to send a spark across the half-inch gap between my hand
and the metal portion of the case; the display would have been cool
were it not extremely painful.

It's generally a good idea, when working around things with large-capacity
or high-voltage capacitors, to discharge the capacitors before handling.
Assuming the power supply is turned off and unplugged, the shock
will most likely not be life threatening, just painful; I do not
recommend anyone risk it, though. (over the years, I've managed
to shock myself with everything from 12V while starting a car, to
277VAC (commercial fluorescent light applications), to high-voltage
spark plug and CRT voltages. I've miraculously survived so far, but I
don't relish the idea of getting shocked again, and certainly have
the memory of many different shocks to know that taking precautions
not to get shocked is a Very Good Idea.)

Just to be safe, before working with power supplies, unplug it, and
using a relatively thick wire, connect any voltage-bearing metal part
to ground to drain it. This can and should include the metal grounding
part of the power supply's case.

-dalvenjah
-- 
 Dalvenjah FoxFire (aka Sven Nielsen) "We've lost the bleeps, the creeps,
 Founder, the DALnet IRC Network       and the sweeps!" "That's not all
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