Troubleshooting Power Supplies for Access Control Systems

July 12, 2011
troubleshooting power supplies for access control systems

Last week I was dispatched over to service an access control installation. The system was comprised of about 12 readers distributed in three different buildings at a private middle school which had a campus and dorms. School was out for summer.

The system was network based. Each door had a reader, a door position sensor and an electric lock which connected to an interface at the door. Each interface was home run back to the head-end controller via RS-485.

The head-end controller was connected to the network. The head-end communicated to the server was also on the network. System management was done with via software installed on a PC, managed by an operator with a valid username and password.

I was told that the readers were ‘dead’ in one of the buildings. The building with the dead readers was the building in which the head-end was located. 

There were two power supplies at the head-end: one for the electric locking devices and the other for the head-end and the readers and door interfaces. The power supply for the head-end and interfaces was down, and the head-end backup battery was depleted to where the head-end crashed. The failed power supply was toast.

I provided two new power supplies to replace the one which had failed. The power supply which had failed had already been recently replaced by one of our techs, and now this one, which was the same model as the original failed unit, had failed again.

The cause for the power supply failure was unknown, but the power supply’s output rating was theoretically more than adequate for the application and it was a regulated design and should have just clamped if a short or overload occurred, and not die.

Once the new power supplies were deployed, the system restarted. But then the customer immediately began complaining that all the doors were locked, and they were scheduled to be unlocked at this time of day.

The system administrator logged in, and sure enough the internal clock in the head-end had stopped due to the loss of power and battery backup, and it needed to be reset. I was surprised how long it took for the system activity log to scroll through as the doors and head-end synced up with the server. The power had been out for quite a while and apparently no one noticed.

Avoiding Problems

Here are some ideas to help you to hopefully avoid situations such as this, or provide you with some effective measures to resolve problems once they manifest themselves.

Several factors need to be considered when selecting a power supply for a particular application.

Select a power supply which is properly rated for the project. Voltage drop must be calculated and compensated for, and the power supply current capacity should be 25 percent more than the calculated load requirement.

Power distribution and power management are best addressed during the planning stages of any project, not as you are driving to the installation, or when you are troubleshooting a system failure.

Power supply monitoring is also a pre-emptive tactic which gives you or your client system feedback in time to avoid costly or perhaps dangerous conditions on site.

Access control and life safety systems are subject to codes, and NEC Article 725 and UL 294 are two of the most important. These relate to "power-limited" power supplies, cabling and devices, and you will usually be required to be in conformance.

Briefly, in a Class 2 circuit, the limit is 100 VA, with a maximum voltage of 30 and current of 8 amperes. (Note that Class 2 circuits are not considered a danger to personnel.)

The gauge and number of conductors, the type of insulation on the cable (Plenum or Non-Plenum) are both critical to the proper performance and safety of the installed system.

Performance of the system is dependent on the correct voltages reaching all the equipment which is a function of the appropriate gauge wire. The safety issues involve the heating and ignition of under-rated wiring, and the toxic fumes non-plenum insulated cable generates when exposed to flame or excessive heat and the possible endangering of lives if this non-plenum cable is inadvertently or deliberately installed in a plenum.

Additionally wire handling and routing during the installation are essential areas of concern. Improper handling of materials will result in wasted material and man-power; and could possibly result in damage to the wire. Inappropriate routing will result in a non-code compliant installation which has legal and ethical ramifications.

So in the case of my service call, I had the opportunity to rethink, or perhaps more accurately actually address the power supply specification and the power distribution and management aspects of the project which had been initially overlooked.

This system was inadequately powered; there was no power distribution scheme, and system and power supply status were not being being monitored remotely or by the end-user.

Having extensive experience with risk assessment, loss prevention and litigation thanked God that we were able to get things re-secured without an attack, robbery or injury occurring first.

The idea is not so much doing it right the first time in case there is a crisis, as it is to follow best practices to avoid a problem from occurring at all.

Product Choices

The access control industry has evolved dramatically, and the choice of product is amazing. For some applications, centrally located power supplies are preferred, while on others, power supplies are best deployed on a door by door basis.

Door by door deployments can be indicated when the system grows door by door over an extended period of time, or when the size of the site would mean pulling long lines that would create voltage drop issues.

Also for certain specialized locking solutions, specifically Electric Latch retraction on exit devices, the in-rush current and manufacturers’ installation guides force power supplies to be deployed near the locking devices.

Where a system is pre-planned, where the structure’s architecture or usage, where system component security are issues (putting a power supply out by a door would invite tampering while securing it in a electrical closet wouldn’t), a centralized approach to power supply deployment may be preferred.

Centrally locating power supplies also simplifies providing line voltage and connection of Fire Alarm interfaces to the power supplies. Fire alarm interfaces are mandated for certain locking arrangements to ensure safe egress in the event of a fire.

Power distribution

Typically an access control system will require power to a door controller, a card reader and an electric locking device at each door.

By providing individually fused circuits, a failure on an individual device does not produce a failure of the entire system, and it also expedites troubleshooting and repair.

The root cause of the system failure in my example was possibly under powering, but perhaps it was an intermittent lock, controller or reader. Or possibly it was a piece of cable which was mishandled or misrouted which is occasionally shorting.

Without individually fused circuits, or a hard component failure, you’re guessing.

If one of the replacement power supplies in my example was shorted out when I repowered the project, the problem would have at least been further isolated, but hardly identified.

Without fused circuits, a failure on one door can possibly bring down the whole system.

We also could have implemented monitoring features available through the access control system to email if power or communications were lost.

Many power supplies are provided with monitoring outputs specifically for this purpose.

Our project had a backup battery on the controller, so the controller continued to function for a while before it finally crashed. A simple loss of AC power alarm would have alerted us in enough time to take action.

Following is a listing of some of the power supplies available for access control applications.


These units convert one 12 to 24 volt AC or DC input into eight independently controlled fused or PTC protected outputs. These power outputs can be converted to dry form “C” contacts (ACM8 & ACM8E only).

Outputs are activated by an open collector sink or normally open (NO) dry trigger input from an Access Control System, Card Reader, Keypad, Push Button, PIR, etc.

The units will route power to a variety of access control hardware devices including Mag Locks, Electric Strikes, Magnetic Door Holders, etc.

Outputs will operate in both Fail-Safe and/or Fail-Secure modes.

Units are designed to be powered by one common power source which will provide power for both the board operation and locking devices, or two totally independent power sources, one providing power for board operation and the other for lock / accessory power.

The FACP Interface enables Emergency Egress, Alarm Monitoring, or may be used to trigger other auxiliary devices. The fire alarm disconnect feature is individually selectable for any or all of the eight outputs.


• 12 to 24volt AC or DC operation (setting not required). (.6 amp @ 12volt, .3 amp @ 24volt current consumption with all relays energized).

• Power supply input options: 

a) One common power input (board and lock power).

b) Two isolated power inputs (one for board power and one for lock/hardware power).

• Eight Access Control System trigger inputs:

a) Eight normally open (NO) inputs.

b) Eight open collector sink inputs.

c) Any combination of the above.

• Eight independently controlled outputs:

a) Eight Fail-Safe and/or Fail-Secure power outputs.

b) Eight dry form “C” 5 amp rated relay outputs (ACM8 & ACM8E only).

c) Any combination of the above  (ACM8 & ACM8E only).

• Eight (8) auxiliary power outputs (unswitched).

RCI PDM-4, PDM-4C, PDM-8 and PDM-8C

The RCI PDM-4, PDM-4C, PDM-8 and PDM-8C boards distribute a single low voltage AC or DC source into 4 or 8 outputs. The output voltage and total current available is determined by the input supply.

When installed into a power supply, servicing time can be greatly reduced by the output indicator LEDs. If an output fuse is blown or PTC is tripped the accompanying LED will be off, simply identifying the device connected to this output will allow for a problem to be quickly corrected. In addition to short circuit protection the input terminals also have a surge protector that can absorb a 1500Watt spike or surge to protect connected devices


[1] Converts a single output to 4 or 8 separately protected outputs

[1] Available with fuses (PDM-4 or 8) or PTC Circuit Breakers (PDM-4C or 8C)

[1] Separate Green power indicator LED for each output

[1] Power switch to shut off all devices for servicing on PDM-4 and 4C only

[1] Circuit breakers up to 1.5A provide Class II power limiting

[1] LED power indicator Green = DC, Red = inverted DC,

Amber = AC on PDM-8 and 8C only


When interfacing with a fire alarm system where the system's auxiliary relay cannot handle the full current potential of the power supply, or when monitoring power systems for AC failure or DC battery failure, the BPSM linear power supply offers additional protection.

During battery operation, if voltage drops below 10.4VDC for 12VDC (20.8VDC for 24VDC) the battery will automatically disconnect to prevent possible damage to the system.


  • 8 polyswitch (PTC) 2A output breakers
  • Slide switches connect or disconnect load from power
  • LED indication (AC and DC) showing power supply status
  • Emergency low current fire release terminals
  • AC status output terminals, SPDT, 3 Amp
  • Battery status output terminals, Form "C" SPDT, 3 Amp
  • Line voltage and DC fuses
  • Sealed lead acid-gel battery charging capability (battery not included)

SCHLAGE PS900 Series

The PS900 Series is a consolidated line of power supplies and accessories that offers enhanced flexibility and functionality. In addition, the PS900 is easy to order and install.

The full line is UL 294 certified, the industry standard for reliability and performance.

The PS900 Series can be used in a variety of applications to convert high voltage AC power into the low voltage DC outputs required by most access control devices.

The PS900 Series of power supplies protects devices downstream by providing Class 2*, filtered, and regulated power. Once power is converted to low voltage DC, the PS900 Series offers a variety of distribution options, including basic fuse protection, simple relay, and advanced logic providing complex sequencing and timing functions.

* PS906 can provide Class 2 rated outputs when used with 900-8P distribution board.

Three models of the PS900 Series are available. All convert high voltage 120 VAC- 240 VAC (50-60 HZ) power to regulated and filtered low voltage power. Output can be field configured to either 12 VDC or 24 VDC.

PS902: 2 amps

PS904: 4 amps

PS906: 6 amps

Note: The Von Duprin PS914 and 900-2RS are available for use with electrified exit devices.


• Constant output rating at both 12 VDC and 24 VDC provides superior performance

• Polarized connectors for option boards eliminate need for racks and side connectors

• Flat mounting of option boards provides easier access to terminal blocks for connection of electrified devices

• High voltage protective cover

• Battery back-up board auto-selects voltage

• Fire alarm relay can be configured to provide either switched or un-switched outputs from a power supply

In addition, Schlage offers several different option boards for the PS-900 series.

Emergency Interface Relay (FA) – Must be installed on distribution boards

Battery Backup

2 relay QEL panic device control board

4 relay distribution board

4 relay distribution board with logic – Field configurable forTime delay function,  Auto operator, or • Security interlock

8 fused output distribution board

8 PTC output distribution board

The Function provided by the 900-8F is to Provide 8 independently-fused outputs. This is an option board designed for use with Schlage PS-900 series power supplies.

Each output is provided with an individual LED to indicate fuse status.

Temperature Range is 32°-120° F (0°- 49° C)

To read additional Locksmith Ledger articles on power supplies for access control applications, visit