Will I need a shunt for this ammeter?

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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
 

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MikeML

Joined Oct 2, 2009
5,444
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...
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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...
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
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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
 

mcgyvr

Joined Oct 15, 2009
5,394
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.
 

Kermit2

Joined Feb 5, 2010
4,162
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.
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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.
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.
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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.
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
 

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Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
Hi Keith,

It's a 400mA FSD AC/DC moving iron meter!

A series resistor would be required for use as a voltmeter and a shunt resistor for use as an ammeter.

The value of the series resistor would depend on the maximum voltage and that of the shunt resistor on the maximum current. The coil resistance (in the case of DC) or coil impedance (in the case of AC) would figure in the calculations.

Regards,

Nandu.
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
 
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