How to Select the Proper Power Supply
Low voltage power is a fundamental requirement for video, access control and fire systems. Some power supplies come with or without battery charging for system backup, and may include diagnostic features. Specific models designed to comply with various standards may have agency approvals such as UL, cUL, MEA, CSFM, FM and CE.
Switch Mode Power Supplies utilize a switching regulator that regulates the output by varying the on-off duty cycle of the power-transistor. While they are highly efficient, have a wider input voltage range and cause less heat dissipation, they have a slightly higher EMI and RFI with slightly higher output ripple, noise, and slower transient response.
Linear Power Supplies utilize a series regulator in which the power-regulating transistor operates in a continuous mode supplying a steady flow of voltage and current. Pros include a steady flow of current and voltage, reduced EMI or RFI, less ripple and noise and faster transient response, while cons include larger size, reduced efficiency and larger heat sinks required.
Determine power demand to match power supply to system requirements. Consider mandatory agency approvals and code requirements, type of devices requiring external power and voltage and current requirements of the devices and system as well as quantity of devices, battery stand-by requirements, surrounding environment, cable length, mount, output type, etc. To determine what size power supply to use for single or multiple devices, multiply the current draw of each device to obtain total power consumption. Should battery back-up be required, the size of the batteries is determined by how much back-up time is required.
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 use.
Fuses protect electrical devices from over-currents and short circuits. They are fast and accurate, but are only good for one time use. PTC (Positive Temperature Coefficient Resistors) function like a circuit breaker. 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 manufacturers specified temperature range. The slow and inaccurate response of a PTC may result in damage to delicate electronic devices.
Typical supervisory features may include output power, AC failure, low battery, battery presence monitoring, ground fault, circuit trouble, and fire alarm trigger notification.
The line voltage that supplies input power may be hard wired 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 utilize plug-in transformers.
It may be determined that the power supply board be installed as a sub-assembly component, 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.
Power supplies are available in a variety of mounting configurations, such as wall mount, rack mount and pole mount. To accommodate future expansion, specify power supplies with spare outputs and more power than you need today.
Once all your power needs are defined, a good rule of thumb is to oversize the power supplies you specify by a margin of 20% as a safety factor to assure proper system operation 24/7. Your top considerations should be availability, breadth of line, track record of the manufacturer, customer support and warranty policies.
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