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

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Discussion Overview

The discussion centers on the design of a series multi-range voltmeter with specified voltage ranges of 0-3V, 0-9V, and 0-30V. Participants explore the necessary parameters, such as full scale deflection current (I fsd) and internal resistance (R m), which are not provided, and how to approach the design without these values.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes the need for I fsd and R m to design the voltmeter and suggests using algebraic forms for these variables.
  • Another participant proposes assuming reasonable values for I fsd (50 microamps) and R m (500 ohms) to facilitate calculations.
  • A different participant questions the use of simulation software (Multisim) for determining these values, implying it may not be a valid approach.
  • One participant provides a method for calculating the total resistance needed for each voltage range using Ohm's Law and suggests a systematic approach to determine the series resistors for each range.
  • There is mention of a useful guide for calculating resistances based on the required current and voltage, indicating a relationship between voltage and resistance values.

Areas of Agreement / Disagreement

Participants express differing views on how to proceed without the specified parameters. Some suggest assuming values, while others emphasize the importance of obtaining accurate data or using calculations based on Ohm's Law. The discussion remains unresolved regarding the best approach to take.

Contextual Notes

Participants acknowledge the absence of specific values for I fsd and R m, which are critical for the design process. There is also a reliance on assumptions and calculations that may not be universally applicable without further clarification of the parameters involved.

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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..
 
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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?
 
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.
 

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