Nematron Pointe Controller - Manual (Page 89)

Pointe Controller User Guide Chapter 4: System Design and Installation
87
4.6.2 Digital Outputs
Digital outputs are used to turn “loads” on and off. “Loads” may be lights,
motors, solenoids, or any type of on/off device found in the “real world.”
Digital outputs in the OptiLogic series come in three types — relay, transistor and
solid state relay. Each type has applications it is best suited for. The following is a
general list of application characteristics for each output type:
Relay
Low contact loss
AC or DC
Moderate to high
current rating
Low cost
Should not be used
for:
o Ultra low current
switching (less than
10mA)
o Sw itching loads at
high frequency
Transistor
DC application only
Low current rating
High frequency
switching
Low cost
Solid State Relay
AC application
Moderate current
Any switching
frequency
Moderate cost
Relay Outputs
Relays are basically electrically controlled mechanical switches. All current
OptiLogic Relay output boards utilize form A relays — i.e. the contact is either
open or closed.
Relays are affected by the type of load that is switched. Inductive loads
(solenoids, motors, etc.) tend to wear the relay much more than resistive loads
(lights, heaters, etc.).
Inductive load wear is due to the fact that inductive loads will continue to
conduct current for a period, even after the circuit is broken. This current flow
builds up opposing polarity charges between the contact segments that just
separated. This makes the two segments attract each other — making opening
the contact more difficult. It also can result in arcing while the contact is being
opened. Arcing, in turn, builds up carbon deposits, i.e. wear.
This situation can be improved for DC inductive circuit loads by the addition of
external diode protection of the circuit. The figure below illustrates diode
protection: