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Old 01-09-2010, 11:17 AM
sparkfishes's Avatar
sparkfishes sparkfishes is offline
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Default Changing the range of an Unknown Ammeter

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 by sparkfishes; 01-09-2010 at 11:19 AM. Reason: spelling error
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Old 01-09-2010, 12:33 PM
Mike33 Mike33 is offline
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Posts: 328

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! :o)
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Old 01-10-2010, 01:09 AM
someonesdad someonesdad is offline
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Originally Posted by sparkfishes View Post
However, I have a 0-10A and so can not make it read lower(?)
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.
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