Convert Analog panel voltmeter to ammeter

Discussion in 'General Electronics Chat' started by naitek, May 21, 2014.

  1. naitek

    Thread Starter Member

    May 31, 2008
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    Hey ;
    i have an old analog voltmeter (20v) and i would like ot conver to a 100µA to use for a project.
    The voltmeter has a 150Ω internal resistance with an inline 20k resistor so basically reads .13Amps . I dotn know about shunting and am asking to make sure i i dont screw anything up. thanks in advance

    -n
     
  2. Senz_90

    Member

    Jul 11, 2013
    70
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    Please browse my thread early. maybe you could get the idea from others comment on those thread.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    if the full sscale on the meter is .13 amps, it will not deflect much with 100 microamps. you will need another meter or a shunt with amplifier.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,777
    1,103
    I don't know where you got your 0.13A figure from, but by my reckoning if the 20V meter has a 20k inline resistor then the meter movement is a 1mA full-scale type. At 100uA it will read only 1/10 scale, so is unsuitable without amplification.
     
  5. naitek

    Thread Starter Member

    May 31, 2008
    15
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    i made a mistake on the amp math; thanks for the correction. So i would need to amplify my output ( eg 400mv ) through an op amp in order to make the meter read properly?
     
  6. #12

    Expert

    Nov 30, 2010
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    It looks to me like a 250 ohm resistor in series with the 150 ohm meter movement will allow 1 ma when you apply .4 volts.
     
  7. naitek

    Thread Starter Member

    May 31, 2008
    15
    0
    i have tried various resistors and i get no movement. I would like to know where t=i can find the correct shunt with amplifier to attempt to make this work. I cant find anything really.

    The thing is if i am applying 100mv - 900mv to this voltmeter; i would need to ramp to about 10v in order to see on the meter so an amplification or gain of 10 would be required.

    Any help would be awesome. thanks again

    -
     
  8. #12

    Expert

    Nov 30, 2010
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    Ohms Law.
    E = I R
    .9V = .001 A x 900 ohms

    You already have 150 ohms in the meter movement so add 750 ohms in series with the meter.
     
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  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Use an active current to voltage converter.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That suggests the meter movement may be damaged :(.
     
  11. #12

    Expert

    Nov 30, 2010
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    I am beginning to get puzzled about this. It seems simple enough from here, but the results aren't happening. I'm about to fall back to seemingly stupid questions.

    Did you shake the meter to see if the needle moves?
    Is the meter on "zero" with no stimulus being applied?
    Have you tried a DMM meter with a diode measuring range on it? That would supply about 1 ma.
    Did you remove the 20k resistor?
     
  12. naitek

    Thread Starter Member

    May 31, 2008
    15
    0
    the voltmeter is perfect for measuring 20v ( dc) Vout, but the circuit i was tring to measure was a 300 - 900mv circuit output which i wanted to read on the same meter. So basically i need an ammeter, but would like to know how to conver this 20v meter ( WHICH STILL WORKS with 20v) with 20kΩ resistor put back in.

    i cant seem to figure that voltage to current converter figured out so trying to read more and leanr about that.

    thanks
     
  13. Brownout

    Well-Known Member

    Jan 10, 2012
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    Current to voltage involves sensing a current, sometimes using a resistor and feeding an op-amp with prescribed gain to output an appropriate voltage. For example, for a 1 AMP current across a .1ohm resistor, the voltage is V=1A*.1ohm = .1v. Then an opamp amplifier with a gain of 50 gives a 5V output. One may also use a hall-effect sensor for the sensing element. Sometimes, the current carrying wire can be wrapped with several loops around the sensor to get more sense current.
     
  14. #12

    Expert

    Nov 30, 2010
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    OK. You don't want the meter to just work, you want it to work with the 20k resistor in the circuit. Try this:
     
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