This is simple VCO ( voltage controlled oscillator) circuit , connected to your current instrumentation by an optoisolator, means that you can measure high voltages of your instrumentation more secure. The component values suit a 0 to 600 V input range (power dissipation in R1 and R2 set a limit on the input-voltage range). The circuit’s linearity is not an issue, because you can linearize its output in software.

The input voltage (VI), charges capacitor C1 until zener diode D1 conducts. Then, the zener diode triggers an “avalanche” circuit that discharges C1 into optocoupler Q1. After Cl discharges, the charging cycle repeats. C1 also averages the sensed-voltage level, which thereby provides noise immunity.

The optocoupler’s output is a pulse train whose frequency increases with increasing input voltage. To develop a linearizing equation for the circuit, measure its output at two convenient, widely spaced input voltages. Then plug the resulting periods into this second-order polynomial approximation and solve the two simultaneous equations for the two constants, k, and k2 :

V2 is zener voltage of D1.

**Parts list :**

R1,R2 : 82K (1/2W)

R3 : 1M

R4 : 1K

R5 : 200

R6 :100K

C1 : 3,3uF

C2 : 470pF

L1 : 10uH

D1 : 1N4691 (6V2)

D2 : 1N4148

T1 : BC557B

T2 : BC547B

Optocoupler : 6N135, 6N136, or 6N137

Tags: isolated high voltage power supply monitor Isolated Voltage Sensors measure high voltages power supply monitor circuit simple VCO voltage controlled oscillator Voltage power supply monitor