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# Overvoltage Sense Circuit

IP.com Disclosure Number: IPCOM000075461D
Original Publication Date: 1971-Sep-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 40K

IBM

## Related People

Duspiva, WS: AUTHOR

## Abstract

Quite often it is necessary to protect electronic components from over-voltage surges at very high levels (500 to 1,000 volts or more). The basic problem is to sense a change in some high voltage at a lower level, in order to use lower voltage devices. If a simple resistive divider is used, much accuracy is lost due to its attenuation.

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Overvoltage Sense Circuit

Quite often it is necessary to protect electronic components from over- voltage surges at very high levels (500 to 1,000 volts or more). The basic problem is to sense a change in some high voltage at a lower level, in order to use lower voltage devices. If a simple resistive divider is used, much accuracy is lost due to its attenuation.

The circuit shown in the figure overcomes this problem by the use of a precision current source formed by transistors Q1, Q2 and Q3 and their associated components. Zener diode D1 and resistors R4 and R5 provide a reference for the current source. The zener diode also serves as a reference for a threshold network (Comparator or SCS).

A function, VY, of the unknown voltage to be sensed, VX, is presented to the threshold network by a resistive divider formed by R1, RX and R2. The attenuation from VX to VY is designed to be small. Ordinarily, the DC level at VY would be quite high but the current source is designed to reduce this level to a much lower value. This value is given by the equation:

(Image Omitted)

The drop across R3 is made equal to the reference node between R4 and R5 by the voltage-follower network action of differential amplifier Q2-Q3 and Q1 as its output stage. R6, R7 and R8 form a biasing network for Q1. R9 is the emitter load for Q2 and Q3, and C1-R10 forms a stabilizing roll-off network.

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