51-040.005. datasheet(39/64 Pages) EAO

39 / 64 page

Application guidelines

01.2010

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 connected in parallel. This free-wheeling

diode provides a path for the inductor current to flow when the current is interrupted by the switch.

Without this free-wheeling diode, the voltage across the coil will be limited only by dielectric break-

down voltages of the circuit or parasitic elements of the coil. This voltage can be kilovolts 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 inductive 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 flowing through the load.

To get an efficient protection, the free-wheeling diode must be connected as close as possible

to the inductive load!

Suppressor circuits

Sveral hundred

to several

thousend volts

OFF

e = L

VDC

Switch

Free-wheeling

diode

Inductive

load

Counter emf

over load without free-wheeling diode

Fig. 2

Switching with inductive load

Fig. 1

51-040.005. Fiches technique(PDF) 39 Page - EAO AG.