- A dc voltmeter is
constructed by a connecting a resistor in series with a PMMC instrument.
- A voltmeter should have a very high resistance Rse and it is normally connected in parallel with the circuit where the voltage is to be measured.
- To minimize voltmeter loading, the voltmeter operating current should be very small i.e., the resistance connected in series with the coil should be high.
The addition of a series resistance or a multiplier
limits the current through the movement so as not to exceed the full-scale
deflection current. The value of multiplier required to extend the voltage
range
The
multiplier is usually mounted inside the case of the voltmeter for moderate
ranges upto 500 V. For higher voltages, the multiplier may be mounted
separately outside the case on a pair of binding posts to avoid excessive
heating inside the case.
Multirange Voltmeter
- Any one of
the several multiplier resistors is selected by means of a rotary
switch.
- The rotary
switch used with the voltmeter should be a break-before make type i.e. moving
contact should disconnect from one terminal before connecting to the next
terminal.
- This system has the advantage that all multipliers except the first have standard resistance values and can be commercially obtained in precision tolerances. The low range multiplier R4, is the only special resistor that must be manufactured to meet the specific circuit requirements.
Precautions
- Observe the
correct polarity. Wrong polarity causes the meter to deflect
against the mechanical stop and this may damage the pointer.
- Place the voltmeter across the circuit or component
whose voltage is to be measured.
- When using a multirange voltmeter, always use the
highest voltage range and then decrease the range until a good upscale
reading is obtained.
- Always be aware of the loading effect. The effect can
be minimised by using as high a voltage range as possible. The precision
of the instrument decreases if the indication is at the low end of the
scale.
