# Basic electric theory #3

#### xbox112005

Joined Oct 30, 2011
9
Please help me with this question.You are an electrician on the job.You have been given a multimeter that has the following ac voltage ranges:30,60 and 150.The meter state that it has a resistance of 5000ohm/v.You need to be able to measure a voltage of 277 volts.How much resistace should be inserted in series with the meter to make the 30-volt range indicate a full scale value of 300 volts?

#### Georacer

Joined Nov 25, 2009
5,182
Voltmeters traditionally measure voltage through the effect of current over a predefined resistor.

For example, in the 30V scale, the internal resistance of 30*5kΩ would have a maximum current of I=V/R=30/(30*5k) excite its needle to the max of its scale.

Now, for your case, you need to increase the scale by 300/30. The maximum current allowed to set the scale to the max is I'=Vmax/R'.
We know that I=I', as the indicator is the same. We also know Vmax, it is 300V. But this time R' is equal to R'=Rinternal+Rseries.

Can you calculate Rseries now?

#### crutschow

Joined Mar 14, 2008
30,755
Voltmeters traditionally measure voltage through the effect of current over a predefined resistor.
...........
This is really a nit, but it's the effect of current through a resistor, not over it.

A meter that's rated in ohms/volt is normally a mechanical meter movement which has a very low intrinsic resistance and is basically a current operated device. Thus the resistor in series with the movement determines its sensitivity. If it has a sensitivity of 5000Ω/V, it's a simple calculation to determine the total resistance you need for 300V full scale.

#### Georacer

Joined Nov 25, 2009
5,182
This is really a nit, but it's the effect of current through a resistor, not over it.

A meter that's rated in ohms/volt is normally a mechanical meter movement which has a very low intrinsic resistance and is basically a current operated device. Thus the resistor in series with the movement determines its sensitivity. If it has a sensitivity of 5000Ω/V, it's a simple calculation to determine the total resistance you need for 300V full scale.
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