Carbon Resistor tests.

[Ed Kotkiewicz]

Recently I acquired a bunch of 1/4 – 2 watt carbon resistors.  These
were new, never installed resistors that had resided in a normal
environment and placed in standard parts box.

Having retired a few years back as an engineer in aerospace, I have
always been familiar with the aspects of degraded carbon resistors.
They “typically” have not been used in aerospace products in well over
20 years.  The key problem being that when a resistor started to go
bad, it went towards “open” and in the process, typically caused other
components to also degrade/fail.

In this case, I set aside all of the red-red-x resistors.  I had 71
resistors.  My selection of two-watt resistors was limited to 22K’s
and the one-watt resistors were a small mix of 2.2K’s and 22K’s.  The
½ watt group made up the bulk of the total range of red-red-x
resistors.  They were all 5% tolerance so the top value should not
exceed 231xx. (2.2 + 5%).  I was using a fairly good autoranging DVM
to check the resistors.

Out of the total of 71 resistors, 43 (61%) tested “out of tolerance”!
These would still be ok for a 10% tolerance requirement, but I sure
wouldn’t use them.  The real surprise for me was that all of the
red-red-black resistors failed but all of the red-red-brown resistors
passed.  I say, surprised since I had always contended that you would
not see the resistance exceeds the limits until you got into the
higher K range of the resistor value.  Well, I learned something new.

With the price of thick film resistors being what they are today, I
would consider it prudent for the hobbyist to change over to the newer
technology resistors.  This is especially true with boat anchor
products that generate internal heat.

Just a thought.

Ed

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