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Will the resistance of a resistor permanently change when heated to high temperatures

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ingram010

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Hi All

The title pretty much says it all.

I have a circuit board that has a couple of resistors that at some point have got very hot, so hot that they have turned black, I want to replace them but obviously I cant identify them visually anymore, can I still rely on the resistance measurement taken across them or are they likely to have changed due to the high temperatures.

Regards to you all

John
 

There is a good chance they have changed slightly but not too much, the value will shift back and forth with temperature but once scorched they tend to stay changed to some degree. Usually, but not always, the value increases after a long overheating session so if you measure it now and go back to the nearest lower standard value you won't be far wrong. The only sure way is to check against the schematic of course.

Brian.
 

betwixt said:
The only sure way is to check against the schematic of course.
Click to expand...

Unfortunately the circuit has been modified from the original circuit so I cant rely on it. the schematic states that there is one 110 Ohm resistor, yet on the actual circuit there are two in parallel, 1500 Ohm and a 47 Ohm. the cooked one measures at 47 Ohm.
 

Data terminators are usually 50 , 60,75,93 etc or 120 ohm for coax and twisted pair or CAT5 for single ended. some are not short circuit protected if used to pullup to Vcc.

Metal,Film,resistors,usually,fail in open circuit mode.

So use two 100 Ohm in parallel and check.
 

I would think it unlikely to overheat if it was a data termination resistor. The combination of those in parallel would make 44.23 Ohms so I would guess it plays some other role in the circuit. The majority of current would pass down the 47 Ohm resistor and that's why it cooked and the others didn't. I think I would take the risk of assuming it should be 47 Ohms and just replace it with a higher power rated one to avoid overheating again.

You could try a better balanced combination of values to share the current more equally, maybe 150, 130 and 120 Ohm in parallel which makes 44.07 Ohms.

Brian.
 

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