Query the Access Control Expert

Oct. 27, 2008
Choosing the Power Supply for Access Control Systems

Deciding on the Power Supply
How do I decide what type of power supply to use with my access system?

A: Locks are designed to be powered by 12 or 24 volts of AC or DC voltage. The higher voltage can be an advantage where distance will separate the lock and the power source because less current is required to operate a 24 volt lock than a 12 volt model.

The Amount of Power Needed
How do I decide how much power I need?

A: Consider the maximum amount of power required for all locks connected to a particular power supply. To determine this, decide the maximum number of devices that could be used at the same time and then determine what the maximum amount of power each could consume.

Say you have a magnetic lock on a front door and a fail safe standard strike. Since the magnetic lock needs power continuously to stay locked, it will need .5 amps at 12 volts. The fail safe electric strike will need .51 amps at 12 volts to unlock. The worst case situation that would draw the most power in this situation is to unlock the strike while the magnetic lock is locked. You will need at least 1.1 amps to accomplish this.

Power consumption varies by lock type and manufacturer. (See the table above). If you added a panic bar release to another door, it would need .5 amps to unlock at 24 volts, but could consume up to 3.8 amps of inrush current to operate. Inrush current is the initial start up current needed to start operation of some devices. While inrush current usually lasts for a very short period of time, it can cause you problems because the power needed can be several times greater than the operating or steady-state current.

Lock Type		Amps for 12 			Amps for 24 
Volt Model    		Volt Model
Continuous	Unlock	Inrush	Continuous	Unlock	Inrush
Magnetic Lock	.25 to .5	0	0	.125 to .25	0	0
Shear	.35	0	0	.175	0	0
Mortise	.4			.2		
Unlatch Strike	.04 	.3	3	.04 	.2	3
Standard Strike- 	.51	0	0	.25	0	0
  (Fail Safe)
Standard Strike- 	0	.51	0	0	.25	0
  (Fail Secure)
Bolts- (Fail Safe)	.5	0	0	.3	0	0
Bolts- (Fail Secure)	0	.5	0	0	.3	0
Panic bar Latch Release	-	-	-	0	.5	3.8

The addition of the panic bar release to your system requires more current and changing to a 24 volt system if you power all the system locks from one source. You could power the magnetic lock (.25 amp max), electric strike (.51 amp max.) and the panic bar release (3.8 amps max) from the same 24 volt source with 4.56 amps, but are probably better off dedicating a power source to the panic bar release because the inrush current will exceed the capacity of most controllers. Therefore, you may need to control another relay designed to handle the higher amperage to make this work.

Brad Shipp is a former Executive Director and Training Director for the NBFAA where he authored several NTS courses, including the Access Control Certification course. His involvement in the access control industry dates back to 1974 and in 1986 he became an instructor for the NBFAA National Training School. Shipp has served on several law enforcement, regulatory and industry association boards and has been honored for his service by the False Alarm Reduction Association and the International Association of Security and Investigative Regulators. Send in your questions on access control to [email protected].