Analog Meters An ammeter is a measuring instrument used to measure the electric current in a circuit.

The tangent galvanometer    Hans Christian Ørsted who, in 1820, observed a compass needle was deflected from pointing North when a current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this effect

The D'Arsonval galvanometer is a  moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field.

Other types of the Ammeter Moving iron  Electrodynamic a hot-wire ammeter Digital ammeter

We said the ammeter can measure only milliamperes at most, so how can measure a higher current?

a resistor called a shunt is placed in parallel with the meter a resistor called a shunt is placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small fraction flows through the meter.

i is im

Hence: Vs=Vm IsRs=ImRm Rs=)Im/Im)Rm I=Is+Im Is=I-Im Rs=(Im/I-Im)Rm

The Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters.

Made By :Abedalhamid (0096294)

Multi-Range Ammeter ( Ayrton Bridge ) Made by : Zaid Al-Ali (0094310)

Small Introduction … In practical terms, ammeters with a single range are not very useful. However there are some exceptions such as Marine meters-voltage, fuel, Power station meters (voltage, frequency) and automotive meters (ammeter, tachometer).All of which have one useful range. To make an ammeter to measure several ranges at once, one approach is to have a separate shunt resistor for each range and we can calculate each resistor value of the shunt.

There’s a problem needed to be fixed By referring to the Figure, the position of the switch, one of R1 to R4 would be connected as a shunt across the meter. However, a problem may have with thus arrangement. At the point when the switch is moved from position 1 to position 2, the PMMC movement will be forced to pass a current that may be more than full scale deflection current, fsd I . This will most likely destroy the meter, or at best blow a fuse.

The solution … Hence, to solve of this problem another ways of connection can be used. Those are a make-before-break switch and a Universal or Ayrton Shunt. The make-before-switch establishes contact with the next contact position before losing contact with the existing connection. In this manner, the shunt resistors are never removed from the circuit and the PMMC movement is always protected.

Design Procedure … Calculation for circuit in the Figure to provide an ammeter with a current range of 0 −100mA,0 −150mA, 0 − 300mA and I A fsd = 100μ . Firstly, voltage across resistors R1, R2 and R3 parallel with the voltage Rm. Hence, VSH = VRm ISH RSH = IM RM But, I = ISH + Ifsd so, ISH = I – Ifsd

For range 0 −100mA, (100m −100μ )(R3 + R2 + R1) = 100μRm 99 For range 0 −100mA, (100m −100μ )(R3 + R2 + R1) = 100μRm 99.9m(R3 + R2 + R1) = 100μRm ----------- (1) For range 0 −150mA, (150m −100μ )(R2 + R1) = 100μRm 149.9m(R2 + R1) = 100μ (Rm + R3) ------- (2) (R2 + R1) = 667.11μ (Rm + R3) ------------- (3) For range 0 − 300mA, (300m −100μ )(R1) = 100μRm 299.9m(R1) = 100μ (Rm + R3 + R2) R1 = 333.44μ (Rm + R3 + R2) ---------------- (4) By solving these four equations , the values of R3, R2 and R1 are :

Simulation Part … Using Multisim Software .... Ammeter

Simulation Part … shunt DC Multi-range ammeter