Will I need a shunt for this ammeter?

Discussion in 'The Projects Forum' started by Keith Robinson, Dec 5, 2014.

  1. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    I would like to put a volmeter and ammeter on the output of my Variac and Im fairly sure my ammeter will need a shunt but I cant seem to figure out what size shunt Ill need, or if I could make one if i do need one. Perhaps the icons on the face of the meter might hold some info. could anyone enlighten me please?
    thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    It looks to me that your meter is 400mA AC Full-scale. I have never tried to put an external shunt on an AC ammeter. Because of the non-linearity due to the rectifier inside it, I'm not sure how to go about it...
     
  3. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    so perhaps it doesnt need and external shunt then? I didnt want to fry the thing by just putting it in series with the load
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Should work as long as your load draws less than 400mA AC. Even a 100W lamp would draw more than that...
     
  5. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    oh right! looks like Ill need a larger meter, or only use it for low power running. Thanks for your help. As you might guess Im very amateur but trying to teach myself. I hate asking for help, but this site is really freindly and helpful.
    thanks again. Keith
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Never seen an simple ammeter that didn't need a shunt..
    But I've also never used an ammeter that didn't go into a system with the capacity for hundreds of amps.
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    the simple ammeter is actually a milli-volt meter which measures the voltage dropped across a shunt resistor, through which, a majority of the electric current passes. it is the shunt resistor which turns the voltmeter to ammeter. small ones have the shunt inside and large value meters have external shunts.

    a series resistor just turns the same simple milli-voltmeter into a voltmeter for a larger/higher voltage range.

    For AC use insert a bridge rectifier/(iron vane meters are better in AC)

    About the OP's picture of his meter.

    Looks like it might be based on a very old type known as a heated filament type ammeter. If OEM and from the teens to the fourties it could be worth more on ebay than on a hobby project.
     
  8. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    Thats the lesson Ive learned here, - use an ammeter with enough capacity to cover whatever might be being measured. especially when dealing with AC. A DC ammeter could be used with the correct resisive shunt in place, in parallel
    thanks for your comments.
     
  9. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    Thank you very much for your excellent explanation, I really think Im starting to get it now! Im collecting a few of these old meters as I love the old technolgy. Ive managed to get one or two quite cheaply from ebay. some of the workmanship is outstanding. Ive restored a couple and am hoping to use them in my simple steampunk projects. the brass one must be getting on for a hundred years old and the workmanship is superb.
    thanks again for your great answer about ammeters regards Keith
     
  10. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    Hi Nandu, thanks so much for your response, things are a little clearer to me now. Kermit2 explained it so very well, and with your response the fog is clearing! I'll refer back to this about impedance and resistance when I know a bit more (hopefully!) Its interesting to see that this particular meter is a Moving Iron meter. perhaps that was depicted in the symbols on the dial? kind regards Keith
     
  11. Keith Robinson

    Thread Starter Member

    Nov 30, 2014
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    this is my variac Im trying to upgrade with better meters
     
  12. Johann

    Senior Member

    Nov 27, 2006
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    A variac is one of the most useful pieces of equipment! I shall never part with mine!
     
    Keith Robinson likes this.
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