Power to the Equipment

October 31, 2008 SSI Staff

Selecting and understanding the operation characteristics of security system power supplies is one of the most critical technical skills for any technician or integrator. Manufacturers have done a good job providing the essential features and tools to help in this selection process with online support, training and calculators. However, it is still up to the technician, specifier and customer to understand what is needed and expected in a power supply selection and application.

Step Up, Down to Transformer

The power supply manufacturer will typically specify the correct type of transformer to use. Substitute transformer models should follow the same specs. Consider features such as power LED indicators and positive temperature coefficient (PTC) resistors in your plug-in transformers. An AC LED is helpful in remote diagnostics when power to the system may be shut off somewhere else in the facility. This simple indicator has been known to save an unnecessary service call.

[IMAGE]11962[/IMAGE]Since a transformer is basically a voltage step-up/step-down device, beware of the quality of the power and voltage levels coming into your transformer. Extreme swings in these areas can give your power supply trouble. Poor input power is very common in industrial areas where large electrical equipment can affect power levels and induce large amounts of power line noise.

In very bad cases, you may have to rely on separate uninterruptible power supplies (UPS) to not only provide temporary power, but clean power as well. Take a close look at not only the output but input specifications of the power supply you are considering. Also check the input when your power supply is at maximum load.

Drops Need Not Lead to Stops

You’ve heard it before, but I still see many in the field having trouble calculating how much voltage is lost as current travels along a cable. A true low-voltage system person may understand it, but I have seen many high-voltage electrical people who think low voltage is just plug-and-play, and IT people who just don’t understand the analog principles of resistance, current, and voltage (drop). The best advice is to understand Ohm’s and Kirchoff’s circuit laws and how they apply to your applications.

Make sure to check the manufacturers’ Web sites for additional support in this area as they have charts and on/offline calculators for determining voltage drop. Simply put, if you have a high current load and a high resistance, small gauge cable covering a long distance, you may have enough drop to cause a diminished voltage level at the device you want to power.

Have you ever noticed you can get dual-voltage level power supplies of both 12VDC and 24VDC? Devices falling in the latter category usually have lower current requirements and can help with your voltage drop issues.

Assault on Batteries

When selecting a battery your best bet is a good lead acid unit as they are designed for a steady floating charge. You will need to do an inventory of all the devices connected to the power supply circuit. It pays to write down each device and how it is used (momentary/continuous). Add up the total load that will be required of the backup battery. Make sure you are covered for things like battery age by adding another 25 percent to the overall load requirement.

If you want to specify how much of a charge will be available during a period of time, check out the battery discharge performance charts from the manufacturer. Make sure backup batteries installed in a parallel and/or series configuration are the same age. Sealed lead acid (SLA) batteries typically last around five years.

The Power Players

There are two types of power supplies. The linear power supply usually has a transformer to step down the input voltage. This voltage, in turn, is electronically rectified to provide DC voltage. The pluses of such a power supply are very clean DC power and providing good current limiting protection.

The other type of power supply is the switched-mode power supply. These devices use high-speed switching and smaller internal step-down transformers to produce low-voltage AC that is then rectified to DC power. These power supplies do not have an input transformer like the linear type. They have a maximum limit on the low end of their output and can produce a lot of noisy power line harmonics due to their very high speed switching (10KHz to 1MHz) circuits.

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