# Changing the range of an Unknown Ammeter

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

1. ### sparkfishes Thread Starter New Member

Oct 19, 2009
16
0
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?

Last edited: Jan 9, 2010
2. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
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
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! )