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[AMPS] Power Handling of Resistors

To: <amps@contesting.com>
Subject: [AMPS] Power Handling of Resistors
From: w7iuv@nis4u.com (Larry Molitor)
Date: Wed, 08 Mar 2000 00:09:10 +0000
At 11:29 AM 3/7/00 -0600, Jon Ogden wrote:
>  If
>enough power hits the resistors in a spike to destroy them, it will do just
>that - DESTROY them as Peter Chadwick suggests.  But is the typical PA
>supply, particularly a commercially made amateur supply capable of supplying
>that much power?
>
>I doubt it.
>
>Jon

Jon,

I have to comment here. Try this experiment:

1) go down to the hardware store and buy a face shield, the kind sold for 
grinding and sawing operations.

2) obtain a 2 watt 10 ohm resistor (carbon or metal film, whatever)

3) connect said resistor with one end on you amp HV and the other end dangling.

4) with all safety gear in place and the HV on, use a suitably insulated 
tool to touch the open end of the resistor to ground.

5) describe the observed effect to us

6) repeat the above experiment with a current probe/storage oscilloscope in 
series with the open end of the resistor.

7) describe the amplitude and duration of the current pulse observed.

Be advised that I have performed the above experiment (albeit without the 
face shield or the oscilloscope) hundreds of times. Not by choice.

The "suitably insulated tool" in these cases was either a 4CX250 or a 
4CX1000 that must have been birthed in a "poor science" lab someplace. 
These tubes have a tendency, without provocation and without damage to 
themselves, to go "big bang" at random intervals. I have had them do this 
with and without cutoff bias applied, usually at HV turn-on (but not always).

FWIW, I have *NEVER* seen a glass envelope tube do this, or a triode tube 
for that matter.

In case you don't really want to do the experiment, I can say that you 
probably don't need the face shield. The effect is approximately what you 
might expect from zapping the resistor with a "Klingon Disintegrator Ray" 
from a Star Trek episode. I have tried to capture the debris by placing the 
resistor in a plastic box, but have never seen anything except a bit of 
dark soot!

Now after all that, go back and repeat the experiment with a piece of wire 
wrapped around the resistor, Nichrome or otherwise. Please describe the effect.

73,

Larry - W7IUV




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