Datasheet

Application guidelines
Suppressor circuits
When switching inductive loads such as relays, DC motors, and
DC solenoids, it is always important to absorb surges (e.
g. with a
diode) to protect the contacts. When these inductive loads are
switched off, a counter emf can severely damage switch contacts
and greatly shorten lifetime.
Fig. 1 shows an inductive load with a free-wheeling diode connec
-
ted in parallel. This free-wheeling diode provides a path for the
inductor current to ow when the current is interrupted by the
switch. Without this free-wheeling diode, the voltage across the
coil will be limited only by dielectric breakdown voltages of the
circuit or parasitic elements of the coil. This voltage can be kilo
-
volts in amplitude even when nominal circuit voltages are low
(e. g. 12 VDC) see Fig. 2.
The free-wheeling diode should be chosen so that the reverse
breakdown voltage is greater than the voltage driving the induc
tive
load. The DC blocking voltage (VR) of the free-wheeling diode can
be found in the datasheet of a diode. The forward current should
be equal or greater than the maximum current owing through the
load.
To get an efficient protection, the free-wheeling diode must be connected as
close as possible to the inductive load!
Several hundred
to several
thousend volts
ON OFF
0
e = L
di
dt
__
V
DC
Switch
Free-wheeling
diode
Inductive
load
Counter EMF
over load without free-wheeling diode
Fig. 2
Switching with inductive load
Fig. 1
+
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