Resistor value dropping after first use

Thread Starter

RogueRose

Joined Oct 10, 2014
375
I have power supply that has selectable voltage with resistors (470 & 1K ohm 1/2w carbon film +-5%). The resistors measures 462 and 970 when they were installed and as soon as I turned on the unit and measured the values again (when it was off) the resistor values were 440 and 900. This happened on 2 different identical supplies with two sets of resistors. This is the first time I have used these resistors so I don't know if that is normal or not.

Can anyone shed some light on why this may be happening and whether I may have bad resistors or what?
 

Alec_t

Joined Sep 17, 2013
11,296
You can't measure resistor values accurately when they are installed in a circuit, because the (possibly unknown) resistance of other connected components will affect the measurement. The lower measured values after power had been applied and removed may have been due to the effects of residual voltage on a capacitor.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
You can't measure resistor values accurately when they are installed in a circuit, because the (possibly unknown) resistance of other connected components will affect the measurement. The lower measured values after power had been applied and removed may have been due to the effects of residual voltage on a capacitor.
Thanks for the info. I measured before installing butnotafter installing (before powering on). I took them off and they were the correct values. Thanks again!
 
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