Author Topic: Measuring series circuits with multimeter  (Read 754 times)

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Offline stolevoleTopic starter

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Measuring series circuits with multimeter
« on: April 27, 2019, 02:27:02 am »
So this might seem like ultra newbie question but i really dont get it. If i connect multimeter in series with potentiometer for example and close circuit with multimeter and then turn potentiometer would i get right reading on multimeter? I know voltage in series is proportional to resistance of each resistor but it confuses me why i always get reading of source voltage when i connect multimeter this way(i tried with ac and light bulb dimming potentiometer).Is it different for dc? Shouldnt multimeter get low voltage reading since it has low resistance? I would very appreciate any help. :-//
 

Offline cs.dk

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Re: Measuring series circuits with multimeter
« Reply #1 on: April 27, 2019, 08:36:49 am »
If im understanding you correct, you are connecting your meters input impedance ~10 meg ohm in series with your circuit.

You can only use your multimeter in series when measuring current. Voltage are measured parallel.
 
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Online iMo

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Re: Measuring series circuits with multimeter
« Reply #2 on: April 27, 2019, 09:23:17 am »
If your potentiometer (wired as a rheostat) is, say 1kohm, and it is wired in series with the internal resistance of the DMM (say 10Mohm = 10000k), and with series with a 10V source for example, the voltages you read for both end positions of the wiper will be:

V1 = 10V * (10000k/(1k+10000k)) =  9.999V
V2 = 10V * (10000k/(0k+10000k)) = 10.000V

With a 100kohm potentiometer:

V1 = 10V * (10000k/(100k+10000k)) =  9.901V
V2 = 10V * (10000k/(    0k+10000k)) = 10.000V

With a 10Mohm potentiometer:

V1 = 10V * (10000k/(10000k+10000k)) =  5.000V
V2 = 10V * (10000k/(       0k+10000k)) = 10.000V
« Last Edit: April 27, 2019, 09:46:28 am by imo »
Readers discretion is advised..
 
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Offline stolevoleTopic starter

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Re: Measuring series circuits with multimeter
« Reply #3 on: April 27, 2019, 05:05:09 pm »
So DMM actualy has high resistance and thats why it gets all the voltage. :-+ But why potentiometer gives voltage even when its off is that because there is no perfect insulator? Thank you for clearing this up. :)
 

Offline Nerull

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Re: Measuring series circuits with multimeter
« Reply #4 on: April 27, 2019, 06:52:13 pm »
Typical potentiometers don't turn 'off', their resistance between the wiper and one of the terminals varies between the rated resistance of the pot (10k, for example) and zero (or as close to it as the materials allow).

In the circuit posted by imo above, the pot would have a maximum resistance of 1k when turned in one direction, and would approach a dead short in the other direction. It has no 'off' position.
 
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