Measuring Resistance in Circuit - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 26, 2019, 06:46:57 pm
content: 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 ?

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2019, 07:02:04 pm
content: Easy: Capacitor.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 26, 2019, 07:12:06 pm
content: 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?

author: Nerull
author_rank: Frequent Contributor
timestamp: September 26, 2019, 07:43:48 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2019, 07:45:37 pm
content: 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.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 27, 2019, 07:17:51 am
content: Quote from: Zero999 on September 26, 2019, 07:45:37 pmExcellent I didn't know this. ThanksRegards

author: fourfathom
author_rank: Super Contributor
timestamp: September 27, 2019, 04:04:41 pm
content: Quote from: sdancer75 on September 26, 2019, 07:12:06 pmIn this circuit the capacitor will still affect the resistance measurement because the battery is a very low-impedance element in the circuit. Replace the battery with a short-circuit or very low-value resistor and you can see how the capacitor still has to charge up before the resistance measurement becomes stable.

author: MosherIV
author_rank: Super Contributor
timestamp: September 27, 2019, 08:02:42 pm
content: Semi conductors such as transistors, op-amps, logic gates, etc can also affect your measurements. If the voltage of the CC used to do resistance measurement exceeds 0.7V (or 0.3 for germainium) will turn on semi-conductors and affect the results.Some ESR meters claim to use 0.3V as the test voltage and claim to be able to do in circuit measurement.Real circuits are generally too complicated to be able to do in circuit resistance or capacitance measurement. There are just too many other connected components that affect the result!