Automation Direct DL205 Manual page 35

Prolonging Relay Relay contacts wear according to the amount of relay switching, amount of spark
Contact Life created at the time of open or closure, and presence of airborne contaminants.
There are some steps you can take to help prolong the life of relay contacts, such as
switching the relay on or off only when it is necessary, and if possible, switching the
load on or off at a time when it will draw the least current. Also, take measures to
suppress inductive voltage spikes from inductive DC loads such as contactors and
solenoids.
For inductive loads in DC circuits we recommend using a suppression diode as
shown in the following diagram (DO NOT use this circuit with an AC power supply).
When the load is energized the diode is reverse-biased (high impedance). When the
load is turned off, energy stored in its coil is released in the form of a negative-going
voltage spike. At this moment the diode is forward-biased (low impedance) and
shunts the energy to ground. This protects the relay contacts from the high voltage
arc that would occur just as the contacts are opening.
Place the diode as close to the inductive field device as possible. Use a diode with a
peak inverse voltage rating (PIV) at least 100 PIV, 3A forward current or larger. Use a
fast-recovery type (such as Schottky type). DO NOT use a small-signal diode such
as 1N914, 1N941, etc. Be sure the diode is in the circuit correctly before operation. If
installed backwards, it short-circuits the supply when the relay energizes.
Adding external contact protection may extend relay life beyond the number of
contact cycles listed in the specification tables for relay modules. High current
inductive loads such as clutches, brakes, motors, direct-acting solenoid valves, and
motor starters will benefit the most from external contact protection.
PLC Relay Output
Output
Common
I/O Wiring and Specifications
Input
Supply
+ – Common
DL205 Installation and I/O Manual, Rev. B
3–11
Inductive Field Device