Resistors dropping in value after first use

[RogueRose]

I installed 2 new resistors (carbon film 1/2watt +-5%, 470 & 1K ohm - measured 462 & 970 before powering on). These are in a power supply and determine output voltage. After powering on and turning off, the values are measuring 440 and 895-900. This happened on two identical PSU's with two sets of resistors. these are new resistors I have never used before so IDF there is something wrong with them or if this may be normal. Does anyone have any ideas as to what may be going on here and if there may be a problem with these resistors? Thanks for any insight on this matter!

 

[FvM]

We have to presume correct measurement.

This may in happen case of considerable (e.g. pulse) overload. Can you show how the resistors are used in the respective circuit?

 

[betwixt]

I agree with FvM and also point out that less than 1% difference could be due to temperature or even humidty differences both in the resistors and the test meter. You need to ensure the measurment conditions are identical before worrying about minor changes. The quoted tolerance is in initial value not variation because of the environment. It is quite possible that the resistors 'dried out' slightly after warming up after first use.

Brian.

 

[Vbase]

Some resistors change value after soldering.
Make sure the resistors are in room temperature and don't touch the probes while measuring.