Resistance measurement using multimeter

Discussion in 'General Electronics Chat' started by vinodquilon, Mar 4, 2013.

Dec 24, 2009
234
0
Consider the below circuit with 28V supply and 5A load.

What voltage would I expect when I connect a multimeter between A & B ?
V(AB1)= V(R6)+V(R8)+V(R9)

What voltage would I expect when I connect a multimeter between C & D ?
V(CD1)= V(R7)+V(R8)+V(R10)

Consider the second circuit.

What voltage would I expect when I connect a multimeter between A & B ?
V(AB2)= V(R6)+V(R8)+V(R9)

What voltage would I expect when I connect a multimeter between C & D ?
V(CD2)= V(R8)

Am I right ? And V(AB2)>V(AB1)

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Last edited: Mar 4, 2013
2. wmodavis Well-Known Member

Oct 23, 2010
737
150
First off you will not get valid results if you connect an ohm meter to a circuit that has live power applied from any other source than the meter itself. It likely will still provide some meter reading but totally invalid as far as anything meaningful and it might damage your meter.

You are not measuring resistance using an ohm meter but rather measuring voltage drop so your topic title is quite misleading. I'll let others deal with specific answers to your misleading topic.

Dec 24, 2009
234
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The aim is to calculate voltage not resistance.

Dec 24, 2009
234
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Can I use four wire voltage measurement to avoid this error ?

5. thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Is this a homework assignment?

The voltage read across the resistors will be a result of the value of resistors. You can use mesh or node analysis to find the symbolic answer.

Some multimeters can measure low resistance values (such as a short) while there is some voltage/current the circuit, but I wouldn't suggest that method.

Kelvin (4 wire) measurement is to isolate the leads and connectors from the measurement itself, not to cancel out existing currents.

The way to calculate the resistance would be to measure each with the circuit turned off. Then doing the node analysis to find all, or measure between the two points when there is no power.

The accepted method to measure an unknown resistance while current is flowing is to insert a very accurate ammeter in series with the unknown resistance. Measuring voltage voltage drop across the unknown resistance when a known current is passing through the resistor allows the value of resistance to be calculated. This method is used for determining specifications such as RDSON in MOSFETs.

Dec 24, 2009
234
0
Not resistance but voltage drop measurement is required.

Last edited: Mar 4, 2013
7. thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The extra resistors leading to the measurement point (R1, R12) aren't carrying any current except when a voltage measurement is being made. The voltage shown will be proportional to R8.

Assuming all resistors are the same value, R3, R16 will change the measured value at C-D to a different value than A-B due to another resistor in series with others in the current path.