Varying Range of Voltmeters and Ammeters
Voltmeter. The range of a voltmeter may be extended by connecting a resistor in series with the instrument. This resistor is called a multiplier (Rm).
Equation used to calculate series resistor.
Where;
Vt – Total voltage to be measured.
IFSD – Full scale deflection current.
RMETER – Resistance of voltmeter coil.
Vt – Total voltage to be measured.
IFSD – Full scale deflection current.
RMETER – Resistance of voltmeter coil.
Ammeter.
Figure 2: Showing how shunt resistors are connected to meter coil.
The range of an ammeter may be extended by connecting a resistor in parallel with the instrument. This resistor is called a shunt (RSH).
Where;
ISH – Current through shunt resistor.
IT – Total current to be measured.
IFSD – Full scale deflection current.
RMETER – Resistance of ammeter coil.
ISH – Current through shunt resistor.
IT – Total current to be measured.
IFSD – Full scale deflection current.
RMETER – Resistance of ammeter coil.
AMMETER SAFETY PRECAUTIONS
When you use an ammeter, certain precautions must be observed to prevent injury to yourself or others and to prevent damage to the ammeter or the equipment on which you are working. The following list contains the MINIMUM precautions to observe when using an ammeter.
· Ammeters must always be connected in series with the circuit under test.
· Always start with the highest range of an ammeter.
· Turn OFF and discharge the circuit completely before you connect or disconnect the ammeter.
· In dc ammeters, observe the proper circuit polarity to prevent the meter from being damaged.
· Never use a dc ammeter to measure ac.
· Observe the general safety precautions of electrical and electronic devices.
· Ammeters must always be connected in series with the circuit under test.
· Always start with the highest range of an ammeter.
· Turn OFF and discharge the circuit completely before you connect or disconnect the ammeter.
· In dc ammeters, observe the proper circuit polarity to prevent the meter from being damaged.
· Never use a dc ammeter to measure ac.
· Observe the general safety precautions of electrical and electronic devices.