Power Supplies (PSUs) in Access Control

The common misapprehension with most Installers is that a power supply is simply just another component part of a security system, however it is the most important part! If it does not work correctly or fails to perform to its specification, then the whole system will fail.

Working out what power supply you need doesn’t need to be a daunting task. This blog will explore power supply units within access control systems with the aim of helping you to select the right unit and Amperage for your requirements.

There are a range of different power supplies in the market with most Access Control systems using 12V DC PSUs, but there are a lot of systems/products that can run on 24V DC including Fire Alarms, CCTV and Entry systems and so specifying the correct product is essential at the time of design.

There are two types of Power Supplies that are commonly used in the Access control design.

Linear – Large Transformer based product, with medium efficiency, but they can run hotter than the switchmode alternative. They tend to be larger and heavier units.

Switchmode - A smaller and lighter overall product with higher efficiency and runs much cooler than the linear products. 

Switch mode power supplies are an alternative to the traditional linear AC – DC voltage power supply units.

Most of these products are regulated at around 13.8vdc or 27.6vdc for battery backed up units.

Non-regulated Power Supplies can provide a DC output voltage that is non-constant. These types of power supplies are affected by variations in mains power surges, temperature changes and changes in the load current, which cause the output voltage to fluctuate.

Regulated Power Supplies however are not affected by the above and therefore have a stable, maintained output voltage. These are the types of power supply that you want in your access control systems because the output isn’t unpredictable, and you know that it is the right output voltage for each product/component within the system.

A very basic definition of a power supply is an electrical component in a circuit that is used to reduce the Mains voltage (230VAC) down to a lower voltage (12/24V DC); which most electronic equipment require to function. 

Selecting the correct product requires a calculation of the products you are connecting to the power supply at the correct voltage.

 All Access Control products should state what voltage and current (mA) it requires to operate correctly and by adding all the combined current requirements together you will be able to select the correct output amperage (A).

Approximate Estimations:
Single Maglock @ 500mA
Basic Keypad @ 120mA
Ancillary Items @ 100mA

= 720mA so a 1Amp PSU would power this, but best to double it to ensure that it’s enough.

Electric Locking in Access Control
Be aware that some electric locks have a high activation current so make sure you take this into consideration when specifying your Power Supply Unit.

Example: Solenoid Locks can have a hold current of around 120mA but on an activation can surge to 740mA on 12V DC.

There is a small transformer located within the shell that reduces the AC Voltage from the mains to a safer level. The voltage then passes through multiple components within the unit in order to provide a DC output. Power Supplies are NOT the same as transformer rectifiers as the current passes through multiple parts (including a transformer) to provide DC Voltage. Transformer Rectifiers are less process based and include other functions such as being able to isolate circuits and change signal voltages in amplifiers.

Power Supply Units (PSUs) for use with access control

Most power supplies used in Access control systems require battery backup in case of Mains failure (230VAC) as the system will require to work 24/7.

So another feature of PSU products is to have some form of monitoring the mains input and also the battery status. These products then give you early warning of issues that could affect the system and give some indication that the customer can action before a total down powering of the system.

Battery Backup

Most Power Supplies designed for access control are supplied with a battery backup output. Battery backup is optional but great for access control where you want to keep the door secure for some time during mains power failure/interruption. 
Note: In the event of a fire alarm activation and your electric lock is failsafe, the door will still open to allow safe emergency egress.

What do you need? – Cables and Calculations

When deciding on the ampere of a power supply unit, as well as taking the distances into consideration; you will need to consider the draw of each unit. It is recommended that you add up the draw of each component and then double it so that you ensure you have enough to power the circuit.

The table below is designed to provide an approximate guide for the wire gauge in relation to the voltage of the PSU and the distance needed to power the locking device furthest away from it: