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[AMPS] Stability of Carbon Resistors

To: <amps@contesting.com>
Subject: [AMPS] Stability of Carbon Resistors
From: k7fm@teleport.com (Colin Lamb)
Date: Tue, 7 Mar 2000 13:38:31 -0800
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Years ago I designed a 10 watt termination type wattmeter.  It was =
designed around  5 x 2 watt carbon resistors and used a frequency =
compensated voltmeter reading the voltage developed across the =
resistors. =20

I was curious about how much power a 2 watt resistor could safely =
dissipate.  In a 70 degree room, I adjusted the voltage so that the =
resistor would dissipate 2 watts.  After a short time (tens of seconds), =
I measured the resistance and found it had changed substantially (an =
increase in resistance).  And, it never returned to the original value.  =
Surprised by the result, I began experimenting with these resistors and =
derated them to 1.5 watts.  Numerous resistors were tried with always =
the same results.  The value kept changing and never returned to the =
starting point.  So, I thought I would cycle them a number of times and =
ultimately the resistance would stabilize and they would be usable.  =
But, they never did stabilize.  My conclusion was that 2 watt carbon =
resistors cannot dissipate 2 watts and that when heated they will change =
value. =20

This evaluation would apply to carbon resistors used in parasitic =
suppressors.  If the carbon resistor will become hot - for any reason - =
it will change value and ultimately will fail.

I then tested Corning Glass metal film resistors (270 ohm values) and =
found that 2 watt resistors could dissipate 2 watts indefinitely - with =
no change in value.  I needed these resistors to be non-inductive only =
to 450 mhz and they satisfied my needs.  Later, I tried carbon film =
resistors in the same value but could only test them to 150 mhz.  The =
film resistors use spiral windings to set the resistance and will have =
some inductance - which varies with resistance values.  The lower values =
will have less inductance. =20

My tentative conclusion is that carbon resistors should be suspected of =
changing value in existing parasitic suppressors if they have ever been =
overheated.  But, before other types of resistors are used, they should =
be tested for inductance and frequency characteristics.

Colin  K7FM

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<DIV><FONT face=3DArial size=3D2>Years ago I designed a 10 watt =
termination type=20
wattmeter.&nbsp; It&nbsp;was designed around&nbsp; 5 x 2 watt carbon =
resistors=20
and used a&nbsp;frequency compensated voltmeter reading the voltage =
developed=20
across the resistors.&nbsp;&nbsp;</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I was curious about how much&nbsp;power =
a=20
2&nbsp;watt resistor could safely dissipate.&nbsp; In a 70 degree room,=20
I&nbsp;adjusted the voltage so that the resistor would dissipate 2 =
watts.&nbsp;=20
After&nbsp;a short time (tens of seconds), I measured</FONT>&nbsp;<FONT=20
face=3DArial size=3D2>the resistance and found it had changed =
substantially (an=20
increase in resistance).&nbsp; And, it never returned to the original=20
value.&nbsp; Surprised by the result, I began experimenting with these =
resistors=20
and derated them to 1.5 watts.&nbsp; Numerous resistors were tried with =
always=20
the same results.&nbsp; The value kept changing and never returned to =
the=20
starting point.&nbsp; So, I thought I would cycle them a number of times =
and=20
ultimately the resistance would stabilize and they would be =
usable.&nbsp; But,=20
they never did stabilize.&nbsp; My conclusion was that 2 watt carbon =
resistors=20
cannot dissipate 2 watts and that when heated they will change =
value.&nbsp;=20
</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>This evaluation would apply to carbon =
resistors=20
used in parasitic suppressors.&nbsp; If the carbon resistor will become =
hot -=20
for any reason - it will change value and ultimately will =
fail.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I then tested Corning Glass metal film =
resistors=20
(270 ohm values) and found that 2 watt resistors could dissipate 2 watts =

indefinitely - with no change in value.&nbsp; I needed these resistors =
to be=20
non-inductive only to 450 mhz and they satisfied my needs.&nbsp; Later,=20
I&nbsp;tried carbon film resistors in the same value but could only test =
them to=20
150 mhz.&nbsp; The film resistors use spiral windings to set the =
resistance and=20
will have some inductance - which varies with resistance values.&nbsp; =
The lower=20
values will have less inductance.&nbsp; </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>My tentative conclusion is that carbon =
resistors=20
should be suspected of changing value in existing parasitic suppressors =
if they=20
have ever been overheated.&nbsp; But, before other types of resistors =
are used,=20
they should be tested for inductance and frequency =
characteristics.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Colin&nbsp; =
K7FM</FONT></DIV></BODY></HTML>

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