Changing the range of an Unknown Ammeter

Discussion in 'General Electronics Chat' started by sparkfishes, Jan 9, 2010.

  1. sparkfishes

    sparkfishes Thread Starter New Member

    Oct 19, 2009
    I have read the tutorial - to refresh my memory-

    I have several small ammeters but want one to indicate
    say 0- 5A or 0- 3 amps
    However, I have a 0-10A and so can not make it read lower(?)
    I have a 50mA which, by the addition of a resistor in parallel (1/10 of value as the resistance of the FS defelctionof the meter - I will check the maths after ) could get the ammeter to read ten times the shown scale .
    My problem is the meter does not show the FS deflection resistance so how do I find that out?

    Or any other solutions please!
    Last edited: Jan 9, 2010
  2. Mike33

    Mike33 Senior Member

    Feb 4, 2005
    You need the current that will cause full-scale deflection. I wire up a 1M pot as variable resistance in series with the meter and an ammeter (DMM), then run a volt or 2 in and take the reading that gives FS.
    You now have the meter's FS deflection in mA. This is probably what you were after...
    Then I measure the internal resistance of the meter to get Rmeter, then use the formula:

    Rshunt = Rmeter/N-1
    where N= your desired FS deflection reading.
    Real example:
    Say you have a FS deflection of the meter at 1mA, and want a 5A FS meter, taking the meter's internal resistance of 58ohms (which I measured on mine), then the shunt resistor Rshunt=(58/5,000mA-1)= .0116ohms

    You can get that value resistor using special-order shunts, or using resistance wire...make sure it can handle the additional current! :eek:)
  3. someonesdad

    someonesdad Senior Member

    Jul 7, 2009
    It may have an internal shunt to make it a 10 A meter. You may be able to open the meter up and remove the shunt; then you'll have a meter movement that probably has a 50 μA or 100 μA or somesuch. Then you can make new shunts.

    A useful number to remember is that 10 gauge (AWG) copper wire has a resistance of about 1 mΩ/ft; that may be helpful to make a shunt.