Datasheet
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ST
Application guidelines
Suppressor circuits
LED polarity
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 efcient protection, the free-wheeling diode must
be connected as close as possible to the inductive load!
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 con
-
nected 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.
When tting the LED elements the polarity has to correspond with
the respective terminals, (+) goes to +.
Several hundred
to several
thousend volts
ON OFF
0
e = L
di
dt
__
VDC
Switch
Free-wheeling
diode
Inductive
load
Counter EMF
over load without free-wheeling diode
Fig. 2
Switching with inductive load
Fig. 1
+
_
+X2
(-X1)
(+X2)
01