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.

The original rectum-frier, no doubt.

Which reminds me of the article I found and appended.....

"default" wrote in message
...

A spark shot from his ass to the scope cart behind him.


-----OOOOO-----

In article ,
writes...
Hi,
Would you please tell me what transistor do and how to do
so? What is the difference between PNP transistor and NPN
transistor? What is the concept of using a transistor as a
switch? Thanks a lot.

SemiConductor Theory
--------------------
(c) 1997 by Gareth Alun Evans

A transistor is rather like the human alimentary canal, after the
typical USA diet of burgers and chips; - it constipates, as do
semiconductor diodes with no applied bias - the available holes get
filled in and nothing can move.
The base current is like a small application of laxative; some of
the constipation passes through, until the effect of the laxative wears
off. The total throughput depends upon the Mobility. By applying a
continuous feed of laxative, then a continuous current passes through.
Applying too much laxative results in saturation - ie, there is a limit
to the maximum throughput, depending on the external circuit; in this
case, the maximum rate at which you can feed in the burgers at one end.
(If you are a customer of MacDonalds's, here in Chippenham, Wiltshire,
UK then this rate is very low - I have been there twice, and both times,
the service was *APPALLING*.)
The difference between PNP and NPN is the direction. In the old
days, PNP was used, whereby one injected from the rear end, using a sort
of huge syringe - hence PNP - "Put-in Near Poo". More recently NPN is
more common, where the laxative is entered via a carrier of some sort,
usually chocolate and so we have NPN - "Now Pleasant Nutrient".
Despite the adverse effects, the USA diet of burgers and chips
carries on, and recourse has to be made once again to the chocolate. Now
the ratio of the carriers of the constipation, the burgers and chips, is
much higher than that of the chocolate. Thus they are referred to as the
Majority Carriers and the Minority Carriers. If you indulge too much,
you find that the vendor will provide you with a paper bag, known as an
Excess Carrier.
More recently, there are problems with impurities and you find that
the opposite effect occurs. You have no time to reach home before
diarrhoea takes over. You have no option but to stop the car and nip
over a gate into a field. Hence the Field Effect Transistor. This time
you have to inject something to STOP the flow. Now, assuming that there
was a certain control over events; nothing happened until the Gate was
encountered, you then became the Source of flow, and the field itself
acted as the Drain. What was originally dirt, became grass, was consumed
by the Cow, you ate it as a burger, and it has now returned to the
topsoil, an effect known in the trade as Surface Recombination.
(Incidentally, did you know that Diarrhoea is hereditary? Apparently it
runs in the jeans.)
Some of the impurities accumulate in your rear end, and no matter how
valiently you try, you cannot rid yourself of them. Hence In-de-Bum is
known as a Try-Valient Impurity. In the same way, Arse-nic, well known
for its ill-effects and accumulation in the body tissues is known as a
Pent-Up-Valient Impurity.

--
73's etc de Gareth G4SDW (nee G8DXY) GQRP 3339