Help - Simple Ammeter and Voltmeter using VU-meter

Thread Starter

Anggi Tri Prasetyo

Joined Mar 17, 2017
4
I was assigned by my teacher to make simple ammeters and volt meter using VU-Meter. but I am confused, what should I do to make accurate ammeters and voltmeters. Rvumeter = + -800ohm, Imaxvumeter = -0.5ampere and Vmaxvumeter = +-0,5volt. eg 10V maximum limit measure. Thank You.
 

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Alec_t

Joined Sep 17, 2013
14,314
Are you aware that the VU-meter has a logarithmic scale?
Are you expected to use opamps or a micro processor as part of your solution?
 

Thread Starter

Anggi Tri Prasetyo

Joined Mar 17, 2017
4
Show how you had the meter connected and describe what didn't work as expected.
I apologize in advance could not give a series of pictures while we're doing a practicum. I have tried following the instructions of my teachers in theory, but to measure the flow of a bigger scale such as 5 amperes was not possible because R shunt in theory be small. if there is less precise than the picture my circuit in order to measure the current 5ampere.
when we do a practicum corresponding circuit images. electric current into the circuit has been set, namely 0,5ma. but the scale on vu meter does not show the maximum scale. how do I solve the problem.
thank you
 

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Thread Starter

Anggi Tri Prasetyo

Joined Mar 17, 2017
4
Are you aware that the VU-meter has a logarithmic scale?
Are you expected to use opamps or a micro processor as part of your solution?
yes I am aware that vu meter has a scale. so far I have not tried opamps or microprocessor. because my teacher did not explain in detail what are the components that should we use. at the practicum we only using Vu meter, power supply, resistor. is there not a critical component that we use in the manufacture of ammeters and voltmeters?
 

MrAl

Joined Jun 17, 2014
11,480
Hi,

Yes sometimes the shunt resistor value will be small, there's no way around that.
If the meter reads 1ma and you need to measure 1 amp, then you should realize that is 1000 to 1 so you need to shunt 99.9 percent of the current through the shunt resistor, and that means it will be of a small value.

Current shunts in use today are common and are rated in terms of the current vs the measured voltage, for example 50mv at 50 amps. If you do the math, you can see right away that the resistance must be 1mOhm which is 0.001 Ohms which is very small. That's not the end of it either, you can get shunts that are 50mv at 100 amps, 50mv at 200 amps for example.

For a quick rough estimate of the resistance with a regular analog panel meter, if the meter is 1ma and has 1000 ohms internal resistance and you want to measure 10 amps full scale, the external resistor would be in the neighborhood of 0.1 Ohms so that when there is (roughly) 10 amps flowing in the circuit there is 1ma flowing through the meter. That's only approximate but shows you how small the shunt resistance can get. There is sometimes a way to adjust it too so you can calibrate the final meter with another meter known to be accurate enough.
The power dissipation in the external resistor has to be checked too. 0.1 ohms with 10 amps is 10 watts.
 
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