Measure resistance of resistor in cirucit

Question

How do I measure accurately with a multimeter the resistance of a resistor that is already soldered as part of a circuit?

In a simple transistor as a switch circuit, I noticed that when I tried to measure the collector resistor I got a value of about 170KOhms, but I knew that was incorrect, so I took the resistor out and measured again and I got the correct value, so that got me curious, how can I measure the resistance of a resistor that is already in a circuit and know for sure that it is the resistor I am measuring and not the rest of the circuit.

Answer 1

With a multimeter and no allowances for the rest of the circuit, you can't tell the difference between the resistor of interest and the effect of parallel resistances and conduction paths.

So, the answer is: you can't take a reliable measurement without knowing what other components could be spoiling it.

Answer 2

If you fully understand and can analyze the rest of the circuit (and know for a fact that it is as it should be with no shorts or damaged parts) sometimes you can infer or calculate, or simply read the value of a part in-circuit, but in general it's not possible.

That does not mean it's not useful. For example, if something reads higher resistance than expected then something is wrong (assuming no charged capacitors or power sources on the board- if resistance reads negative then it's usually something producing voltage and might damage your meter). ESR (equivalent series resistance) can usually be read accurately enough on electrolytic capacitors in-circuit, which is a major troubleshooting advantage.

In the case of (say) a 300K resistor, if you read 650K then something might be wrong since external paths can only lower the resistance.

Often (because of the way they are connected) you can check diodes and other semiconductors in-circuit well enough to detect bad parts. Not always.

Even if you have not figured out much about the circuit, sometimes comparing a faulty unit to a known-good unit will turn up something of interest to help pinpoint the problem.

Answer 3

If you still want to measure resistances without desoldering, you can do at least the following:

• Take two measurements in the straight and reverse directions of the ohmmeter and take the higher value of the resistance (to eliminate the influence of shunt p-n junctions)

• Wait a while (to eliminate the influence of shunt capacitances)