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Diagram A
This circuit consists of
a relay coil being
switched by a
transistor.
When a coil is switched
off, a large BACK EMF
appears across the coil.
This back emf may be
several thousand volts
in value, enough to
destroy the transistor.
The diode, which is
normally reverse biased,
is forward biased by the
back emf, and conducts,
its low resistance short
circuiting the back emf
and protecting the
transistor.
Diagram B
This circuit consists a
meter movement with two
germanium diodes across
it.
Typical movements
require only 100 mV for
full scale deflection of
the pointer.
If a voltage, exceeding
about 0.3 volts, appears
across the circuit then
one of the diodes will
conduct and protect the
movement from excessive
voltage.
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