Powering Up Your Access Control System with Command Access

Electrified lock mechanisms require different amounts of amperage. Electrified mortise locks, cylindrical locks, magnetic locks, electric strikes usually require low current DC, requiring almost no surge (inrush) current. The same is true for keypads and readers.

The reason is the size of the solenoid and operation of the electronics. Most electrified locks are controlled by a solenoid that either locks (prevents) or unlocks (allows) the mechanism by moving the locking bar/tail piece into or out of a stop position. The solenoid does not move any part of the latch mechanism.

Electric latch retraction products have a larger, more powerful solenoid that actually retracts the latching mechanism, which is either the latch or the rim device bolt. They require a significant amount of surge current to get the solenoid armature and latching mechanism moving. Because the solenoid has a larger coil, it will also require greater holding current (amperage) to keep the latch retracted, permitting the door to be opened.

Surge current is short duration (less than one half a second), high amperage current draw by an electrical device when first operated. For example, the surge current provides the amperage necessary to start a large solenoid moving from the latched to the unlatched position. Once the solenoid has moved, it does not require high amperage current draw to remain in the unlocked position.

Most electromechanical lock mechanisms, keypads and readers have their power requirements printed on the advertisement or at a minimum, the installation manuals. For example, a continuous duty solenoid for a non-inrush applications typically requires 150-350mA @ 24VDC to operate the solenoid. The actual amount is determined by the characteristics of the solenoid and the specific application. Holding current for the solenoid would be the same.

However, to retract a typical latch retraction device would require high in-rush of 12-16 Amps @24VDC for 150-500 milli-seconds. Once the latch has been retracted, the holding current would be 125 to 500mA. Holding current keeps the latch retracted.

To choose the proper power supply, we need to understand how they operate, the ratings of the power supply, the type of power supply and have an understanding of the major components that make-up a power supply.

At this time, we should discuss “filtered” and “regulated” power supplies. “Filtered” power supply means the output voltage current that leaves is for all intent and purposes direct current (DC). However, the output from a switching power supply and a linear power supply are different as different methods are used to “filter” the output.

A “regulated” power supply is designed to maintain constant output voltage under changing load. The regulated power supply monitors the load condition and makes modification to minimize variations. All power supplies are rated to a + voltage and current percentage.

UL294 is the typical testing standard in the Access Control arena. The test protocol verifies the durability and dependability of the power supply through variety of tests including extended “fully loaded” duration, electric shock resistance, and battery backup (if applicable) conditions. UL294 compliant power supplies will be filtered, regulated, and include an indicator showing the presence of power from outside the enclosure. UL 294 Power Supplies are rated at +10 of the stated voltage.

A power supply consists of a transformer and the circuitry necessary to provide the amperage and the “clean” current. The transformer can be the largest component of the power supply. A transformer is a twin coil wired apparatus for reducing or increasing the voltage of alternating current. For access control applications, it lowers the voltage from the AC line current to the usable voltage. Every power supply has a reducing transformer that converts the AC line current to 12, 16, 24, or 48 VAC depending upon the transformer’s design. Some power supplies offer the ability to have multiple voltages, such as 12 and 24 volts.