EC 217 MEASUREMENTS AND INSTRUMENTATION Chapter #3 D’arsonval meter movement used in DC meter Ammeter.

D’ ARSONVAL METER MOVEMENT USED IN DC METER A MMETER. Since the windings of the moving coil are of very fine wire, thus it can carry only very small current, therefore the basic d’arsonval meter movement has only limited usefulness without modification. One desirable modification is to increase the range of current that can be measured with the basic meter movement. This is done by connecting a low resistance in parallel with the meter movement resistance (R m ).

D’ ARSONVAL METER MOVEMENT USED IN DC METER A MMETER. This low resistance is called a shunt ( R sh ) as shown in Fig.: R sh I sh ImIm RmRm + - I

D’ ARSONVAL METER MOVEMENT USED IN DC METER A MMETER. The voltage drops across the shunt and the movement coil must be the same, i.e I m. R m = I sh.… (1) Since I sh = I-I m Then Therefore for each required value of full-scale meter current we can solve for the value of the shunt resist.

M ULTI – RANGE A MMETER : Simple type multi range ammeter: RaRa RbRb RcRc RmRm ImIm 1A 5A 10A - +

S IMPLE T YPE M ULTI - RANGE A MMETER Disadvantages of this type:- In this type of switching of the multirange ammeter there is no any resistance connected in parallel with the meter coil. Thus if the input current is greater than I m (I>I m ) the coil will burn or disconnected.

T HE A YRTON SHUNT ( MULTI – RANGE A MMETER ) The Ayrton shunt (multi – range Ammeter) RcRc RbRb RaRa 10A 5A 1A - + RmRm

T HE A YRTON SHUNT ( MULTI – RANGE A MMETER ) Advantages of the Ayrton shunt: It eliminates the possibility of the meter movement being in the circuit without any shunt resistance. Another advantage is that it may be used with a wide range of meter movements. Note: The individual resistance values of the shunts are calculated by starting with the most sensitive range and working towards the least sensitive range. The most sensitive range is the lowest one.