National Instruments NI PCI-8254R - Manual (Page 25)

Chapter 2 Digital I/O
NI PCI-8254R User Manual 2-14 ni.com
Protecting Inductive Loads
When an inductive load, such as a relay or solenoid, is connected to an
output, a large counter-electromotive force may occur at switching time
due to energy stored in the inductive load. This flyback voltage can damage
the outputs and the power supply.
To limit flyback voltages at the inductive load, install a flyback diode across
the load. Mount the flyback diode as close to the load as possible. Use this
protection method if you connect any of the isolated outputs on the
NI 8254R to an inductive load.
The following figure shows an example of using an external flyback diode
to protect inductive loads.
Digital
Output
V
C
Vcc
NI 8254R
External
Flyback
Diode for
Inductive Loads
Load