Automation Direct DL205 Manual page 37

If the contact is switching a DC inductive load, add a diode across the load as near to
load coil as possible. 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 as the contacts are opening.
For best results, follow these guidelines in using a noise suppression diode:
•
DO NOT use this circuit with an AC power supply.
•
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.
S
Be sure the diode is in the circuit correctly before operation. If installed
backwards, it short-circuits the supply when the relay energizes.
I/O Wiring and Specifications
DL205 Installation and I/O Manual, Rev. B
3–13