Once the voltage has been lowered, the power supply electronics changes the voltage from alternating current to variations on direct current. The type of power supply describes the method used for the conversion. There are linear power supplies and switching (switch mode) power supplies. A linear power supply creates “straight line” direct current (battery power). Linear power supply operation is less complicated, produces cleaner power, and is less susceptible to producing electrical noise problems.
A switching power supply creates pulsing current by having full wave rectification. A switching power supply turns on and off, removing the lower portion of each alternating current wave. The switching power supply is more energy efficient and produces less heat during the conversion to direct current. A switching power supply is chosen over a linear power supply when higher efficiency, smaller size or lighter weight (smaller transformer), is required.
If you have a 24 VDC power supply, you want to have an output very close to 24VDC even when there is current draw such as the lock mechanism being powered in order to gain access. A switching power supply is susceptible to having poor (low) power factor. A low power factor draws more current resulting in overpowering. To compensate for overpowering which results from a large in-rush current draw, a switching power supply would need to have large capacitors.
Switching power supplies and linear power supplies operate differently to get to the voltage and amperage specifications. For the purpose of this article we will discuss how a linear power supply operates.
When AC line power enters a power supply, there can be an input fuse. Two common types of fuses are used by power supplies. The glass fuse that blows immediately after the current exceeds the specified value. The PPTC device (also known as resettable fuse) is a self-resetting over current protector that stops the current flow when a short or high amperage current inrushes by expanding and opening the circuit. Once the over current stops, the resettable fuse resets, permitting the flow of current. The purpose of an input fuse is to protect the power supply and the connected electronics from an incoming surge. In addition, this fuse can protect the AC line from a short within the access control system.
From the input fuse, the AC line current goes to the transformer. The linear power supply transformer changes the input voltage to the desired output (24V) voltage.
The linear rectification components convert the 24VAC to continuous 24VDC. Note: There can be additional circuitry in the power supply to remove any AC current remaining after the change to DC.
The current flows into a large single capacitor or twin capacitors that regulates the output voltage variations. A capacitor is like a temporary battery. It fills up with current and releases initially when the load is powered and additionally in order to maintain the specified current.
The additional circuitry of the power supply monitors the output voltage and makes adjustments. It controls the operation capacitor(s), and if the power supply is equipped with battery backup, battery power can also be drawn upon to maintain the constant output.
Protection can be built into the power supply to prevent damage to the attached hardware and the power supply itself. If too much current or power is drawn, the over protection (feedback) circuitry will either shut down the power supply or attempt adjustments. In addition, there can be circuitry that recognizes stable power.
This can include glass or resettable fuses attached to each output or one fuse for all outputs. If constructed with a fuse at each output and there is a problem with a specific door, only that output will stop providing power to the door. All other doors will still be powered.
Looking at power supplies manufactured by Command Access, we can see 24VDC linear and switching power supplies ranging from one Amp to five Amps. Each power supply comes with a three year, “no hassle” warranty. All of the power supplies are equipped with Euro style connectors that are removable for ease of connecting wires. They are also equipped with a light pipe that indicates if there is power. A light pipe is a standalone piece of optics that draws the illumination from within the enclosure. The other end is visible through the hole in the enclosure notifying there is power. The light pipe eliminates wiring from the circuit board to the enclosure door.