Basic DC Ammeter

An ammeter is an instrument used for measuring the electric current in a branch of an electric circuit. It must be placed in series with the measured branch, and must have very low resistance to avoid significant variation of the current it is to measure. The ammeter is also used the principle of d’Arsonval meter movement with slight modification. A low value resistor (shunt resistor) is used in DC ammeter to measure large current.

Voltage across meter resistor ( Rm ) is given by,

Vm= Im Rm

Voltage across shunt resistor ( Rsh ) is given by,

Vsh= Ish Rsh

Meter resistor and shunt resistor are connected in parallel, so voltage drop across resistor is same,

ImRm= IshRsh

I = (Ish + Im)

Ish = (I –Im)

Rsh =( ImRm)/(I –Im)

Where,

Rm= internal resistance of the movement.

Rsh= shunt resistance

Ish= shunt current

Im= full scale deflection current of the movement

I = full scale current of the ammeter + shunt