A moving-coil instrument gives a full-scale deflection when the current is 40mA and its resistance is 25 Ω. Calculate the value of shunt to be connected in parallel with the metre to enable it to be used as an ammeter for measuring currents up to 50A

Vm = 0.04 *25 = 1.0 volt across meter @ full-scale.

Rs = Vm/I = 1.0/(50-0.04) = 0.020 Ohms.

.04 * 25 = (50 - .04) * s

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A moving coil instrument gives a full scale deflection when the current is 40mA and it's resistance is 25, Calculate the value of the shunt to be connected in parallel with the meter to enable it to be used as an ammeter for measuring current up to 50A.

To calculate the value of the shunt needed to convert the moving-coil instrument into an ammeter, we can use Ohm's Law (V = I * R) and current division in parallel circuits.

1. First, let's determine the voltage drop across the meter when it is connected in parallel with the shunt resistor. We know that the full-scale deflection of the meter occurs at 40mA (0.04A) and its resistance is 25 Ω. Therefore, the voltage drop across the meter at full-scale deflection can be calculated using Ohm's Law:

V_Meter = I_Meter * R_Meter
= 0.04A * 25 Ω
= 1V

2. Next, let's determine the current that must flow through the shunt resistor (I_Shunt) in order to measure a maximum current of 50A. Since the meter and the shunt resistor are connected in parallel, the total current flowing through the parallel combination is equal to the sum of the currents flowing through the meter and the shunt resistor.

I_Total = I_Meter + I_Shunt

We want the maximum current to be 50A, so:

I_Total = 50A
I_Meter = 0.04A (meter full-scale deflection)

Therefore,

I_Shunt = I_Total - I_Meter
= 50A - 0.04A
= 49.96A

3. Now, let's use Ohm's Law to calculate the value of the shunt resistor (R_Shunt) that will allow a current of 49.96A to flow through it with a voltage drop of 1V:

R_Shunt = V_Shunt / I_Shunt
= 1V / 49.96A
= 0.02004 Ω

Therefore, a shunt resistor with a resistance value of approximately 0.02004 Ω should be connected in parallel with the meter to enable it to be used as an ammeter for measuring currents up to 50A.