In the case of an ammeter, the shunt in parallel with the ammeter (which
also may be measuring the voltage drop across a resistor) is necessary
because in an ammeter the current is typically too large, and several
amps may "blow" the internal circuitry. Therefore it is more important
to have a shunt resistor(s) in parallel with the meter to "siphon off"
the large share of the electric current.

so how does a ammeter measure the full circuit current,if it is letting the minimum current passing through its deflection part,while rest of current is passing in the shunt resistor?

 

Hello,

Please read the page on Ammeter design in the eBook:
http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

Bertus

 

thnax....but thats not the way to reply?

 

Try to think of the Ammeter as "measuring the voltage drop across the shunt resistor which is carrying the full current of the circuit".

Then it is a matter of ohms law to label the meter's face correctly.

 

so how does a ammeter measure the full circuit current,if it is letting the minimum current passing through its deflection part,while rest of current is passing in the shunt resistor?

That's easy.

The ammeter needle will deflect to a certain degree when current is flowing.

One just pass the current say 10Amp through the meter(with shunt) and then mark the position of the needle to be 10A.

This doesn't means 10A is actually flowing in the shunt(it is a bit less as some go through the meter coil) but 10A is flowing via the meter & the shunt combination.

 

The simplest way is for the OP to read a multi-meter manual. The classic would be a Simpson 260.

Section three covers the functions of the parts. An ammeter is illustrated.

http://www.simpson260.com/downloads/simpson_260-2_user_manual.pdf

 

If the meter requires 1mA for full scale deflection, shunting 9.999 mA around the meter and 1mA driving the meter to full scale will represent a current of 10A. Any reduction in current will be shown proportionately on the meter scale. The resistance of the shunt is determined by the internal resistance of the meter divided by the shunts multiplication factor. In the above case this would be 9.999. A digital meter will measure the mV drop across the shunt.

 

Should read 9.999A