Extending ammeter range

Discussion in 'General Electronics Chat' started by ronph, Mar 9, 2013.

  1. ronph

    Thread Starter New Member

    Feb 4, 2013
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    I have an old Triplet milliampere meter that reads between 0 & 1 milliamp @55 ohms based on the online specs. Is it possible to extend the range to 1 amp reading using a resistor at one end of the lead coming out of the meter or parallel to the legs of the meter? Its not critical as to the exact output - close enough would do. Working voltage would be between 9 & 30 volts (if this helps).

    Thanks
     
    Last edited: Mar 9, 2013
  2. ScottWang

    Moderator

    Aug 23, 2012
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    1mA Meter for V = 1mA * 55Ω = 55mV.
    Total R = 55mV / 1A = 0.055Ω.
    Parallel R of I = 1A - 1mA = 999mA
    Parallel R = 55mV / 999mA = 0.055055Ω.
    Power dissipation of Parallel R = V * I = 55mV * 999mA = 0.055W

    Assuming above formular is correct.
    So if you can get a 0.055055Ω resistor in parallel with 1mA Meter then you can expanding the measuring range.

    If you can't get the 0.055055Ω resistor, then you better buy a new one.
     
    Last edited: Mar 9, 2013
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    If an approximate reading is OK, then you can easily make a resistor by using a measured length of wire of some known diameter. For instance, 24Ga wire has resistance of .026 ohms/foot, so if you want .055 ohms, just over 2 feet ought to do it.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    I never heard 24Ga wire, but that is a new knowledge to me.
    I just heard someone talking about the Manganin shunt, and below is a 2mΩ Manganin shunt, that is used for big current sensor.


    [​IMG]
    The picture was copy from here.

    Manganin Shunts.
     
    Last edited: Mar 9, 2013
  5. antonv

    Member

    Nov 27, 2012
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    Why do you mention a working voltage of 9-30V? Do you want to measure voltage or current?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    A .05Ω resistor or two 0.1Ω resistors in parallel will be within 91% of the correct value, if that's close enough for you.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    We used to buy manganin wire of various diameters on spools to make current shunts. It's resistivity is well known and has zero TC.
     
  9. ronph

    Thread Starter New Member

    Feb 4, 2013
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    Thanks for all your input. I want to use it to measure current (not voltage). I will try as suggested with a lenght of wire and see if that will do the trick.

    Will update if I make any progress on this.
     
  10. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Toroidal current transformer...
     
  11. crutschow

    Expert

    Mar 14, 2008
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    That'll work for AC but not DC, which I think is what the OP is measuring.
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    A manganan shunt has no bandwidth limitations and is good all the way to DC.
     
    PackratKing likes this.
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