Converting 500mA analog meter to a uA meter.

Discussion in 'The Projects Forum' started by hyper9, Mar 31, 2016.

  1. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    Hello hello everyone.

    I was at Fry's electronics just now and passed some analog panel meters and remembered a project that required one. i went with the 500mA meter cuz it looked familiar and i thought it was the one i needed... must have been a different project cuz looking it up just now, and turns out i need a .5mA meter.

    now i'm looking in my text on how to put a shunt across the meter to measure quantities higher than your meter can read but how would i do this so that when a full scale deflection on my 500mA meter would actually be 500uA???

    i cannot seem to figure this out.. thanks for your time everyone.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Look to see if there is already a shunt resistor on the meter.
     
  3. #12

    Expert

    Nov 30, 2010
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    I don't think I've ever seen a meter movement wound for 500 ma, but I've seen a lot that were designed for 1ma and lower. There is probably a shunt resistor already there. If you take that resistor out you can find out what is the original current it was designed for. I remember 35 ohms and 1 ma, so that one would need 35 millivolts. The point is, be really careful not to let it have much current of you'll peg the needle.
     
  4. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    Here is the meter:


    IMG_4697.JPG

    IMG_4698.JPG
     
  5. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    oh maybe not... ok let me figure out how to post up some pictures
     
  6. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    just posted pictures, how would i do this on this meter?
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
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    There is usually a shunt. Open it up.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    They are ALL voltmeters. A voltmeter which measures the "voltage drop" across a low value resistor or "shunt" is often referred to as an "amp" meter or ammeter.
    There is also a common spec sometimes found on a meters face in small print or code. This is the "full scale deflection" current.
    Common values are 1mA and 500uA.
    The first step to open it would be to pry off the clear cover and see how the movement is mounted to the back case. You may have to remove the printed backplate as well.
     
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    Coil meters are by definition ammeters.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Analog meters are generally ammeter/voltmeters, i.e. they measure either current or voltage (while obeying Ohm's Law).
    They generally are very sensitive, for example, 50μA full-scale. It is very possible that there is a shunt resistor inside that meter.
     
  11. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    awesome! thanks for all the help so far fellas, i'm on this right now. i'll be back with the details.
     
  12. hyper9

    Thread Starter New Member

    Jul 13, 2014
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    well these are the resistors in the meter:

    IMG_4701.JPG

    so the shunt resistors i cant really tell what they are...

    Black, violet, green, silver, brown it looks like.. but that wouldnt make any since
     
  13. Marcus2012

    Member

    Feb 22, 2015
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    0.75 ohms 1% I think but 2 in parallel, 0.5-1W maybe?
     
  14. vk6zgo

    Active Member

    Jul 21, 2012
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    Moving coil meters measure current-period!

    Voltmeters are simply current meters with series resistors so the voltage source can cause current flow through the series circuit of that resistor & the meter's own internal resistance,obeying Ohm's Law.
    The value of the series resistor is usually calculated to enable the highest voltage which it is required to measure to produce a "Full scale deflection" (FSD)in the meter.

    Current meters commonly use "shunt" resistors of very much lower resistance than the meter internal resistance,so that in the resultant parallel circuit,the largest proportion of the circuit current is through the shunt,with a (usually) much smaller current through the meter movement.

    Moving coil meter movements are commonly designed to have quite low FSDs,with higher current scales achieved with the use of "shunts".
    If these are internal,careful dismantling will give access to these shunts.
     
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