Peak Reading AC Voltmeter

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Peak Reading AC Voltmeter

  • In some cases peak valve of an ac wave from is more important. (necessary to obtain maximum dielectric strength of insulating solids etc.)
  • When waveform is not sinusoidal RMS valve of the voltage multiplied by √2 is not correct. Hence separate peak valve instrument is needed.

Series Capacitor Peak Voltmeter :

  • When a capacitor C2 is connected to a sinusoidal voltage source, the entering 0 to current is Where V – RMS valve of voltage and co angular frequency.
  • If half wave rectifier is used the arithmetic mean of rectifier current is proportional to the peak valve AC voltage.
  • The DC meter reading is proportional to peak value of valve Vm or Vm = Vm /2 πfc Chaftfrotscue method.
  • Not suitable when waveforms is not sinusoidal or more than are peak on each half cycle.
  • P – Protective device and I – Indicate meter.
  • Very high voltage cannot reduce directly and are potential divider preferably of capacitance type. 
  • Figures A and B shows a simple peak voltmeter circuit with capacitor voltage divider which reduces the voltage V to be measured to a low voltage Vm.
Peak Reading Voltage for C Potentiometer with or Resistances Divider
Figure A

Capacitor Divider
Figure B

  • Voltage across changes the storage capacitor CS. CS is charged to a voltage proportional to the peak valve to be measured voltage can be measured by an electrostatic voltmeter or other suitable (V.T.V.M.) voltmeters with high input impedance. Rd is a discharged resistor employed to permit variation 0 and Vm. whenever V2 is reduced. The discharged time constant CS  is desired to be about 1 to 10 sec this gives rise a discharge error which depends on the frequency of the supply voltage.
  • To compensable for charging and discharging error due to resistance circuit can be modified as shown in Figure B.

    Peak Reading Voltmeter for Impulse Voltage

    • Methods are similar to employed for ac crest valve measurement.
    • Instrument is normally connected to low voltage arm of potential dividers.
    • Basic circuit and equivalent circuit are shown in Figure C.
    • Circuit consists of any valve rectifiers.
    • Diode D conducts for the voltage only for negative pulses to diode due to connected in reverse.
    • When a voltage impulse V (t) appears across the low voltage arm of potential divider the capacitor Cm is charged to the peak valve of the pulse.
    • When amplitude of signal starts decreasing the diode becomes reverse biased and prevents the discharging to the capacitor Cm
    • The voltage developed across Cm is measured by a high impedance voltage (Electrostatic or an electrostatic).
    • As diode D has finite forward resistance the voltage to which Cm is charged will be less than the actual peak of the signal and is measuring capacitances em 10.
    • The error can be estimated if waveform is known.
    • Actual forward resistance of diode D is difficult to estimate and hence bobo meter is calibrated using an oscilloscope.
    • With potential divider voltage of either polarity is transferred into a proportional the measuring signal by a resistance or capacitor voltage divider and a diode circuit.
    Basic Circuit
    Figure C

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