Rectifier Ammeters

Rectifier Ammeters Points : Rectifier Ammeters, Ranges of Rectifier Ammeters , Precautions of Rectifier Ammeters For current measurement they find their greatest use as milliammeters, the circuit employed being the same as in voltmeter. The current to be measured causes an alternating potential drops across the input terminals, its’ magnitude is a fraction of a volt: depending on the input impedance of the bridge circuit. This ac. voltage drops give unidirectional pulses of current, the peak value of which depends on the forward resistance of the moving coil instrument, which is generally designed to five scale deflection with 500mV, with as low a resistance as possible.

It is not possible to employ a shunt (as in d.c. moving coil instruments), as the proportional of total current passing through the movement would vary widely with the total current,, due to change of rectifier resistance .with current. The resultant scale shape would depart so far from uniformly as to be practically useless. Hence for higher currents, the current transformers are employed. When a current transformer is used the secondary (instrument) current is proportional to the primary current (the measured current) whatever- the value of the instrument resistance, provided this resistance is not too high. The current transformer employed for this purpose differ in some particulars from those which will be described in further chapter.

The effect of temperature variations, capacitance, and form factor discussed in case of rectifier voltmeters also apply to rectifier ammeters. Ranges of Rectifier Ammeters (i) 0-100µA; up to 0-100mA
(ii) 0-1.0V up to 0-250V with out external resistance
(iii) Frequency range 20 to 20,000Hz Precautions of Rectifier Ammeters The circuit to the instrument should never be opened while the rectifier is connected to the line, because this will result in full voltage impressed across the rectifier plates, which may get damaged due to it. Protective fuses should not be provided in the instrument circuit.