DL06 Micro PLC User Manual, 3rd Edition, Rev. E
Chapter 2: Installation, Wiring, and Specifications
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2-21
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.
LOGIC
Koyo
06
AC
Supply
L L
L L
C0
C4
C2
X1
X3
X4
X6
X11
X13 X14
X16
X21
X23 N.C.
C1
C3
X2
X5
X7
X10
X12
X15
X17
X20
X22
X0
N.C.
L L
L L
AC(N) 24V
0V
N.C.
C1
C3
Y0
Y15
Y12
Y10
Y17
Y7
Y5
Y2
C0
C2
Y16
Y14
Y13
Y11
Y6
Y4
Y3
Y1
LG
G
AC(L)
D0-06AR
40VA
50-60Hz
PWR: 100-240V
2.0A
OUTPUT: 6-240V
50 - 60Hz
2.0A, 6 - 27V
Y
X
7 - 15mA
INPUT: 90 - 120V
0
1
2
3
4
5
6
7
10
11
12
13
14
15
16
17
20
21 22
23
fuse
+24 VDC
+
-
N
L
Oscilloscope
Relay Coil
(24V/125mA/3W,
AutomationDirect part no.
750R-2C-24D)
24 VDC
+
-
160
140
120
100
40
20
-20
Volts
80
60
0
Example: Circuit with no Suppression
