testing ohms law - issue

[eleboy]

Hi, I am testing ohms law with resistors in serial connection like below:
V = 8 ( from 9 v battery )
R1 = 1k
R2 = 3k
When testing this in a bread-board, Voltage correctly divides ( ie 6V at R2 and 2V at R1 ). But the R ( which supposed to be R1+R2 = 4k ) is not verifying when the circuit is on. If I diconnect the circuit ( remove the negative end from breadboard ), i am able measure the R as 4k.
So, can't we test the resistance of a Resistor when the circuit is ON or do I miss something?

Thanks in advance!

 

[keith1200rs]

You cannot measure resistance with multimeter when is circuit is powered. Resistance is normally measured by applying a voltage and measuring current or vice versa. That is what the multimeter is trying to do. By powering the circuit you will mess that up.

Keith

 

[eleboy]

You cannot measure resistance with multimeter when is circuit is powered. Resistance is normally measured by applying a voltage and measuring current or vice versa. That is what the multimeter is trying to do. By powering the circuit you will mess that up.

Keith

Thanks. Won't it make the testing of complex circuits harder? What else components can't be tested when the circuit is powered?

 

[rfredel]

Hello eleboy,

you can't test anything in a circuit when it is powered, because you can not verify what components around affect the measuring device.

You can only test the result, like voltage with a multimeter or pulses and signals with an oscilloskop. Therefore you need details of the circuit and the knowledge of the function.

Regards

Rainer

 

[jeffrey samuel]

using a multi meter when the ckt is turned on to measure the resistance is not a fine idea cos it interferes with the internal battery operation of the ohm meter

try using some different devicesto do just that

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Thanks. Won't it make the testing of complex circuits harder? What else components can't be tested when the circuit is powered?

this is not feasible only current and voltage can be measured in on condition all the others are done in off mode only

 

[albert22]

As jeffrey said measuring with an ohm meter is a bad idea because it applies a know current and measures the voltage and displays the resulting resistance value by applying the ohms law.
Doing that in a live circuit not only interferes the circuit operation and the measuring but can produce damages to either the circuit or the multimeter itself. Modern multimeters are internally protected to avoid damage for doing that by accident. But you can ruin an analog instrument. Check the multimeter user manual to use it correctly. although is probable that the manuals may take the issue for granted.
As has been said too. You can measure voltages and currents directly by using voltage probes or current probes. Or indirectly. For example you can know the current thru a resistor by measuring the voltage and doing the calculation.
Voltmeters and oscilloscopes can interfere with the operation of sensitive circuits (high impedance) under measurement because of their internal "resistance" but thats another subject.
hope that helped

 

[jeffrey samuel]

real true To measure resistance of the circuit in live condition there are some devices i guess

i have not come across them so worth googling for it