Pick Your Alarm Power
October 1, 2007
The type of power supply you may select is determined by your application and the requirements of the installation. For fire and burglar alarm installations, codes and standards affect selection of the products.
“In general, every power supply takes line voltage and reduces it to the voltage digital electronics need,” explains Gene Pecora, general manager of Honeywell Power Products, Northford, Conn. “There’s lots of different ways to do that.”
Some power supplies also report their status to the alarm panel so the panel knows they are ready for use in an emergency. Additionally, they recharge backup batteries for the alarm system that are used in the event of a power failure.
In the past, fire and burglar alarm systems were driven by the power supplies built into their control panels. “What happened over time was the need for the system to command more power than what the internal power supply could drive,” Pecora relates. “Today, buildings have gotten larger, and codes and standards have made it so virtually every structure needs a fire system.
“So rather than build the same panel with medium, large or small power supplies, we separate the power from the control,” he continues. “The panels still have a small power supply built into them so they can be used to set up a relatively small system.”
In the past, fire and burglar panels supplied their power from 12 VDC power supplies, but as systems have gotten larger, they use 24 VDC power supplies.
“They’ll extend the control circuit to what is called a fire alarm power supply, so the alarm panel can activate that power supply to drive other devices,” Pecora explains. “When you need auxiliary power, it is a function of the rating of the capacity of the panel.”
Switching or Linear?There are two basic types of power supplies â€” switching and linear. The two produce different amounts of electrical noise â€” radio frequency interference (RFI) and electromagnetic interference (EMI) â€” and vary in the efficiency of their conversion of alternating current (AC) to the direct current fire and burglar alarm systems require. They also differ in cost.
Switching power supplies often produce more electrical noise, which can affect aspects of some alarm systems. They can use forward converters or offline switching.
The conversion efficiency of offline switching approaches 90 percent, Pecora maintains. He also thinks noise is not much of a problem any longer with switching power supplies.
“The first switchers that came out were noisy,” he says. “They got onto the power circuit and caused digital electronics to behave erratically, but that issue has been largely eliminated.
“And the same is true with forward converters â€” we use pretty sophisticated filtering circuits,” Pecora insists. “We now have to pass FCC testing on switching power supplies. What the FCC requires is that you don’t create noise either on the lines or out in the air, so you don’t disturb other devices around the supply.”
Motion detectors have been affected by electrical noise in the past, points out Don Lamb, design engineer for Elk Products Inc., Hildebran, N.C.
“Sometimes those are sensitive but not so much anymore, because they’ve gotten better filtering,” Lamb declares. “But in past years, it used to be a concern. It’s just something that the installer might want to look at in terms of choosing a power supply.”
Compared to switching power supplies, linear power supplies produce less noise and are less expensive, but they also are less efficient.
Because of their lower efficiency, they dissipate up to 50 percent of the power supplied to them as heat. Therefore, heat sinks to absorb that heat are required.
Linear power supplies also are an older technology, take up more space and are being used less often, except in applications where low noise is required, such as access control, where some card readers may be sensitive to electrical noise.
“Switchers can deliver more current in a smaller footprint than a linear power supply, but linears cost less money to build, and some linears require less filtering to get a smoother output,” Lamb notes.
“Switchers require better filtering on the output â€” that’s one reason for the higher cost of switchers,” he points out. “The drawback of a linear is the heat dissipation. The more of a current requirement you have, the more heat that linear supply will generate.”
So what is the best type of power supply to use for a fire or burglary installation? “If they find a linear power supply that meets their requirements, they most likely would choose that, because it’s probably going to be less expensive than switcher power supplies, but that’s a personal preference from the installers’ point of view,” Lamb concedes.
Says Pecora, “There is really a mix used in burglary and fire. What you could say is depending upon the applications, the larger power supplies tend to be offline, the middle size tend to be forward converters and the small tend to be linear, but that’s really painting with a broad brush.”
Sidebar: What Size Power Supply Is Needed?It depends on the number of devices being used by the fire or burglar alarm system. When designing a system, the amount of power required by each sensor is totaled to determine the power supply needed.
For example, if a PIR requires 0.1 A, depending on its size and manufacturer, and 9 are being used, a power supply with more than 1.0 A is necessary, according to the National Electrical Code (NEC), because power supplies are only supposed to be loaded to 80 percent of their capacity. So the next highest power supply, perhaps 1.5 A, would be required.
“Mainly what the installers are looking for is if it can meet the current requirements and the voltage requirements of their system,” declares Don Lamb, design engineer for Elk Products Inc., Hildebran, N.C. “It depends on the system you’re configuring.”