Innovations in Power Supplies
No matter the complexity or size, there is an operational premise that all security systems are subject to: if the power goes down, nothing works.
Low-voltage power is an elemental requirement for video surveillance, access control and fire/life-safety systems — and yet the focus on power supplies is often overlooked. The fact is power supplies are the foundation for any system’s operation and selecting and installing the right model involves a crucial decision-making process. In its most basic form, power supplies convert incoming AC line voltage to low voltage AC or DC that remains consistent within a range regardless of line, load or environmental conditions. Power supplies are available in many shapes and sizes. Some come with or without battery charging for system back-up and may include diagnostic features.
Specific models designed to meet various standards may have product compliance agency approvals such as UL, CE and FM. With so many options, how does a security professional go about selecting the correct power supply for an application and what considerations must be taken into account? A good way to help make the correct determinations is to gauge the pros of cons of power supply types and apply fundamental application guidelines.
2 Types Of Power Supplies
There are two types of power supply technologies available, switch mode and linear. Each kind presents security contractors with inherent advantages and disadvantages on which to decide application decisions. Switch mode power supplies utilize a switching regulator that controls the output by varying the on-off duty cycle of the power transistor. Among its advantages the switch mode type: is highly efficient; offers less heat dissipation; has a wider input voltage range; and the unit is smaller in size. The disadvantages: slightly higher electromagnetic interference (EMI) and radio frequency interference (RFI); slightly higher output ripple and noise; and slower transient response.
Switch mode power supplies are available in two different versions. The first utilizes the low-voltage output of a separate step-down transformer as its primary input. The other being an offline design, in which the low-voltage transformer is integrated into the power supply board. This design utilizes primary line voltage 115/230VAC 50/60Hz input directly to the power supply and eliminates the need for the use of a separate low-voltage transformer.
Linear power supplies use a series regulator in which the power-regulating transistor operates in a continuous mode, supplying a steady flow of voltage and current.
The advantages of this type: steady flow of current and voltage; reduced EMI or RFI; less ripple and noise; faster transient response. The disadvantages: reduced efficiency; larger heat sinks are required to remove heat; precise AC voltage input is critical; and the unit is larger in size.
Calculating Power Demand
When planning for any new security system installation, one of the foremost considerations is power demand. This initial determination allows efficient matching of the power supply to the system requirements. The same holds true when adding additional devices or expanding an entire system.
With this in mind, an overall assessment and power demand calculation must be completed. The following are issues to be considered based on the magnitude of the project: mandatory agency approvals and code requirements; type of devices requiring external power; voltage and current requirements of the individual devices and of the total system (commonly used voltage is 12VDC, 24VDC or 24VAC).
Given the numerous factors involved to correctly specify a system’s power supply equipment, it’s easy to understand just how vital power supplies are to the operational well-being of any solution. To calculate the quantity and specifications of the correct power supplies for a system, the following considerations must be taken into account:
- Quantity of devices
- Fused or positive temperature coefficient (PTC) protected outputs
- Operating voltage and maximum current draw of the devices
- Battery stand-by requirements
- Special features (e.g. power supervision and fire alarm interface)
- Surrounding environment (e.g. operating temperature and humidity)
- Line voltage fluctuations
- Power safety factor
- Internal isolation
- Ability to accommodate high in-rush currents from a panic device
- Cable length and gauge
- Physical characteristics of the power supply (e.g. wall mount or rack mount)
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