Resistor changing value when in circuit? Possible issue?

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Rybow

Well-known member
Joined
Aug 18, 2009
Messages
782
Location
Edmonton, Alberta
Hey all,

I am troubleshooting a circuit right now, and one of the things I am doing is replacing my 1/4 watt resistors with 1/2 watt ones. Anyways, I was checking the values of my replacements and one came up different. It was supposed to be an 8.2K, and my meter was reading 5.6K. I thought I'd put the wrong one in, so I checked the colour code against another 8.2K, and it was the same. Thinking it was bad, I replaced it with one that I knew was good, and the same thing happened. The reading also took awhile to settle on the reading of 5.6K.

I measured with the probes as close to the resistor as possible. Does this point to another issue in the circuit? Should I be focusing attention on other components that are connected to this resistor?

Also my 1M resistors don't measure properly unless my meter is set to 200M range. When in 2M range, it measures 0.014. They measure fine when not in the circuit.

Thanks in advance!
 
When in a circuit, resistors can be in parallel with other resistive components (or just resistors).
Do a search for  "resistors in parallel" and you'll know what I´m talking about...
This is why you can't meassure resistors when they are in a circuit (i.e. soldered on a pcb).
Desolder 1 leg and lift it. Now meassure again.....
 
Reading you, I understand that you are measuring these resistors in circuit.
Any residual DC voltage will impair the measurement (sometimes you may read negative resistance values!). You need to fully discharge all decoupling caps (you must short them with a low value resistor for about a minute).
Also electrolytic caps, even fully discharged interfere with resistance measurement. You must do the measurement long enough to obtain a stable reading.
And then, all semiconductor junctions interefere with measurements.
If you want to take any error out of the process, you must unsolder one side of the res.
 
Thanks for the replies!

I wasn't sure because my other resistors were measuring fine. I just wanted to double check that other components could still interact with the resistor even with no power going into the circuit.

Thanks again!

EDIT: I realize now the above statement is a dumb one. What I meant is that I can't see anything in line this resistor that would cause the resistance drop, but I'll keep looking.
 

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