Converting voltmeter to ammeter.

Thread Starter

rahul411

Joined Feb 19, 2018
244
I have an old voltmeter from a voltage stabilizer, that can measure AC/DC voltage from 0 to 300 volts.
Is it possible to convert it to measure current of micro-amps range? Specifically 0 to 50uA for esr meter. Link is below
https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/
Although I don't want it to be precise AF but atleast it can measure something in microamps range, thats it.

Is the conversion possible?
 

Alec_t

Joined Sep 17, 2013
11,150
Depends on the meter type. Some old-school moving-coil voltmeters consisted of a 50uA full-scale-deflection microammeter with a high value resistor in series. The scale would typically be marked "20kΩ/V". A meter of that type could be converted by simply removing the resistor. I doubt if any other type could be easily or economically converted.
 

KeithWalker

Joined Jul 10, 2017
813
I have an old voltmeter from a voltage stabilizer, that can measure AC/DC voltage from 0 to 300 volts.
Is it possible to convert it to measure current of micro-amps range? Specifically 0 to 50uA for esr meter. Link is below
https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/
Although I don't want it to be precise AF but atleast it can measure something in microamps range, thats it.

Is the conversion possible?
I doubt whether it would be sensitive enough to measure microamps. Is it a moving coil meter?
 

Thread Starter

rahul411

Joined Feb 19, 2018
244
Depends on the meter type. Some old-school moving-coil voltmeters consisted of a 50uA full-scale-deflection microammeter with a high value resistor in series. The scale would typically be marked "20kΩ/V". A meter of that type could be converted by simply removing the resistor. I doubt if any other type could be easily or economically converted.
There isn't any marking on it but has a resistor of 6.8kohm in series with the coil.
I read about the conversion stuff but they are only making low amp rating ammeter to higher not the other way around. And when i put my values in their derived formula i get negative shunt resistor value. What's happening here?
 

SLK001

Joined Nov 29, 2011
1,543
Measure the resistance of the coil, then add the 6.8k to that value. You have the full scale value of the meter. With these values, you can calculate the current required for a full scale deflection.
 

Thread Starter

rahul411

Joined Feb 19, 2018
244
Measure the resistance of the coil, then add the 6.8k to that value. You have the full scale value of the meter. With these values, you can calculate the current required for a full scale deflection.
At 12 volts the current calculated is 1.5 mA
Is this right?
 

BobTPH

Joined Jun 5, 2013
2,349
Are you sure that is the value of the resistor? That would make the full scale deflecton 37.5 ma and it would be dissipating 11W at 300V, which seems unlikely.


What are the color bands on the resistor?

Bob
 

Thread Starter

rahul411

Joined Feb 19, 2018
244
Are you sure that is the value of the resistor? That would make the full scale deflecton 37.5 ma and it would be dissipating 11W at 300V, which seems unlikely.


What are the color bands on the resistor?

Bob
1563118287017901773789703742485.jpg

This is that resistor. Marked as 10k but it isn't.
 

BobTPH

Joined Jun 5, 2013
2,349
Okay, that is an 11W resistor, so it really does draw that much current. Going by the marking, it is actually more like 30mA. The value nay have changed over time.

Bob
 

BobTPH

Joined Jun 5, 2013
2,349
Oh, and the answer to your original question is no, you cannot convert it to read 50uA full scale. Well, you could, but it would involve using a DC amplifier,

Bob
 

SLK001

Joined Nov 29, 2011
1,543
Also, looking at your scale, it says "AC/DC" which means to me that there is some circuitry inside to rectify the AC to DC, which is screwing up the calculations.
 
AC/DC makes things wierd. You might find two scales on the meter face. There is likely circuitry inside the meter like diode bridge.

The scale should be linear for your re-purposing.

NEVER measure the resistance of a microammeter with an ohmmeter. It has to be measured indirectly,

f you assume 50 uA, get a variable resistor that will get you 50 uA with some fixed power supply. Set the resistor to maximum. Turn on the power supply (slowly if it variable). Adjust the resistor and.or power supply such that you read 1/2 scale. Measure the variable resistor. That's the resistance of your meter.

You can make it read current with a shunt resistor or voltage with a series resistor.
At a minimum you need to know meter resistance and full scale current.

You can find 1/2 scale current by measure the voltage across the pot.

Then what you need to measure.

==

It's easy to slap together a feedback ammeter with an OP-amp. The voltage drop is extremely small like under a mV.
 

Thread Starter

rahul411

Joined Feb 19, 2018
244
That's how you do it. But you use the full scale deflection value as your "V" value. You now have your "V" and your "R" values - now calculate your "I", or full scale current of your meter.
The full scale deflection is at 44.6 volts or around that value, because the needle keeps on moving back and forth at end of scale.
So taking the V as 44.6V and R as 1159 ohm we get a current of 3.848mA.
Next what?
 
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