Ammeters for Current Measurement

Demonstrative Video

Ammeters

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  • The ideal ammeter has zero internal resistance.

  • But practically the ammeter has small internal resistance.

  • The measuring range of the ammeter depends on the value of resistance.

Working Principle & Connection

  • The working principle of an ammeter depends on the current flowing along with its resistance.

  • Very little impedance is used inside the ammeter as it must drop the least amount of voltage attached to it.

  • It is connected to the series of circuits as the current in the series circuit is the same.

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Types of Ammeter

  • Depending on their design

    • Permanent moving coil ammeter.

    • Moving iron/coil ammeter.

    • Electro-dynamometer ammeter.

    • Rectifier type ammeter.

  • By the type of current flowing

    • DC ammeter

    • AC ammeter

  • Permanent moving coil ammeter

    • used only for the measurement of the direct current

    • conductor is placed between the pole of the permanent magnet.

    • When the current flows through the coil, it starts deflecting.

    • The deflection of the coil depends on the magnitude of current flows through it.

  • Moving iron/coil ammeter

    • measures both the alternating and direct current

    • coil freely moves between the poles of a permanent magnet. When the current passes through the coil, it starts deflecting at a certain angle. The deflection of the coil is proportional to the current passes through the coil.

  • Electro-dynamometer ammeter

    • measurement of both AC and DC

    • The accuracy of the instrument is high as compared to the PMMC and MI instrument.

    • The calibration of the instrument is same both for AC and DC, i.e. if DC calibrates the instrument then without re-calibration, it is used for AC measurement.

  • Rectifier type ammeter

    • used for measuring the alternating current.

    • The instruments using the rectifying instrument which converts the direction of current and pass it to the PMMC instrument.

    • Such type of instrument is used for measuring the current in the communication circuit.

Ammeter Shunt

  • The high current directly passes through the ammeter which damages their internal circuit.

  • This problem can be removed by connecting the shunt resistance in parallel with the ammeter.

  • The major portion of the current passes through the shunt resistance.

  • The shunt resistance will not affect the working of the ammeter, i.e., the movement of the coil remains same.

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Effect of Temperature in Ammeter

  • Ammeter is sensitive to the surrounding temperature which causes the error in the reading.

  • This can reduce by swamping resistance.

  • The resistance having zero temperature coefficient is known as the swamping resistance.

  • It connects in series with the ammeter.

  • The swamping resistance reduces the effect of temperature on the meter.

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  • The ammeter has the inbuilt fuse which protects the ammeter from the heavy current.

  • If substantial current flows through the ammeter, the fuse will break.

  • Ammeter would not measure the current until the fuse is replaced