[AMRadio] Power Resistor


Jim candela jcandela at prodigy.net
Sun Mar 20 20:23:38 EST 2005


Byron,

   I recall someone in your situation using a lower wattage pot, and a HV
NPN Horizontal output transistor as an emitter follower. A resistor from
emitter to ground is required, maybe 47K 2 watts. You might think about that
option. This transistor will need to be heat sinked, and the case isolated
from ground with a mica insulator, and a little "owl sh_t" grease. If the
transistor beta is 100 at say 20ma emitter current, then the base current is
.02/100 or 200 ua. Your pot will stay cool as the transistor does the work.
In case the output gets shorted, maybe add a 470 ohm series resistor that
should fry in the case of a short. I'd put that on the collector side going
to the low B+ since it might quench out a parasitic before it gets started.

Regards,
Jim

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Byron Lichtenwalner
Sent: Sunday, March 20, 2005 10:31 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Power Resistor


I have been following the discussion on Jim's problem, and have a similar
"quest".

I have been trying to find a 4 watt, 20 k pot. to use in a Ranger power
control circuit as described by W3AM.  Have had no luck in finding on.  Any
ideas?

Byron, W3WKR

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