Managing Inductive Load Challenges - Schlage RC Pure IP User Manual

Appendix B: Additional wiring considerations | Managing inductive load challenges

Managing inductive load challenges

Most door latches use a relay coil that powers up and
down, when the door is unlocked and locked. When
this happens, a pulse of electrical energy is produced
by the lock's coil. This pulse is called back EMF, and
can interfere with the reader's operation.
Switching off a typical 12 VDC relay coil can produce
a back EMF pulse of 300 volts or more. If this voltage
pulse is allowed to flow back into the reader, it can
cause the reader to "brown out" and the reader will
reboot.
Figure 5.2: Inductive load control (two options
shown) shows a solution. You can virtually eliminate
back EMF by installing a transient suppression
device (diode). Each Reader Controller is supplied
with a diode assembly, which simplifies the
installation process. A standard diode, from any
electronic supply store, can also be used. 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.
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. lightning), we recommend using a
fast-switching transient voltage suppressor.
20 • Schlage • Reader controller
Standard diode
Figure 5.2: Inductive load control (two options shown)
Diode
assembly
(+DC)
(1N4007 or
equivalent)
(-DC)
(+DC)
(-DC)