D.C.Chopper type Voltmeter

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D.C.Chopper type Voltmeter

It has sensitivity of one micro -volt per scale division.

It has  high impedance of input of the order of hundred mega ohms.

construction and working:-

In chopper amplifier dc voltmeter ,Photo diodes change its resistance under different illumination conditions, this property of photo diode is used in chopper amplifier. Its resistance changes from the order of few mega-ohms to few hundred ohms when it is illuminated by a light source in the dark place.

In dc voltmeter circuit two neon lamps are used , these are supplied by an oscillator for alter­nate half cycles. Two photo diodes are used in input stage which acts as half-wave modu­lators because of its alternate switching action by the neon lamps at the frequency of oscillator.

Output of chopper modulator is a square wave voltage (proportional to the input signal) which is supplied to the ac amplifier through a capacitor and Amplified output is again passed through a capacitor and then fed to chopper demodulator. Capacitor is used to remove dc drift from the signal.

The Chopper demodulator gives a dc output voltage (proportional to the input voltage) which is passed through the low pass filter to remove any residual ac component and this dc output voltage is supplied to the PMMC meter for measurement of input voltage.

 It is highly sensitive dc electronic  volt­meters (in this Chopper type dc amplifier is used)

input impedance is of the order of hundred mega-ohms.

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