| Author | Message | | | eric | | Posted: Fri Sep 01, 2006 6:08 pm Post subject: Supply voltage monitor | | | | Site Admin Joined: 01 Jan 1970 Posts: 311 | 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 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 6.3 mA 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
D1 which then
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