Supply Voltage Monitor

A circuit for monitoring
supply voltages of ±5 V and ±12 V is readily constructed as shown in
the diagram. It is appreciably simpler 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 T1 and T3 must be seen as current sources. The drop across
resistors R1 and R2 is 6.3 V (12 –5 –0.7). This means that the current
is 6.3mA and this flows through diode D1 when all four voltages are
present. However, if for instance, the –5 V line fails, transistor T3
remains on but the base-emitter junction of T2 is no longer biased, so
that this transistor is cut off. When this happens, there is no current
through D which then goes out.

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