Analogue Meters

Done by : Bana makahle &

1.Analogue display:

Analogue displays have a pointer which moves over a graduated scale. They can be difficult to read because of the need to work out the value of the smallest scale division.

• Example: The scale in the picture has 10 small divisions

between 0 and 1 so each small division represents 0.1. The reading is therefore 1.25V (the pointer is estimated to be half way between 1.2 and 1.3).

The maximum reading of an analogue meter is called full-scale deflection or FSD (it is 5V in the example shown).

• 2.Digital display:

Values can be read directly from digital displays so they are easy to read accurately. It is normal for the least significant digit (on the right) to continually change between two or three values, this is a feature of the way digital meters work, not an error! Normally you will not need great precision and the least significant digit can be ignored or rounded up.

For general use, digital meters are the best type. They are easy to read, they may be connected in reverse and they are unlikely to affect the circuit under test.

Ammeters:

An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters.

• Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems.

1.DC Ammeter

a. Shunt resistor ammeter: The basic movement is permanent magnet

movement coil .

Permanent Magnet Moving Coil: Principle of

Working: When a current carrying conductor is placed in

a magnetic field, it experiences a force and tends to move in the direction as per Fleming's left hand rule.

• Fleming left hand rule:

If the first and the second finger and the thumb of the left hand are held so that they are at right angle to each other, then the thumb shows the direction of the force on the conductor, the first finger points towards the direction of the magnetic field and the second finger shows the direction of the current in the wire.

Fleming left hand rule

• One of the types of shunt resistor ammeter is the Galvanometer.

Galvanometer is used to measure small value of current along with its direction and strength. It is mainly used onboard to detect and compare different circuits in a system.

A Galvanometer

• How to design an ammeter:

Equations:Rm: internal resistance of the moving coil.

Rs: shunt resistance (your design).Im: full scale deflection of the moving current.Is: current shuntI: full scale current of the ammeter including the shunt (the current

to be measured).

B. Multirange Ammeter:

Several values of shunt resistors and a rotary switch are used to select the desired range of current to measure. A make-before break rotary switch insures that there is a shunt resistor connected at all times.

C. Universal shunt: A shunt used to increase the range of a

galvanometer without changing the damping. Also known as Ayrton shunt. An Aryton shunt uses a combination of resistors.

• NOTE : 1. Make sure it is placed in the circuit series so

you can measure the electrical current at a specific point.

2. Make sure that the polarities are correct so not to get a negative value.