Operating Principal. Use in accordance with supplied product documentation.

The 667 High Voltage monitor is designed to measure dc voltages and de-energise a relay if the measured potential rises above a pre-set limit. It functions by comparing a reference voltage generated by R9 and ZD1 on pin 3 (non-inverting input) of the op amp IC1 fitted to the high voltage monitor pcb, to a signal voltage derived by D1, R22, CR3, VR1 & R8 which is fed onto pin 2 (inverting input). CR4 provides approximately 250mV of hysteresis.

The output voltage on pin 1 of the op amp is normally high (15V) and switches into a low condition when the signal voltage on pin 2 (inverting input) rises above the reference voltage on pin 3 (non-inverting input).

When pin 1 switches from high to low, a negative going pulse on pin 2 of U1 triggers the timing circuit into count mode. The signal voltage on pin 5 will drop to approximately 3VDC whilst the potential on pin 6 remains at 2V DC.

At the end of the preset timed period the output signal from pin 1 of U1 switches from low to high, forcing pin 6 (inverting input) of the op amp more positive than pin 5 non-inverting input). The output voltage from pin 7 switches from high to low, pin 5 is pulled down to approximate 1VDC via D4 & R20 and T1 is switched off causing the relay RL1 to de-energise.

The 16V electronic supply is generated by components CR1 and ZD3 whilst C4 and C5 smooth the rail and maintain a steady dc level devoid of noise and switching spikes that can occur on the dc supply pins 2 (negative) and 10 (positive).

The potentiometer VR1, adjusts the signal voltage on pin 2 (inverting input) of the op amp which adjusts the voltage detection level. Potentiometer VR3, adjusts the frequency of the timer oscillator which provides adjustment of the fault detection - relay de-energisation delay. The third potentiometer VR2, is used where two separate detection levels are required, Boost high volts alarm for example.


To set the high voltage monitor a DC voltage at the required tripping level must be applied to the alarm using pins 2 (-) and 10 (+).

The potentiometer VR1 is adjusted until the red PCB mounted led switches off and the green led switches on. The potentiometer should then be slowly adjusted in the opposite direction until the two LED's change state. Verification of a correct setting is made by adjusting the supply voltage 0.5V negative to the required trip point where the relay (if already de-energised) should energise and the green led will illuminate. The red led should switch on and the green led should switch off when slowly increasing the supply voltage to the required trip point and after a short delay, the relay should de-energise.


Using a suitable counter, measure the frequency of the oscillator on C3/R15/VR3. Adjust VR3 until the reading displayed on the instrument is at the required level.


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