Resistor testing

[zooterman]

How do you personally test a resistor in circuit, I have been looking at ways people do it (testing that is) , check with a resistance meter, done that and half the resistors on the PCB fail, but raising a leg and testing again and they pass, so how do you do it.

[G8HQP Dave]

Two ways:
- raise a leg and check out of circuit
- calculate what the in-circuit result should be and compare with measurement

If a circuit has some life in it then DC voltage checks will usually tell you what is going on. Then you only need to check a few resistors.

[JHGibson]

If you measure a resistor in circuit in an unpowered circuit and the value is way too high then that resistor is suspect. If you get a value that is too low, then you can draw no conclusions because there may be external components in parallel.
John.

[ukcol]

Any faulty resistor will usually come under suspicion during normal fault finding in the circuit with a meter.

The only reliable way to check the resistance of a resistor is with an ohm-meter out of circuit. As has been said, it is sufficient to just lift one leg of the resistor for out of circuit measurement.

Parallel components (as has been mentioned) can cause incorrect readings when measuring in circuit, but as well as this problem, unless the equipment has been powered down for a long period, there will still be charge left in the power supply capacitors causing voltages that will make any measurement inaccurate.

An ohm-meter will not identify a resistor that is noisy or one that changes value with temperature. In these circumstances testing by substitution is appropriate.

Colin M

[Station X]

Almost all resistors in valve radio sets can be tested in circuit. They may be shunted by leaky capacitors, but unless they're very leaky the voltage from a DMM won't be enough to make them conduct.

With transistor sets it's a different story as almost all resistors are shunted.

[Radio Wrangler]

Some fancy meters on their ohms range apply low voltages, less than 0.6V so they don't turn on silicon junctions. They can be quite useful for checking resistors. Another trick is to try the probes each way round, but you still come across plenty of cases where lifting a leg is needed. The probe reversal trick makes it possible to guess electrolytic values as well.

David

[GMB]

The third way to do it is to use an in-situ bridge such as the Marconi TF2701.

This can measure a component (R,C or L) with stuff connected to it!

Obviously there are limits as to what it can do but it does often save having to disconnect things.

[vinrads]

I always lift one leg this is the sure fire way of testing ,you could have things in parallel ,also they may be a small voltage across the resistor giving you a false reading .

[Skywave]

Originally Posted by vinrads

I always lift one leg this is the surefire way of testing: you could have things in parallel. Also they may be a small voltage across the resistor, giving you a false reading.

Agreed: I do the same. Sometimes I measure with an AVO 8 and then again with a Fluke 77 (or vice-versa)!

Al.