A Guide To Power Supply Units in Access Control Systems

This blog explores power supply units within access control systems with the aim of helping you to select the right unit and amperage for your requirements.

There is a range of different power supplies in the market with most access control systems using 12V DC PSUs. However, there are a lot of systems and products that can run on 24V DC including fire alarms, CCTV and entry systems so specifying the correct product is essential.

What is a Power Supply Unit?

A basic definition of a power supply is an electrical component in a circuit that is used to reduce the mains voltage (230V AC) down to a lower voltage (12/24V DC) which most electronic equipment requires to function correctly. A power conversion takes place from the higher input voltage power source, usually alternating current AC, to a steady output at a low voltage direct current DC. These are known as AC-DC power supplies.

Selecting the right power supply product requires a calculation of the products total amperage of all the items of access control that you are connecting to the power supply at the correct voltage. All access control and electric locking products should state what voltage and current (mA) it requires to operate correctly. By adding all the combined current requirements together you will be able to select the correct output amperage required (A).

Approximate Estimations:

Single Maglock @ 500mA

Basic Keypad @ 120mA

Ancillary Items @ 100mA

= total of 720mA so a 1Amp PSU would power this setup, but best practice is to double it to ensure that it’s sufficient.

Two Types of Power Supply Unit

• Linear – A large transformer based product with medium efficiency. A linear regulator can run hotter than the switchmode alternative and a linear power supply tends to be a larger and heavier unit.
• Switchmode - A smaller and lighter overall product with higher efficiency that 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 unregulated power supplies are affected by variations in high voltage mains power surges, temperature changes and changes in the load current, which cause the output voltage to fluctuate.

Regulated power supplies 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. The output is steady, and you know that it is the right output voltage for each product/component within the system.

Access Control Batteries

Most power supplies used in access control systems require battery backup in case of a mains failure power cut as systems are required to work 24/7. As such, the majority of boxed power supplies designed for access control solutions are supplied with a battery backup output. Encased PSUs have room inside of the case to install a backup battery.

Battery backup power supply is optional but great for access control where you want to keep the door secure for some time during mains power failure/interruption. In the event of a fire alarm activation and your electric lock has failsafe function, the door will still open to allow safe emergency egress.

Another feature of PSU products is to have some form of monitoring the mains input and also the battery status. These products 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.

Access Control Battery Backup

A fully charged 7aH backup battery working with the setup we mentioned earlier, a single maglock @ 500mA, a basic keypad @ 120mA, plus ancillary items @ 100mA, would provide just over 9.5 hours of backup battery power.

Calculations should be based on the capacity of the fully charged battery and the total of the system’s components, as above, this is 720mA. Divide the battery rating by the system load, so 7aH/0.72A, totals the 9.5+ hour figure.

Once you have calculated the system load, it is best practice to derate the calculation by 10-20%, as a battery will stop working at a set voltage before it is fully drained, so in our example, it would be safe to assume a 7.5-8.5 hour battery life.

Backup batteries will need to be replaced at intervals. There is not always a fixed time period for replacement but with regular internal system checks and annual servicing of alarm and access control systems, any issues with the batteries will be flagged and they can be replaced as required.

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 bolts can have a hold current of around 120mA but on activation they can surge to over 1000mA on 12V DC. So, if you’re running two of these types of electric door locks on a pair of doors, then you would require either a 3 Amp or 5 Amp power supply unit.

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.

Regulated Transform Rectifiers

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.

A 24V DC regulated transformer rectifier power supply is ideal to use with hold open magnets/maglocks or door closers. With switched mode power technology, short circuit protection, and multiple cable entry, they’re a versatile option. However, they are not suitable for use with a backup battery. So, if you’re installing electromagnetic hold open door closers, or swing free function closers then a regulated transformer rectifier is the best option to use with these units.

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. As mentioned before, 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 DC power supply and the distance needed to power the locking device furthest away from it:

Best practice is to fit one spur and one PSU per door so any failure is limited to one door, battery life will be extended to back up system, and it’s the best for maintenance. Of course, multiple doors can be run off of one spur and PSU, but upon power failure or fault, then all doors would fail and battery life is severely limited, but it is a useful solution for convenient access control.

You can view our range of power supply units, but if you’d like any assistance, please contact our sales team on 01305 263300 or send them an email at [email protected].