Electronics > Beginners

Measuring Resistance in Circuit

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sdancer75:
Hi,

Trying to measure an smd resistor in Circuit, I get a non constant value. Instead, the measurement  increases trying to reach the nominal value.

Out of the circuit it's ok.

So, the question is which components in the circuit cause that behavior to the resistor ?

soldar:
Easy: Capacitor.

sdancer75:
Yes of course but it makes me wonder because I test the component directly at the both ends, how can a capacitor influence the measurements in this case?


For example, at the attached schematic I will get a different value for my resistor if I measure it on the circuit?

Nerull:
If everything is in series you shouldn't see a difference, but few real life circuits are that simple.

Put your capacitor in parallel with the resistor and see what happens.

Zero999:
Assuming no the capacitor is at 0V, you'll read the resistor value, when the resistance meter is initially connected to the circuit. After awhile, the capacitor will charge up and the reading will increase towards open circuit, or whatever the effective parallel resistance is. Unfortunately, it takes time to take a resistance reading, so you'll always read a slightly higher value, than expected.

Hint: very low voltages in the circuit can interfere with resistance measurements, because the meter injects a small current, measures the voltage and applies Ohm's law. This effect can be cancelled by noting the resistance value, reversing the meter's probes, recording the value again and take the average of the two readings, which will be close to the actual resistance value.

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