Converting voltmeter to ammeter.

Discussion in 'General Electronics Chat' started by rahul411, Jul 14, 2019 at 8:22 AM.

  1. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    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?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    9,694
    2,341
    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.
     
  3. KeithWalker

    Member

    Jul 10, 2017
    262
    81
    I doubt whether it would be sensitive enough to measure microamps. Is it a moving coil meter?
     
  4. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    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?
     
  5. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    Moving coil.meter means coil placed between magnets that makes it move. Right? I don't think mine is of that type. See the picture.
     
  6. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    Coil's resistance is 1.159 kohms
     
  7. SLK001

    Senior Member

    Nov 29, 2011
    1,325
    515
    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.
     
  8. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    At 12 volts the current calculated is 1.5 mA
    Is this right?
     
  9. SLK001

    Senior Member

    Nov 29, 2011
    1,325
    515
    No. Your full scale deflection is 300V.
     
  10. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    I simply applied ohms law...
    How to do it then?
     
  11. SLK001

    Senior Member

    Nov 29, 2011
    1,325
    515
    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.
     
  12. BobTPH

    Senior Member

    Jun 5, 2013
    1,732
    441
    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
     
  13. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    1563118287017901773789703742485.jpg

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

    Senior Member

    Jun 5, 2013
    1,732
    441
    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
     
    rahul411 likes this.
  15. BobTPH

    Senior Member

    Jun 5, 2013
    1,732
    441
    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
     
  16. SLK001

    Senior Member

    Nov 29, 2011
    1,325
    515
    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.
     
  17. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
    3,347
    609
    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.
     
  18. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    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?
     
  19. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    I don't have h decent voltage source, for this I'm using 2 lm317 based regulator which does the work fine.
     
  20. rahul411

    Thread Starter Member

    Feb 19, 2018
    160
    11
    No there is nothing but a coil.
     
Loading...