Chapter 8: Installation and Wiring
Do-more T1H Series PLC User Manual, 1st Edition, Rev. C
8-31
Relay Output Guidelines
Several output modules in the Terminator I/O family feature relay outputs: T1K-08TR,
T1K-16TR, and T1K-08TRS. Relays are best for the following applications:
•
Loads that require higher currents than the solid-state outputs can deliver
•
Cost-sensitive applications
•
Some output channels need isolation from other outputs (such as when some loads
require different voltages than other loads)
Some applications in which NOT to use relays:
•
Loads that require currents under 10mA
•
Loads which must be switched at high speed or heavy duty cycle
Relay output module contacts are available in
Form A type, or SPST (single pole, single throw)
normally open.
Some relay output module’s share common relay
terminals, which connect to the wiper contact in
each relay of the bank. Other relay modules have
relays which are completely isolated from each
other. In all cases, the module drives the relay coil
when the corresponding output point is on.
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.
Relay with Form A contacts
