Back To Basics: Selecting The Right Power Supply

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...

When we refer to electronic access control systems, we are 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 controlling 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.

Types Of Power Supplies

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 from causing 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.

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