24 VDC
DC MOV or TVS Diode Circuit
Sinking
Sourcing
+
_
ZL-TSD8-24
Transorb Module
AC Coils:
Two options for AC coils are MOVs or bi-directional TVS diodes. These devices are most
effective at protecting the driver from a transient voltage when connected across the driver
(PLC output) but are also commonly connected across the coil. The optimum voltage rating
for the suppressor is the lowest rated voltage available that will NOT conduct at the supply
voltage, while allowing a safe margin.
AutomationDirectβs ZL-TSD8-120 transorb module is a good choice for 120VAC circuits. It
is a bank of eight bi-directional 180V TVS diodes.
VAC
C MOV or Bi-Directional Diode Circuit
ZL-TSD8-120
Transorb Module
NOTE: Manufacturers of devices with coils frequently offer MOV or TVS diode suppressors as an add-
on option which mount conveniently across the coil. Before using them, carefully check the suppressor
ratings. Just because the suppressor is made speciο¬cally for that part does not mean it will reduce the
transient voltages to an acceptable level.
For example, a MOV or TVS diode rated for use on 24-48 VDC coils would need to have a
high enough voltage rating to NOT conduct at 48V. That suppressor might typically start
conducting at roughly 60VDC. If it were mounted across a 24V coil, transients of roughly
84V (if sinking output) or -60V (if sourcing output) could reach the PLC output. Many
semiconductor PLC outputs cannot tolerate such levels.
Two more common options for DC coils are Metal Oxide Varistors (MOV) or TVS diodes.
These devices should be connected across the driver (PLC output) for best protection as
shown below. The optimum voltage rating for the suppressor is the lowest rated voltage
available that will NOT conduct at the supply voltage, while allowing a safe margin.
AutomationDirectβs ZL-TSD8-24 transorb module is a good choice for 24VDC circuits. It is
a bank of 8 uni-directional 30V TVS diodes. Since they are uni-directional, be sure to observe
the polarity during installation. MOVs or bi-directional TVS diodes would install at the same
location, but have no polarity concerns.
CLICK PLC Hardware User Manual, 5th Edition, Rev. F β C0-USER-M
3β36
Chapter 3: Installation and Wiring
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