How to Choose the Right Power Supply for Your Security System
Tech Talk columnist Bob Dolph shares his observations on power supplies so we can all be a better security system designer, seller, installer and service technician.
Most would agree that electrical power is an important part of security systems. Without it nothing would work. More important is the use of the correct power supply for each particular installation.
Only the true alarm professional will take the time to study the many power supply options available. This month I thought I would discuss some trade observations on power supplies so that we can all be a better security system designer, seller, installer and service technician.
Ohm’s Law Review
As one starts out in the security industry it would appear that all components of a security system are 12 volts direct current (VDC). The control panels have an auxiliary power output of 12VDC to power system sensors and annunciators.
At times one would notice that some power supplies also come with the option of 24VDC for devices that designers specified at 24VDC. But have you asked yourself, “Why have this optional higher voltage?”
Let’s take a glance back at one of our favorite electrical formulas, Ohm’s Law or E = IR. Remember, volts (symbolized by the letters E or V) equals current measured in amps (symbolized by the letter I) multiplied by resistance measured in ohms (symbolized by the letter R).
Also, we must look at a complete electrical circuit, which includes the resistance of the cabling. A 12VDC circuit that has a .5 amp load with a cabling distance of 150-200 feet would need 14 gauge wire.
That same circuit at 24VDC would only need 18 gauge wire. So with the 24VDC you would not only save in the cost of less copper, but miss out on all the fun of working with the heavier 14 gauge wire.
Considering these two factors together will help us better understand the reasoning behind optional 24VDC power and devices.
Battle of the Surge
There are basically two types of power supplies: switching and linear. Switching power supplies are typically designed for use with resistive or capacitive loads. They are compact and economical and often found in alarm panels, cameras and signaling devices.
Linear power supplies are also designed to power inductive loads. These are often used to power electrified door hardware that utilize coils and solenoids.
Inductive loads require a large inrush current when powered up, which can often be four to 20 times the device’s normal operating current. Have you ever noticed that these power supplies are often located very close in proximity to the electrified hardware?
Now you get the picture. Devices with coils not only have a large in-rush current but also have a reverse voltage spike when the power is removed and the field collapses.
Don’t forget to correctly install those tiny diodes the door strike manufacturer supplied you. These are designed to clamp the reverse voltage spike and protect your equipment.
PTC to the Rescue — Not Always
A handy voltage compensation tool for every technician’s tool box is the VB1 Voltage Booster from Altronix, a long established power supply manufacturer. This device will allow you to connect that occasional 24VDC device onto a 12VDC power circuit. It will also allow you to compensate for voltage drops due to long wire runs. It has a regulated output and built-in overload protection. The device output is 24VDC @ .75 amps.
It is very rare that you find a home today that has a fuse box to protect its power circuits. While fuses work great they are a pain because you have to replace them.
Circuit-breakers have won the circuit protection war since they are easy to reset with a flip of the switch. Similarly, the low voltage industry was introduced to the Positive Temperature Coefficient resistor, or PTC.
These auto-reset electronic circuit-breakers can now be found in such popular low voltage power sources as the plug-in wall trans-former, sometimes referred to as a wall wart.
But if you look at many power supplies such as for CCTV you will still see fuses. Why not use the new PTC technology? Because the PTC is slow acting and has an inaccurate trip thresh-old that may not protect camera circuits as well as a fast-acting fuse.
You may also have to manually reset a PTC by removing power to the circuit, which could mean a service call. Now for some exciting power supply news.
Companies like Sargent and Corbin Russwin have developed devices using the EcoFlex technology. These are ultra low-powered access control devices.
These devices offer total door power consumption of only .3 watts vs. typical 20W. That is a 99% reduction. Also check out the complementary EcoPower power supplies from Securitron. All of these companies are group brands of ASSA ABLOY.
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