Relay Outputs - Transient Suppression For Inductive Loads In A Control System - AutomationDirect C0-USER-M Manual

Relay Outputs – Transient Suppression for Inductive Loads in a Control System
The following pages are intended to give a quick overview of the negative effects of transient
voltages on a control system and provide some simple advice on how to effectively minimize
them. The need for transient suppression is often not apparent to the newcomers in the
automation world. Many mysterious errors that can afflict an installation can be traced back to
a lack of transient suppression.
What is a Transient Voltage and Why is it Bad?
Inductive loads (devices with a coil) generate transient voltages as they transition from being
energized to being de-energized. If not suppressed, the transient can be many times greater
than the voltage applied to the coil. These transient voltages can damage PLC outputs or other
electronic devices connected to the circuit, and cause unreliable operation of other electronics
in the general area. Transients must be managed with suppressors for long component life and
reliable operation of the control system.
This example shows a simple circuit with a small 24V/125mA/3W relay. As you can see, when
the switch is opened, thereby de-energizing the coil, the transient voltage generated across the
switch contacts peaks at 140V.
Example: Circuit with no Suppression
+
24 VDC
-
In the same circuit, replacing the relay with a larger 24V, 290mA, 7W relay will generate a
transient voltage exceeding 800V (not shown). Transient voltages like this can cause many
problems, including:
• Relay contacts driving the coil may experience arcing, which
can pit the contacts and reduce the relay's lifespan.
• Solid state (transistor) outputs driving the coil can be damaged if the
transient voltage exceeds the transistor rating. In extreme cases, complete
failure of the output can occur the very first time a coil is de-energized.
• Input circuits, which might be connected to monitor the coil or the
output driver, can also be damaged by the transient voltage.
A very destructive side-effect of the arcing across relay contacts is the electromagnetic interference
(EMI) it can cause. This occurs because the arcing causes a current surge, which releases RF
energy. The entire length of wire between the relay contacts, the coil, and the power source
carries the current surge and becomes an antenna that radiates the RF energy. It will readily
couple into parallel wiring and may disrupt the PLC and other electronics in the area. This EMI
can make an otherwise stable control system behave unpredictably at times.
CLICK PLC Hardware User Manual, 5th Edition, Rev. F – C0-USER-M
Oscilloscope
Relay Coil
(24V/125mA/3W,
AutomationDirect part no.
750-2C-24D)
Chapter 3: Installation and Wiring
Volts
160
140
120
100
80
60
40
20
0
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3–33