[AMPS] Stability of Carbon Resistors
Colin Lamb
k7fm@teleport.com
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. It was designed around 5 x 2 watt carbon =
resistors=20
and used a frequency compensated voltmeter reading the voltage =
developed=20
across the resistors. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I was curious about how much power =
a=20
2 watt resistor could safely dissipate. In a 70 degree room,=20
I adjusted the voltage so that the resistor would dissipate 2 =
watts. =20
After a short time (tens of seconds), I measured</FONT> <FONT=20
face=3DArial size=3D2>the resistance and found it had changed =
substantially (an=20
increase in resistance). And, it never returned to the original=20
value. Surprised by the result, I began experimenting with these =
resistors=20
and derated them to 1.5 watts. Numerous resistors were tried with =
always=20
the same results. The value kept changing and never returned to =
the=20
starting point. So, I thought I would cycle them a number of times =
and=20
ultimately the resistance would stabilize and they would be =
usable. But,=20
they never did stabilize. My conclusion was that 2 watt carbon =
resistors=20
cannot dissipate 2 watts and that when heated they will change =
value. =20
</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>This evaluation would apply to carbon =
resistors=20
used in parasitic suppressors. If the carbon resistor will become =
hot -=20
for any reason - it will change value and ultimately will =
fail.</FONT></DIV>
<DIV> </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. I needed these resistors =
to be=20
non-inductive only to 450 mhz and they satisfied my needs. Later,=20
I tried carbon film resistors in the same value but could only test =
them to=20
150 mhz. The film resistors use spiral windings to set the =
resistance and=20
will have some inductance - which varies with resistance values. =
The lower=20
values will have less inductance. </FONT></DIV>
<DIV> </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. But, before other types of resistors =
are used,=20
they should be tested for inductance and frequency =
characteristics.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Colin =
K7FM</FONT></DIV></BODY></HTML>
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