4 1 Channel Relay Breakout (30A) – User Gui e
A itional Information
Using relay mo ules with in uctive loa s
It is important to take a itional care when using relays with in uctive loa s. An in uctive loa is
pretty much anything that has a coil an works base on magnetic principles like Motors, Solenoi s
an transformers. In uctive loa s pro uce back emf when the magnitu e of the loa current changes.
The back emf can be in the or er of tens or even hun re s of voltage (See this Wikipe ia article
http://en.wikipe ia.org/wiki/Counter-electromotive_force). This effect is most severe when power is
isconnecte from in uctive loa because the rate of change of current is maximum at that point.
Even though the back emf lives only for a very short time (a few millisecon s) it can cause sparks
between the relay contacts an can eteriorate the contact quality over time an re uce the life span
for the relays consi erably.
So it is important to take countermeasures to suppress the back emf to acceptable levels to protect
relay contacts. Usually this requires connecting electronic evices in parallel with the loa such that
they absorb the high voltage components generate by the loa . For solenoi s, connecting a io e
(fast switching io e is recommen e ) in parallel to the
loa (in reverse irection to the loa current) is very
effective. A io e use for this purpose is usually calle a
freewheeling io e. Please see the iagram on the right
for connection etails.
A capacitor with proper rating is recommen e for protecting the relay contacts when a motor is use
as loa . The capacitor shoul be rate enough to withstan the back emf that is generate by the
motor. Please see the iagram below for connection etails.
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