Design a Series Multi-Range Voltmeter: 0-3V, 0-9V, 0-30V

[wanzstar]

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..

 

[vk6kro]

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.

 

[wanzstar]

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]

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.

 

[alphy]

In order to design a series multi-range voltmeter with the given voltage ranges, it is important to determine the value of the full scale deflection current (I fsd) and the internal resistance (R m). These values are crucial in calculating the appropriate multiplier resistors for each range. Without knowing the values of I fsd and R m, it is not possible to design an accurate and functional voltmeter.

To determine the value of I fsd, the sensitivity of the voltmeter must be known. This sensitivity is defined as the amount of current required to produce a full scale deflection on the voltmeter. It is typically expressed as a fraction, where 1/I fsd is the sensitivity. Once the sensitivity is known, the value of I fsd can be calculated by dividing 1 by the sensitivity.

Next, the internal resistance (R m) must be determined. This is the resistance of the voltmeter itself, which is usually in the range of 10-100 ohms. It can be measured using a multimeter or it can be estimated based on the type of voltmeter being used.

Once the values of I fsd and R m are known, the multiplier resistors can be calculated using the formula R = Sensitivity X Range - Rm. The sensitivity and range will vary for each voltage range, so the appropriate values must be used for each calculation. These multiplier resistors will ensure that the voltmeter is accurate and provides the correct readings for each range.

In conclusion, designing a series multi-range voltmeter with the given voltage ranges requires the determination of the full scale deflection current and internal resistance. These values are crucial in calculating the appropriate multiplier resistors for each range and ensuring the accuracy of the voltmeter.