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April 2004

AN319

APPLICATION NOTE

RELAY DRIVE PROTECTION

Rev. D2A - 3569

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

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.

a

L

R

r

C

A