e997022.pdf

supply voltage monitor

022

12V

G. Kleine

A circuit for monitoring supply voltages of ±5 V and ±12 V is

readily constructed as shown in the diagram. It is appreciably sim-

pler than the usual monitors that use comparators, and AND

gates. The circuit is not intended to indicate the level of the inputs.

In normal operation, transistors T 1 and T 3 must be seen as cur-

rent sources. The drop across resistors R 1 and R 2 is 6.3 V

(12–5–0.7). This means that the current is 6.3 mA and this flows

through diode D 1 when all four voltages are present. However, if

for instance, the –5 V line fails, transistor T 3 remains on but the

base-emitter junction of T 2 is no longer biased, so that this tran-

sistor is cut off. When this happens, there is no current through

D 1 which then goes out.

BC558

green

groen

grün

vert

BC548

BC558

[994026]

1N4148

BC548

12V

994026 - 11