Back To Basics: Selecting The Right Power Supply

Sept. 3, 2013
A basic door control system may be comprised of a power source, wiring which connects the system elements and a locking device. It could also be several locks and controls. Or the system power supply may have multiple outputs, supplying locks and controlling electronics.

When we refer to electronic access control systems, we are speaking actually speaking of ‘circuits.’ A circuit is a path, and an electronic circuit is a path where electrons flow from the power source (power supply) through conductors (wiring) to the load (controls, sensors and locking devices).

An open circuit is where there is an incomplete path so the electrons are unable to flow, perhaps due to a broken wire or a switch in the off position; electrons are not flowing.

A short circuit is an undesirable condition where the electrons are able to flow between the positive potential of the power supply’s output to ground bypassing the load(s) in the circuit. A system with a “short” does not typically perform the desired action such as unlock a door.

A closed circuit is one where the electrons are permitted to flow and the system is performing its intended purpose (perhaps controlling a locking device).

Each element of the circuit is critical to and dependent upon the others.

Circuits are categorized according to their purpose. A circuit can be referred to as a system, although it is also common for multiple circuits are combined to form a system.

A basic door control system could ostensibly be comprised of only a power source (power supply), the wiring which connects the system elements and a locking device. It could also be comprised of several locks and controls. The system power supply may have multiple outputs, supplying locks, and controling electronics, etc.

Some power supplies can provide more than one output voltage, or specialized outputs for special purposes such as for an electronic latch retraction (EL) device, or sequenced for electric locks and door/gate operators.

The system may have more than one power supply. For example, multiple lock power supplies could be located throughout the facility adjacent to the controlled doors.

A common requirement for door control systems is that it is interfaced with the premises fire alarm to ensure that building occupants can egress the structure freely in the event of a fire or emergency.


The type of power supply you use on a project will be determined by a number of factors.

If you are servicing an existing system, then your selection will be guided by the type of power supply that was initially used. If the original power supply failed because it was inadequate for the system, then of course you won’t put in the same thing again. If the failure was due to short (circuit) or other failure in the system, you may consider power supplies that utilize fuses or isolation devices to prevent a short circuit to cause permanent damage to the power supply as well as other system elements.

Power supplies produce outputs rated in Volts and Amps. Volt is the unit of measurement of electrical potential, differential or pressure. Amp is the unit of measure for the amount of power flowing in a circuit. Volts are pushed through the circuit, while Amps are pulled or drawn through the circuit.

A power supply whose rating is 24VDC @ 5 Amps has that voltage available at the output terminals of the power supply. The amount of current the power supply provides is determined by what the load draws.

Theoretically a power supply cannot provide more than its rated current output. Best practices is to select a power supply whose output current is 25 percent higher than the calculated load of the circuit.

Power Surges

Surge protection may be employed to further protect the system. Surge protection may be built into components, or be a separate device added by the installer.

The two largest causes of electronic system failure are contamination of the hardware and power surges. Power surges are also referred to as “spikes” or “transients” (high amplitude, short duration electrical pulses).

Transients last for a very short time (generally less than one millisecond (1/1,000 of a second)) and the damage they cause may or may not be immediately apparent.

It is estimated that the life cycle of electronic equipment can be increased up to 30 percent with the use of surge protection measures, longer with the proper protection. A surge can be induced onto ANY metallic conductor.

Induction is caused by changing the magnetic flux inside a loop of wire and is a phenomenon commonly used and found in electrical and electronic technology. Transformers use induction. Extraneous voltages can be as easily induced as the intended ones.


Isolation is the electrical and sometimes magnetic separation between two circuits in close proximity to each other.

Isolation is accomplished by devices such as transformers, fuses, relays and opto-isolators.

Isolation is used:

  • To protect system elements from overload
  • For safety: Transformers used within power supplies function as a safety isolation transformer.
  • For Voltage Level Shifting: Isolation is often required to achieve the potential level shifting
  • To Obtain Multiple Outputs: The isolating characteristics of a transformer also allow the design of power supplies with multiple outputs by adding windings to the transformer along with rectifier and filter components. A very common example of this type of power supply is the desktop computer supply with +12, +5, and +3.3 volt outputs.
  • To prevent Grounds Loops which occur when two or more circuits share a common return path.

Underwriters Laboratories (UL) tests the quality of Power Supply Isolation as part of its Listing and recertification of power supplies by performing a “Hi-Pot” test. This test applies either a DC or an AC voltage across two isolated circuits. The resulting leakage current is measured. Another method is to measure the resistance between two isolated circuits.

All insulator material used in isolation applications exhibits capacitance resulting where two circuits may be DC isolated but not AC isolated. If the circuits are physically close enough to each other, and the frequency components are high enough, one circuit will be able to disrupt the operation of the other circuit. This is known as “crosstalk.”

Insulation is a closely related to isolation. An electrical insulator is a material which does not conduct an  electric current, under the influence of an electric field. A perfect insulator does not exist, but some materials such as glass and paper, which have high resistivity, are very good electrical insulators. Electrical insulation is the absence of electrical conduction.

Hard Wired vs Plug-In

If the old power supply was a hard-wired type, then it will be easiest to simply replace the circuit board or step down transformer inside the power supply’s metal enclosure so you do not have to get involved with electricians to rewire and the removal and reinstallation of the sheet metal enclosure.

Hard-wired is a term which refers to when the power supply line voltage is permanently attached to the power supply using flexible metal sheathing or rigid conduit. The transformer is typically inside the enclosure and steps the voltage down from line voltage (120 VAC) to whatever the operating voltage range is of the equipment being used. Typically this voltage is 24 Volts. The transformer transforms the 120 down to 24 volts AC, and the AC voltage is input into the power supply circuit board where it can be rectified, filtered and regulated and further mangled.

Plug-in transformers (also referred to as wall warts) step down the line voltage as described above, but they plug directly into a line voltage receptacle. They are not necessarily inside a metal enclosure.

There are drawbacks to using plug-in transformers:

  • If you do not verify the branch circuit is non-switched, the power may be turned off inadvertently from a wall switch.
  • If the receptacle you use is on a branch circuit shared by other receptacles on the same circuit breaker, you need to be sure your equipment does not exceed the load capacity for the circuit.
  • If the receptacle you select is convenient for you, it may also be convenient for others. Callbacks resulting from plug in transformers being removed so other appliances can be connected (like vacuum cleaners) or kicked loose are pretty common where step down transformers are used.

Plug-in power supplies are plug-in transformers with additional circuitry inside the enclosure to rectify, filter and regulate the voltage to the desired DC (Direct Current) voltage.

Power supplies may also be connected to line voltage by means of a line cord.

The same constraints which apply to the wall mount transformer and line cord operated equipment also apply to the wall mount power supply.

According to the National Electric Code (NEC), receptacles found above dropped ceilings should not be used for plug-in power or line cords. You or your customer should not have a receptacle installed for permanently powering a system above a dropped ceiling. Doing so is a violation of the electric code, and if ever a fire or personal injury occurs, or an electrical inspector finds it, you may be subject to censure or lawsuits.