Nematron Pointe Controller - Manual (Page 90)

Chapter 4: System Design and Installation Pointe Controller User Guide
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When the contact is closed, the diode is reverse biased and no current flows
through it. When the contact opens, current will continue to flow through the
inductive load. The diode provides a path for current flow. The result that is the
energy is dissipated in the inductive coil and not the relay contact.
NOTE: Do not use this circuit for AC loads.
Transistor Outputs
An NPN transistor sinking output provides a path to ground. A typical circuit is
shown below:
There is a small voltage drop across the transistor in such a circuit. The voltage
drop will generate heat in the transistor. Therefore NPN transistor outputs are
generally limited to lower current applications.
Transistor outputs can be operated at high frequency. There is no effective wear
on a transistor output from switching, as there is in a mechanical relay.
Diode protection applied to inductive loads is recommended in cases where the
load current approaches the rated current limit of the output. In most cases
OptiLogic outputs are designed to withstand voltages of at least twice the rated
output voltage. However, diode protection like that shown above will ensure that
turn off voltage spikes will never get to that level.