# Multi Range Voltmeter

how to design a series multi-range voltmeter with its voltage range is given but its value of full scale deflection current (I fsd) and internal resistance (R m) is not given?

the voltage range given is 0-3V, 0-9V and 0-30V.

since to find the value of every single multiplier resistor is: R = Sensitivity X Range - Rm
Sensitivity, S = 1/I fsd
but the value of Rm and I fsd is not given..

## Answers and Replies

vk6kro
Science Advisor
You need to know those things to design a voltmeter.

So, you can:

1) Go back to your teacher and ask for more data if this is an assignment.

2) use them in algebraic form like Ifsd and Rint.

3) assume some reasonable values and use those.
Maybe 50 microamps for the FSD and 500 ohms for the internal resistance.
Just explain what you are doing.

thanks vk6kro..

maybe I'll try to assume those values as you advised..
hmm..how about if I figure out those values by using Multisim?

vk6kro
Science Advisor
Wouldn't that be cheating?

With a calculator, work out the total resistance needed to get 50 microamps to flow with 3 volts applied. (that is Ohms Law.). 3 volts divided by 0.00005 amps is ? ohms.

Then subtract the internal 500 ohm resistance.

Then work out the resistance for 9 volts and subtract the internal resistance and the 3 volts resistor. Put this new resistor in series with the 3 volt one.

Do the same with the 30 volt range.

You now have a meter with three resistors in series with it and you can select the range with a switch.

There is a useful guide you can use. 3 volts giving 50 uA requires a 60000 ohm resistor. That is 20000 ohms per volt.
So, a 50 volt range would require a 50 times 20K resistor or 1 megohm. Very handy to work these resistors out with mental arithmetic.