Permanent Magnet Moving Coil Instrument (PMMC) as D.C. Ammeter

Permanent Magnet Moving Coil Instrument (PMMC) as D.C. Ammeter Points : Permanent Magnet Moving Coil Instrument (PMMC) as D.C. Ammeter The basic movement of a d.c. ammeter is a PMMC D. Arsonval galvanometer. The coil winding of a basic movement is small and light. It can sense very small currents since the construction of an accurate instrument with a moving coil to carry current greater than 100 mA is impracticable due to the bulk and weight of the coil that would be required.

When heavy currents are to be measured, the major part of the current is bypassed through a low resistance called a “Shunt”. Following fig. Shows the basic movement (meter) and its shunt to produce an ammeter.

Let,
R_m = Internal resistance of movement (coil)
I_m I_fs = full scale deflection current of movement
R_sh= Resistance of shunt
I_sh = Shunt current
I = current to be measured