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If the AC supply fails
then the equipment is
automatically powered by
the standby battery.
While the AC supply is
present, D1 has +15
volts on its anode, and
is forward biased.
Since there is 0.6 volts
across the diode, there
is +14.4 volts on its
cathode.
This voltage powers the
equipment.
D2 is reverse biased
since its anode is less
positive than its
cathode.
D2 is non conducting and
the battery is isolated.
If the supply fails then
the +15 volts
disappears, and D2
becomes forward biased
and conducts, to power
the equipment.
The diodes STEER the
voltages.
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