Relay Output Guidelines
Several output modules in the DL205 I/O family feature relay outputs: D2–04TRS,
D2–08TR, D2–12TR, D2–08CDR, F2–08TR and F2–08TRS. 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 10 mA
• Loads which must be switched at high speed or heavy duty cycle
Relay outputs in the DL205 output modules are available in
two contact arrangements, shown to the right. The Form A
type, or SPST (single pole, single throw) type is normally open
and is the simplest to use. The Form C type, or SPDT (single
pole, double throw) type has a center contact which moves and
a stationary contact on either side. This provides a normally
closed contact and a normally open contact.
Some relay output modules relays share common 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.
Surge Suppression For Inductive Loads
Inductive load devices (devices with a coil) generate transient voltages when de-energized with
a relay contact. When a relay contact is closed it, “bounces”, which energizes and de-energizes
the coil until the “bouncing” stops. The transient voltages generated are much larger in
amplitude than the supply voltage, especially with a DC supply voltage.
When switching a DC-supplied inductive load, the full supply voltage is always present when
the relay contact opens (or “bounces”). When switching an AC-supplied inductive load, there
are two (2) points when the voltage is zero (0) in one complete cycle of a sine wave; therefore,
there are two (2) chances in 60 (60 Hz) or 50 (50 Hz) to stop the current flow at a zero
crossover point. If current flow isn’t stopped, the relay contact will open (or “bounce”). If the
voltage is not zero when the relay contact opens, there is energy stored in the inductor that is
released when the voltage to the inductor is suddenly removed. This release of energy is the
cause of the transient voltages.
When inductive load devices (motors, motor starters, interposing relays, solenoids, valves,
etc.) are controlled with relay contacts, it is recommended that a surge suppression device be
connected directly across the coil of the field device. If the inductive device has plug-type
connectors, the suppression device can be installed on the terminal block of the relay output.
Relay with Form A contacts
Relay with Form C contacts
Chapter 3: I/O Wiring and Specifications
3–9
DL205 Installation and I/O Manual, 2nd Edition
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