Peak Reading AC Voltmeter

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 V_m or V_m = V_m /2 πf_c 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 V_m.

Figure A

Figure B

• Voltage across changes the storage capacitor C_S. C_S 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. R_d is a discharged resistor employed to permit variation 0 and V_m. whenever V₂ is reduced. The discharged time constant C_S  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 C_m 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 C_m
• The voltage developed across C_m 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.

Figure C