Installation, Wiring,
and Specifications
2--24
Installation, Wiring, and Specifications
DL105 PLC User Manual, 3rd Edition
This figure shows the same circuit with a transorb (T
S) across the coil. Notice that
the voltage spike is significantly reduced.
+24 VDC
-- 2 4 V D C
Module Relay Contact
-- 4 2 V D C
+24 VDC
Use the following table to help select a TVS or MOV suppressor for your application
based on the inductive load voltage.
Surge Suppressors
hhVendor / Catalog Type (TVS, MOV, Diode) Inductive Load Voltage Part Number
AutomationDirect
Transient Voltage
Suppressors
www.automationdirect.com
TVS
TVS
110/120 VAC
24 VDC
ZL--TD8--120
ZL--TD8--24
General Instrument Transient Voltage
Suppressors and LiteOn Diodes: from-
Digi--Key Catalog: www.digikey.com;
Phone: 1--800--344--4539
TVS, MOV
TVS, MOV
TVS
Diode
110/120 VAC
220/240 VAC
12/24 VDC or VAC
12/24 VDC or VAC
Contact Digi--Key, Corp.
catalog or website
Relay contacts wear according to the amount of relay switching, amount of spark
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 c urrent. 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.
Inductive Field Device
+--
PLC Relay Ou tput
Output
Common
Input
Common
Supply
Prolonging Relay
Contact Life
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