MULTI RANGE (OR) EXTENDING THE RANGE OF DC AMMETER

 

The current range of the dc ammeter may be further extended by a number of shunts, selected by a range switch. Such meter is called a multi range ammeter.

The multi range ammeter as shown in below figure.

Figure shows in multi range ammeter

The circuit has four shunts,R₁, R₂, R₃ and R₄ which can be placed in parallel with the movement to give four different current ranges I_1,,I_2,,I₃ and I₄.

 R₁=I_m R_m / I₁-I_m for current range I_1;

R₂= I_m R_m/ I₂-I_m for current range I_2;

R₃= I_m R_m/ I₃-I_m for current range I_3;

R₄= I_m R_m/ I₄-I_m for current range I_4.

Switch “S” is a multi position switch, make before break type switch. It is used for range changing. This switch protects the meter movement from being damaged without a shunt during range changing.

Multi range ammeters are used for ranges up to 50A. When using a multi range ammeter, first use the highest current range, then decrease the range until good upscale reading is obtained. The resistance used for the various a ranges are of very high precision values, hence the cost of the meter increases.