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Understanding
Relays
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19 of 27
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RELAYS
WITH DE-SPIKING DIODES
A de-spiking (clamping) diode is connected in parallel with the relay
coil. It is in the reverse biased position when the relay is turned
on; therefore no current will flow through the diode. When the relay
control circuit is opened (turned OFF), current stops flowing through
the coil, causing the magnetic field to collapse. The magnetic lines
of force cut through the coil and induce a counter voltage (a voltage
in reverse polarity) into the winding. The counter voltage begins to
raise. When the bottom side of the diode sees .7 volts more positive
voltage than the top, the diode becomes forward biased, allowing the
excess voltage to pass, completing the circuit to the other end of the
coil. The current flows around in the diode and coil circuit until the
voltage is dissipated.
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