2.3 DC AMMETER

WHAT IS DC AMMETER??

If this electric charge flows only in one direction, then the resultant current is called Direct Current (DC)
The instrument, which is used to measure the Direct Current called DC ammeter.
If we place a resistor in parallel with the Permanent Magnet Moving Coil (PMMC) galvanometer, then the entire combination acts as DC ammeter.
The parallel resistance, used in DC ammeter is also called shunt resistance.

Rs is the shunt resistance

Rm is the internal resistance of galvanometer

I is the total Direct Current that is to be measured

Im is the full scale deflection current

The range of the dc ammeter is extended by a number of shunts, selected by a range switch.
The resistors are placed in parallel to give different current ranges.
Switch S protects the meter movement from being damage during range changing.

Multiposition Switch (S)

Have low contact resistance and high current carrying capacity, due to its contacts are in series with low resistance shunts.
'make before break ’ type switch is used for range changing.

also known as universal shunt.
Compared to multirange ammeter, the Ayrton shunt is frequently a more suitable design.
R1 parallel with series combination of R2, R3 and the meter movement.
Current through the shunt is more than the current through the meter movement.
R1 and R2 in parallel with the series combination of R3 and the meter movement.
The current through the meter, is more than the current through the shunt resistance.
R1, R2 and R3 in parallel with the meter.
Maximum current flows through the meter movement, and very little through the shunt. This will increase the sensitivity.

the shunt resistances should be stable and constant with time.
the temperature coefficient of shunt and the meter should be low and equal.
the shunt resistances should not carry currents which will cause excessive temperature rise.

ammeter resistance is very low, is should not be connected across any source of e.m.f.
the polarities must be observed correctly.
while using multirange ammeter, first use the highest current range and decrease the current range until sufficient deflection is obtained.

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