A meter has a resistance of 75 ohms and gives full-scale deflection when it passes a current of 1.0 mA. How would you adapt the meter to read
a) Currents up to 1A
b) voltages up to: 5V
c) Voltages up to: 100V
E = IR
To adapt the meter to read different currents or voltages, we need to consider the meter's resistance and its full-scale deflection current.
a) To measure currents up to 1A:
The meter's resistance is 75 ohms and it gives full-scale deflection at 1.0 mA. If we want to measure currents up to 1A, we need to change the meter's range. The current through the meter will cause a voltage drop across its resistance, which should be proportional to the current. To achieve that, we can use an external shunt resistor in parallel with the meter.
To calculate the shunt resistor value, we can use Ohm's Law. The shunt resistor value (Rs) can be calculated as the voltage drop across the shunt resistor divided by the maximum current we want to measure. In this case, the maximum current is 1A, and we want the meter to give full-scale deflection for this current. Therefore, the voltage drop across the meter's resistance should be equal to the voltage drop across the shunt resistor.
So, for a full-scale deflection of 1A, the voltage across the meter's resistance should be 75 ohms * 1A = 75 volts. To achieve this, connect a shunt resistor (Rs) with a resistance of 75 ohms in parallel with the meter. Now, the current that is going to pass through the meter will also pass through the shunt resistor, causing an equal voltage drop across it. This voltage drop will be measured by the meter, resulting in a full-scale deflection.
b) To measure voltages up to 5V:
In this case, since we are dealing with voltages, we need to convert the voltage into a current using a series resistor. The resistor should be selected in a way that for a full-scale deflection of the meter, the voltage drop across the resistor is equal to the maximum voltage we want to measure.
To calculate the series resistor value (Rv), we can use Ohm's Law again. The series resistor value can be calculated as the maximum voltage we want to measure divided by the full-scale deflection current of the meter. In this case, the maximum voltage is 5V, and the full-scale deflection current is 1.0 mA.
Therefore, the series resistor value (Rv) is calculated as 5V / 1.0 mA = 5 kilohms (kΩ). By connecting a 5 kΩ resistor in series with the meter, we can convert voltages up to 5V into a current that the meter can measure.
c) To measure voltages up to 100V:
Similar to the previous case, we also need to convert the voltage into a current using a series resistor. However, in this case, the maximum voltage we want to measure is 100V.
Using the same formula as before, the series resistor value (Rv) can be calculated as 100V / 1.0 mA = 100 kilohms (kΩ). By connecting a 100 kΩ resistor in series with the meter, we can convert voltages up to 100V into a current that the meter can measure.