Inductive Loads in Life Safety Applications
Key considerations in eectively
balancing power integration
August 2016
WHITE
PAPER
WHITE
PAPER
July 2014
2
Contents
Introduction ......................................3
Inductive Loads...................................3
Protecting Against Back-EMF ......................5
Practical Applications .............................6
WHITE
PAPER
WHITE
PAPER
Inductive Loads in Life Safety Applications
3
Introduction
Many loads in the life safety industry are inductive in nature.
Inductive loads are devices which use coils of wire to gen-
erate a magnetic field - these include magnetic locks, door
strikes, solenoids, crash bars, and electric motors as well as
other devices. Unlike a simple resistive load, inductive loads
can present some challenges when integrating them into a
DC power system.
Inductive Loads
To understand the eects of an inductive load, we must first
understand the basic operation of an inductive load. Ignoring
the function of the load, we will just view it as a simple coil of
wire for this discussion.
When an inductive load is powered, a magnetic field is gener-
ated in the coil of wire. Faraday’s Law states that a change
in the magnetic field around a circuit will generate an elec-
tromotive force (voltage) in that circuit. This means that the
magnetic field generated causes the inductor to resist chang-
es in current through it by generating a voltage to oset the
current change (called a back electromotive force, or back-
EMF). This voltage is determined by the amount of change in
current and the time in which the change occurs - increasing
the change in current or shortening the time period in which
the change occurs will both result in an increased back-EMF.
WHITE
PAPER
July 2014
4
In a life safety application, devices such as magnetic locks
are typically controlled by a relay contact. When the relay
contact in the circuit of a simple magnetic load is opened,
the current instantly drops to zero. This large, fast change in
current causes the magnetic field to collapse, briefly causing
the coil to become a high voltage source and feeding a short
duration, negative high voltage spike back into the wiring.
This returned high voltage spike can cause arcing of relay con-
tacts as the high voltage looks for a path to dissipate, greatly
shortening relay life. The negative voltage can also travel to
other sensitive devices in the system, causing problems such
as lockup of microprocessors, false triggering of overcurrent
protection, and possibly damaging devices within the system.
+
+
–
–
I
I
+
+
–
–
I
I
WHITE
PAPER
WHITE
PAPER
Inductive Loads in Life Safety Applications
5
Protecting against Back-EMF
Now that we understand back-EMF and the negative eects
it can have on a power system, how do we handle it? Typi-
cally, the EMF is returned to a lock or other device by placing
a reverse-biased diode across the lock circuit, after the switch
(relay).
A diode is an electrical “check valve”, allowing current to only
flow in one direction from the anode to the cathode.
Under normal powered conditions, the diode is reverse bi-
ased and not conducting current, allowing the current to flow
through the lock normally.
current ow
Anode Cathode
+
+
–
–
I
I
WHITE
PAPER
July 2014
6
When the switch is opened, the negative voltage will flow
back through the diode, returning to the lock to be dissipated
by the lock’s internal resistance.
Be aware that this return of voltage to the lock does result in
power remaining on the lock for a very brief moment as the
voltage continues to circulate. In a fail-secure door strike ap-
plication, this means a slightly delayed relocking of the strike.
In a mag lock or fail-safe door strike application, it means a
brief delay in RELEASE of the lock. Typically, this delay is well
under 1/4 of a second and poses no problem, but in extreme
cases may result in a perceptible delay between card swipe
and the ability to open the door.
Practical Applications
Many of today’s locks handle back-EMF internally, either by
reverse diode or other methods. These locks do not require
external protection. However the internal back-EMF diodes
built into locks often fail over time, causing problems to ap-
pear months, or even years down the road. Many new locks
do not have any back-EMF protection . Older pre-installed
locks also may have no protection, or the protection may
have failed years ago. A good practice is to place a reverse
diode on all locking circuits as cheap insurance, regardless of
the protection that may or may not be present internally to
the lock.
+
–
I
I
WHITE
PAPER
WHITE
PAPER
Inductive Loads in Life Safety Applications
7
LifeSafety Power’s C8 lock control boards now have back-
EMF diodes across each set of output terminals from the fac-
tory, eliminating the need for external diodes.
External diodes for back-EMF protection should be placed as
close to the lock, electrically, as possible. Given an ideal situ-
ation, this means placing the diode at the lock itself. Obvi-
ously this is not always possible, especially in retrofit applica-
tions where the locks are already installed. In these cases, it
is acceptable to place the diodes at the power supply side of
the lock circuit (after the relay).
When using an external diode, most any general purpose
power diode will work. Avoid small signal diodes, as they
cannot handle the voltage and current levels generated by
the back-EMF of most locks. The 1N4000 series is inexpen-
sive and easily obtainable and works well as back-EMF pro-
tection.
+
–
Power
Supply
Mag Lock
Ideal
+
–
Power
Supply
Mag Lock
Acceptable
WHITE
PAPER
July 2014
8
Conclusion
LifeSafety Power’s intelligent power solutions provide com-
prehensive and proactive built-in measures to address poten-
tial challenges integrating inductive load inputs to DC voltage
circuits, eectively minimizing voltage spikes and other pos-
sible system compromise.
WHITE
PAPER
WHITE
PAPER
Inductive Loads in Life Safety Applications
9
About LifeSafety Power — Power is Knowledge™
LifeSafety Power is the leader in Smart Power Solutions and patented
remote monitoring capabilities, providing modular AC, DC, and PoE pow-
er systems that meet the growing needs of the life safety and security
industries. Realizing that network technology presents new opportuni-
ties for active monitoring and management of power supplies connected
to access control systems, fire systems, video surveillance and more, the
company has built its products from day one with intelligence and func-
tionality in mind. LifeSafety Power’s current product oering and planned
future innovations in battery test, display and diagnostics represent an
important step in providing overall system reliability and uptime.
All of the product features discussed in this white paper are available
within LifeSafety Power’s product line.
Visit www.lifesafetypower.com for more information.
Company Contacts:
Joseph Holland
VP of Engineering
jholland@lifesafetypower.com
John Olliver
Sr VP of Sales
jolliver@lifesafetypower.com
Factory
PH 888.577.2898
EM techsupport@lifesafetypower.com
For more information about the FlexPower Power System, visit
www.lifesafetypower.com
© 2016 LifeSafety Power Inc. All rights reserved. LifeSafety Power, and FlexPower are
registered trademarks of LifeSafety Power Inc. or its aliates. Product specifications are
subject to change without notice. This material is provided for informational purposes
only; LifeSafety Power assumes no liability related to its use.
10
