AN319 Datasheet, PDF (1/7 Pages) STMicroelectronics – RELAY DRIVE PROTECTION

AN319
APPLICATION NOTE
RELAY DRIVE PROTECTION
INTRODUCTION
A fast switch-off in an inductive circuit causes overvoltages and electromagnetic interferences that can
damage peripheral elements. When a relay drive circuit is not protected, it is frequent to find some con-
tacts destroyed by the arc due to the overvoltage or failed transistors after initial use. Various solutions
exist which limit the voltage at the terminals of the switching circuit in order to prevent any damage.
There follow some examples.
PASSIVE COMPONENTS
Resistive network, capacitor (see circuit a, Figure 1).
This is an efficient solution in many applications but generates current peaks that can be inconvenient at
switch-on.
Figure 1. Relay Drive Protection by R.C. Network
a
L
R
r
A
C
ELECTRONIC COMPONENTS
Fast Diodes And Transil
Standard protection which puts a diode in antiparallel with the inductive load (see circuit b, Figure 2) offers
many advantages: negligible overvoltage at switch on (forward voltage of the diode) reduced space, low
price, good reliability and negligible permanent losses.
Unfortunately the current through the inductor does not stop immediately at switch-off; instead it decays
with a time constant L/R. This is a disadvantage in some applications, for example a relay coil driver. The
relay contacts will remain closed for some time after the switch is opened due to the current stored in the
relay coil. It would be desirable to speed up the current decay rate while retaining the advantages of re-
covery diode protection.
April 2004
Rev. D2A - 3569
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