Conversion of Anolog Voltmeter into Ammeter

[caspian]

Dear Members,

I have an old analog voltmeter that can correctly measure DC voltage between 0v to 3v. The voltmeter is made by Toyokeiki depicted in the photos. It does not need any battery. It works using a coil and a magnet as any other analog voltmeters.
I am going to convert this voltmeter into a DC ammeter that can measure DC current between 0A to 15A.
To do this, I use a 0.2 Ohm resistor for current sensing in the circuit. Then, I use the setup depicted in the photo to measure current.
For example, if the power be 12v and the load be 8 ohms, the voltmeter shows 0.3v that indicates I = V/R = 0.3/0.2 = 1.5Amps of current.
This measurement is correct. The voltmeter measures current correctly. But It has a problem.
The voltmeter works only if I touch one pin of it by my finger. Otherwise, It shows 0v and cannot show the real voltage drop across the current sensing resistor. It seams there is some electricity that needs to get out of the voltmeter.

Please help me solve this problem so that the voltmeter can work fine without requiring to touch its pins.

Thanks

[sam_sam_sam]

I am not sure if the meter coil needs to be setup for current because there meters that are setup this way I can not tell you what the difference between them is

[caspian]

The internal resistance of the voltmeter is about 3.5KOhms.
I did not modified the internal components of the voltmeter.
Is my simple test setup correct?
Do I have to use a more complicated test setup to make the voltmeter work fine in current measurement?

[sam_sam_sam]

I think should try it with out the resistor 3.5 k ohms and see what happens but only do 100 milliamperes or less and see if you get meter movement that is close to what you think it should be with out touching the circuit in any way

[R_J]

That is battery tester and likely has an internal load resistor to load the battery to get a proper reading when testing 1.5v batteries. It likely won't move with .3v
With your current setup 12v, 8Ω load, the shunt resistor would need to be 2Ω to get full scale deflection (3v) or 1Ω to get half scale. with only .3v it won't deflect, touching it with your finger adds a bit more voltage so the meter moves.

[redwire]

Your test setup looks good, I think there is a poor connection or maybe a switch is inside the meter so you must press it down to take a reading.
Usually panel meter specs are two things - the coil resistance and the current required for it to read full scale. It's a very nice panel meter. A resistor gets added probably inside the meter housing, to calibrate and give 3V full scale.

With 3.5k ohms and 3V is about 0.85mA for full scale. This is a bit odd, usually panel meters are 1mA, 250uA, 300uA etc full scale. edit: it's apparently a battery tester, so extra load resistor is inside as well.

If you wanted to use a much lower shunt resistor than 0.2R (because of the losses and heat in the shunt) you'd have to take the meter apart and post pics of the circuit inside. You could probably go down to 0.05R or 0.1R for the shunt.
I think there is an internal resistor you could eliminate to get higher sensitivity, if you wanted. It could be a 0.3V full scale meter with extra resistor to make it 3V, does that make sense?

[caspian]

Thanks for all the answers, @sam_sam_sam, @R_J, and @redwire.
All the connections of the components are tested in the circuit and none of them is poor.
I do not know if any resistor is inside the meter. I just measured the resistance of the analog voltmeter using a multimeter and it showed 3.5KOhms.
I will disassemble the meter to see what is inside.
If I use a lower shunt resistor (such as 0.05 Ohm), then the voltage drop across that shunt resistor will become too low (such as 0.075v), so that the meter will not move enough to show it.

[caspian]

I disassembled the analog voltmeter and depicted its schematic in the picture. You were right. There is a resistor inside the meter.
I used a 2 Ohm resistor as the shunt in my test setup circuit. But I discovered that the meter still needs to be touched to work.
In another test circuit where I used the meter as a voltmeter to measure voltage across an 8 Ohm resistor, then meter worked great and it measured low voltages such as 0.1v and 0.3v properly.
So I guess the problem is not due to the low voltage. I guess the meter has that problem only when the resistance of the shunt resistor is very low such as 2 ohms or 0.2 ohms.

[caspian]

I do not want to remove the 3kohm resistor inside the meter. because then it becomes too sensitive and can measures only low currents.
Is there any solution to use this nice analog voltmeter as an ammeter for measuring currents 0A - 15A?

[redwire]

"the meter still needs to be touched to work".
Find the bad connection, then worry about the design issues.

[R_J]

You would need to remove the 3k in series and then use a shunt across the movement.

As the meter is now, 3 volts gives FSD, with a resistance of 3500Ω the current through the movement is about 0.857 Milliamps or 857 microamps
Without the 3K resistor and just the (500Ω) movement , a rough guess for the meter to measure 15 amps at FSD would be a shunt value of ABOUT 0.0286Ω (I could be wrong)

[caspian]

After opening the frame of the meter and closing it, I tested the meter again using my test setup circuit depicted in Post#1. Then, I found out that the meter works fine with the 0.2 shunt resistor without requiring to touch its pins. So I guess the problem is solved. However, I do not know what caused this problem. Therefore, the conversion circuit of Post#1 is correct.
Note: I did not removed the internal 3kohm resistor of the meter.
Thanks for all the answers.