- A multi-range
ammeter can be constructed simple by employing several values of shunt
resistances, with a rotary switch to select the desired range.
- When an
instrument is used in this fashion, care must be taken to ensure shunt
does not become open-circuited, even for a very short instant.
- If the shunt resistance remains open-circuited for a
fraction of time, a very large current may flow through the ammeter and damage the
instrument. When the switch is moved from position ‘B’ to ‘C ’ or moved to
any positions, the shunt resistance will remain open-circuited for a fraction of time, resulting a very
large current may flow through the ammeter and damage the instrument.
- To avoid such situation, one may use the
make-before-break switch. The
wide-ended moving contact connected to the next terminal
to which it is being moved before it loses contact with the previous terminals. Thus, during
the switching time there are two resistances are parallel with the instrument and finally the
required shunt only will come in parallel to the instrument.
Multirange Ammeter Circuit
Make-before-break switch
Precautions
- Never connect an ammeter across a source of emf.
Because of its low resistance it would draw damaging high currents and
destroy the delicate movement. Always connect an ammeter in series with a
load capable of limiting the current.
- Observe the correct polarity. Reverse polarity causes the meter to deflect against the mechanical stop and this may damage the pointer.
- When using a multirange meter, first use the highest current range; then decrease the current range until substantial deflection is obtained. To increase accuracy of the observation, use a range that will give a reading as near to full-scale as possible.
