# Strange resistor behaviour



## UlyssesBlue (Apr 19, 2011)

I'm noticing some strange behaviour with a resistor, which I was hoping someone could explain. A google search hasn't uncovered anything, but that might just be my choice of search terms.

This resistor is meant to be 470k-ohms. This is what the coloured bands claim it to be, and this is what the documentation on the electronics say it should be. Yet when I put a multimeter across it, it says 130k-ohms.

This resistor forms part of a resistor chain which distributes voltage between various components.

I found an identical resistor that appears unused, i.e. was never built into this electronic device or put under the same operating conditions, and was probably part of the same batch. It reads the correct amount.

Is it possible for a resistor to change resistance through wear and tear or similar? What could have possibly occurred in this situation to cause me to be getting these measurements?


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## octaneman (May 13, 2009)

Hi UlyssesBlue



All components break down over time that change their values but its also important to understand two things: 1) Resistor values are not made with every value that is why they are rated with a tolerance in percentage. 2) How the resistor chain in your circuit is setup, is it series, parallel, series parallel etc. Remember that resistors are used to limit the current within a circuit.



Note: Is the color band of your resistor yellow, violet, yellow, gold ?


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## gcavan (Aug 13, 2009)

Components will go out of spec with age. Usually resistors will either short or open, but not always. If its out by more than a reasonable amount, replace it.

That said, I have to ask: Are you measuring in-circuit or with one leg lifted?


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## UlyssesBlue (Apr 19, 2011)

The bands on the resistor are:
yellow - violet - black - orange - brown - red
This translates as: 4 - 7 - 0 - x1000 - 1% - 50ppm
i.e. 470k-ohms, with a tolerance of 1% and a temperature coefficient of 50ppm per Kelvin.

I remeasured today and got different values. This was done "in circuit" i.e. the circuit was not broken to take these measurements, although no power was supplied.

For comparison I took measurements on an identical resistor chain used on a different device. All measurements were within error of the corresponding measurements on my first resistor chain, meaning the same discrepancies were noticed on both.

This suggests the issue I'm seeing is not wear and tear, but something else. Could anyone explain what could be going on here?


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## gcavan (Aug 13, 2009)

Taking your reading with the resistor in circuit, you are measuring the impedance of the surrounding circuit as if it were in parallel with the resistor. The only way to accurately measure the resistance of a single resistor is to take out of the circuit first.

Think about the total resistance of two resistors in parallel. It will always be less than the value of the lowest resistor.

Prove this to yourself by taking any two resistors. Confirm their values individually, then tie their ends together and measure. If you use two resistors of equal value, the total will be 1/2 of the value of one of them. Use two of vastly different value, (say 1k + 100k) the total will be just less than the lower value.


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## octaneman (May 13, 2009)

*Re: Strange resistor behavior*

gcavan is right , if your 470k/ohm resistor is in parallel with 130k/ohm resistor, its total value will be less than the lowest value resistor which in this case is 130k/ohms. If you do the calculation on paper with the 1% tolerance your answer will come out to 101k/ohms in a perfect world. Here's the kicker, when you test the resistor in circuit there is also the discrepancy of the margin of error on the meter set by the manufacturer, when you factor it in the tolerance of the resistor with the meter's pre-set margins the meter will come to the closest value of the resistor being tested.


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