On 25 Jan 2005 05:52:49 -0800, "Andrew VK3BFA"
wrote:


Airy R.Bean wrote:
That's news to me! Would you care to expand on it?

If one could look at a carbon atom in isolation, how is
the conduction mechanism changed by the size of the
applied electric field?


The paint on the resitors is not a perfect insulator - it breaks down
as the voltage increases. This is why there are high voltage resistors.
Not rocket science exactly.

And if you are going to stuff around with HV, make sure there is
someone aound who can do CPR when you fry yourself......
73 de VK3BFA Andrew

I don't buy the paint theory much . . . The original answer specified
a few hundred volts per resistor and that isn't exactly high voltage .
.. . The common radio shack flame proof 1/2 or 1 watt sizes would be
my choice for ruggedness.

A long time ago I repaired vacuum tube TV sets. I'd find an
occasional high value resistor with a short in it, or very lower than
tolerance reading. These were usually high value resisters with
relatively low current, across higher voltages ~400-500 and up.

When I asked about it, I was told that when resistors dropped high
voltage DC (these were carbon comp 10%'rs - back then) for long
periods of time, the copper in the leads migrated into the carbon and
lowered the resistance. In the case of something like a screen
resistor - it could go for quite some time before it caused a
noticeable problem - or at least severe enough for the customer to
have the set looked at.

It was a frequent problem in the old tube type CB transmitter output
stages - so RF may have accelerated the problem?

I have an old "boat anchor" style variable HV supply I picked up at a
ham fest. It has a 100 ua meter on the front measuring the 1 KV
output. The dropping resistor is part of the bleeder string - six one
watt resistors.

Stay safe. If it can kill you - respect it.

I saw one of my colleagues take a hit from an ancient high power Navy
AM transmitter. The thing had every safety device imaginable on it
(in fact the safety interlocks were the most troublesome part of the
devices). He followed protocol and picked up the shorting bar in the
compartment and put it to the HV caps - no arc so he assumed it was
safe (it had crowbar relays that short the supply when the covers are
removed, the shorting bar was extra safety).

He took it from his hand on the shorting bar (the ground was broken).
A spark shot from his ass to the scope cart behind him. He suffered
no physical damage - but he went into shock and wouldn't go near that
transmitter again.