User's Manual

How to Install the ISONAS IP-Enabled Reader-controller 35
2.2.11: MANAGING INDUCTIVE LOAD CHALLANGES
Most door latches use a relay coil that powers up and down to open and close the
door. When this happens, electricity enters the connected circuit. This problem,
known as back EMF, produces network interference that usually becomes more
pronounced when the device is switched off.
Switching off a typical 12 VDC relay coil can produce a back EMF of 300 volts or
more. If this relay is switched via an output, that voltage appears across the
terminals of the output. The problem gets worse as switching voltage/current rises.
Figure 19 shows a solution. You can virtually eliminate back EMF by installing a
transient suppression device (diode). Always check that the diode is correctly
rated for the circuit voltage. For optimum performance, the diode should be installed
at the lock or close to the lock. Standard diodes have a stripe-band marking on one
side. That side of the diode should be connected to the “+” wire of the lock circuit.
Figure 19
Protect the Digital
Output
Which type of transient
suppressor should you
install? This depends
mainly on the type of
inductive load being
switched. Some locks
have Back EMF protection
built into the lock itself.
For Back EMF in low-
voltage DC applications, a
1N4007 diode will suffice.
However, for protection
against other transient
voltages (i.e. lightening),
we recommend using a
fast-switching transient
voltage suppressor, such
as a bipolar TranZorb.