Innovations in Power Supplies
Once you have established the need for any of the above, it is then necessary to determine the size of the power supply to ensure reliable system operation. To resolve what power supply to use for single or multiple devices, multiply the current draw of each device to obtain total power consumption. For example, an access control system with 16 devices operating at 24VDC with a current draw of 300mA each can be calculated as such:
16 (devices) X 300mA (current) = 4.8A
Taking voltage-drop issues into account and other safety factors, it is prudent to increase this result by 20 percent to assure proper system operation 24/7. The calculation subsequently becomes:
4.8A X 20% = .96A (power safety factor)
4.8A + .96A = 5.76A (total power required)
Therefore, our access control system example will require a 24VDC, 16-output power supply rated 5.76A minimum.
A Laundry List To Mull Over
So, all the variables have been calculated to come up with the correct total power required, but a security contractor’s work is still not complete. There are other factors to contend with when specifying a solution. Among them:
Battery back-up — Should battery back-up be called for, the size of the batteries is determined by how much back-up time is required. The use of a battery calculation tool will help in the calculation of a proper size battery.
Device location — To eliminate the possibility of voltage drop, it may be necessary to decentralize the power source based on the distance between devices and the length and gauge of cable runs. It is important to calculate the proper wire gauge. To assist in this task, and to work out the voltage drop over a specified distance, special calculators are readily available online (such as the one offered from Altronix Corp.) to determine the correct numbers.
Fused, PTC protected outputs — Fuses protect electrical devices from over-currents and short circuits. When current reaches a predetermined magnitude for a fixed period of time, the fuse’s internal element melts and current flow is interrupted. Fuses are fast and accurate, but are only good for one-time use.
PTC resistors function like a circuit breaker. As the PTC current rating is exceeded, it slowly heats up until the 
temperature threshold is reached causing a dramatic increase in resistance that opens the circuit. PTCs are temperature-sensitive devices. They will trip at lower-than-rated currents at higher temperatures, and higher-than-rated currents at lower temperatures. PTC ratings can vary +/-50 percent from the manufacturer’s specified temperature range. The slow and inaccurate response of a PTC may result in damage to delicate electronic devices.
Supervision — Typical supervisory features may include but are not limited to output power, AC failure, low battery, battery presence monitoring, ground fault, circuit trouble and fire alarm trigger notifi
cation.
Plug-in vs. hardwired — The line voltage that supplies input power may be hardwired, which requires a licensed electrician to make the connection. In some cases, a power supply may incorporate a grounded line cord that simply plugs into a dedicated receptacle. Smaller power supplies typically use plug-in transformers.
Power supply integration – It may be determined that the power supply board be installed as a subassembly component in order to provide the operating power for a system and/or its complementary devices. In this case, if a UL listing is required, the complete system must be submitted for approval.
Installation location – A major consideration is the physical and geographical location of the power supplies. This greatly affects the accessibility and serviceability of the units, as extreme environmental conditions can affect performance and operation.
Mounting options – Power supplies are available in a variety of mounting configurations, such as wall mount, rack mount and pole mount. Physical constraints and system design will dictate which option is suitable.
Future expansion – To accommodate future expansion, it is prudent to specify power supplies with spare outputs and more power than is presently necessary.
Although cost-effectiveness is a factor, it shouldn’t be your highest priority. More important considerations should be availability, breadth of line, track record of the manufacturer, customer support and warranty policies.
Unfortunately there are no set rules when shopping for power supplies that apply to every system or device. Consult the manufacturer for assistance. Unless your selections are based on carefully defined parameters, you could be in for a shock down the road.
Paul Rizzuto is a technical sales manager for Brooklyn, N.Y.-based Altronix Corp., a leading designer and manufacturer of low-voltage electronic components. Rizzuto can be reached via E-mail, [email protected].
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