Author Topic: Measuring Resistance in Circuit  (Read 1871 times)

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Offline sdancer75Topic starter

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Measuring Resistance in Circuit
« on: September 26, 2019, 06:46:57 pm »
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 ?

« Last Edit: September 26, 2019, 07:12:35 pm by sdancer75 »
 

Offline soldar

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Re: Measuring a Resistance in Circuit
« Reply #1 on: September 26, 2019, 07:02:04 pm »
Easy: Capacitor.
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Offline sdancer75Topic starter

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Re: Measuring a Resistance in Circuit
« Reply #2 on: September 26, 2019, 07:12:06 pm »
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?
« Last Edit: September 26, 2019, 07:17:08 pm by sdancer75 »
 

Offline Nerull

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Re: Measuring Resistance in Circuit
« Reply #3 on: September 26, 2019, 07:43:48 pm »
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.
 
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Offline Zero999

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Re: Measuring Resistance in Circuit
« Reply #4 on: September 26, 2019, 07:45:37 pm »
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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Offline sdancer75Topic starter

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Re: Measuring Resistance in Circuit
« Reply #5 on: September 27, 2019, 07:17:51 am »

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.

Excellent I didn't know this. Thanks

Regards
 

Online fourfathom

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Re: Measuring a Resistance in Circuit
« Reply #6 on: September 27, 2019, 04:04:41 pm »
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?

In 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.
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Offline MosherIV

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Re: Measuring Resistance in Circuit
« Reply #7 on: September 27, 2019, 08:02:42 pm »
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.  :palm:

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!
 
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