A (0-50)A moving coil ammeter has a voltage drop of 0.1V across its terminals at full scale deflection. The external shunt resistance (in milliohms) needed to extend its range to (0 - 500 A) is –
Concept:
- Shunts are used for the range extension of ammeters.
- A shunt is a low-value resistance having minimum temperature co-efficient and is made up of manganin. Because manganin has a very low value of temperature coefficient.
- It is connected in parallel with the ammeter whose range is to be extended. The combination is connected in series with the circuit whose current is to be measured.
- Shunt provides a path for extra current as it is connected across (in parallel with) the instrument.
- These shunted instruments can be used to measure currents many times greater than their normal full-scale deflection currents.
In the figure
I is total current flowing in the circuit
Ish is the current through the shunt resistor
Rm is the ammeter resistance
As the shunt and meter are in parallel, therefore, the voltage drop across them will be equal
Calculation:
Given,
The voltage drop across the ammeter is 0.1 V for full-scale deflection
Current flowing through the meter at full-scale deflection is 50 A
Now the range is extended to 500 A,
Current flowing through shunt Ish = 500 - 50 = 450 A
As the shunt and meter are in parallel, therefore, the voltage drop across them will be equal
The voltage drop across shunt = Shunt resistance × shunt current
⇒ 0.1V = Rsh × 450 A
⇒ Rsh = 0.22 mΩ
Important Questions from Basic DC Ammeter
The range of an ammeter can be extended by using:
To measure which of the following is an ammeter used?
Name the tool which is used to measure the current in any electronic circuit.
- Why we use low internal resistance in series of an ammeter?
Two ammeters x and y have resistances of 1.2 Ω and 1.5 Ω respectively and they give full scale deflection with 150 mA and 250 mA respectively. The ranges have been extended by connecting shunts so as to give full scale deflection with 15 A. The ammeters along with shunts are connected in parallel and then placed in a circuit in which the total current flowing is 15 A. The current in amperes indicated in ammeter x is-
