Power supply for video surveillance systems.
One of the main components in the design and construction of a video surveillance system is, of course, the organization of power supply and communication lines. This extensive topic directly affects not only power supplies together with power lines, but also various lightning protection modules and grounding elements. As practice shows, it is the correct organization of the power supply for the video surveillance system that helps to avoid a large number of troubles in the subsequent process of operating these systems.
When designing and installing video surveillance systems, the main and most common mistake is an attempt to reduce the cost of the project by using suboptimal or even unsuitable power supplies (PSU) and communication lines. This often happens despite the established practice of installers in terms of noise immunity and reliability of the equipment as a whole. The consequences of such errors can be both minor failures in the operation of individual elements of the video surveillance system, and serious failures leading to equipment failure. Often, after a burnt-out power supply system, expensive cameras and recorders fail.
In practice, it looks like this: 99.9% of circuit solutions for the output stage of cameras do not provide galvanic, optical or other isolation between the common power supply wire and the common video output wire. This means that problems with such power supply circuits directly provoke problems in the recording and monitoring systems for receiving video signals. As a result, the entire video surveillance system fails because of one cheap power supply that does not meet the technical specifications.
By now, three types of circuit solutions for organizing a power supply for video surveillance systems have become widespread: linear, pulse, backup.
Considering the first of the above-mentioned design solutions, it is impossible not to clarify that the possibilities for its practical application should be limited to testing the equipment or, in extreme cases, checking the equipment before direct commissioning. Such a limitation is due to the fact that linear stabilizers are most sensitive to interference, inductance, reactive components and, in principle, to any force majeure changes in power supply. The principle of operation of a linear stabilizer is based on reducing the input voltage, for example, from a 30-volt transformer, to an acceptable 12-volt direct current at the output. In the event of a malfunction of the stabilizer, alternating current with an amplitude of up to 50 volts instantly disables video cameras and recorders, and the presence of a fuse in the circuit saves only from a fire, that is, it is not completely wrong to completely rely on its «protective» functions. Also, one cannot help but mention another important drawback of linear stabilizers — low efficiency.
Moving on to pulse power supplies, we must pay tribute to the development engineers, including Russian ones, for developing reliable and stable systems. All the above-mentioned disadvantages of linear stabilizers are completely eliminated in pulse converters. The following critical moments have minimal impact on power supplies: cable length, its inductance, «sparking contacts», external interference environment, potential difference. When using pulse power supplies, you can get a system that is not only excellent in reliability, but also the most advantageous in terms of price/quality ratio.
The use of backup power sources is usually the prerogative of specialized facilities, where the video surveillance system receives energy directly from backup batteries (AKB) or from other alternative energy sources. In this case, communication line problems are minimized, and the only condition for the absence of troubles is the fuse of the electrical circuit.
Communication lines.
To build communication lines in video surveillance systems, depending on the installation conditions, three main types of cables are used: two-wire, twisted pair, combined.
The most common of them is a two-core cable of the ШВВП type. This cable is best suited for use in video surveillance systems due to its temperature parameters and protection from various influences, however, its parallel conductors can accumulate potential differences when laid over long distances. In this case, when constructing a network, it is necessary to take into account the absence of other power conductors nearby. Particular attention should be paid to the inadmissibility of parallelism of power and low-voltage lines, otherwise, when switching a powerful load in power conductors, quite strong interference may occur in the power lines of the video surveillance system. It should also be taken into account that such two-core cables are mostly not shielded and do not have conductors twisted together. It is for this reason that a long cable works as an antenna, collecting all external interference.
Twisted pair, in turn, is not intended for transmitting power to equipment, but, nevertheless, is widely used by installation organizations. The main error in the circuit design of the power supply network when using twisted pair is the simultaneous transmission of a signal, be it analog or digital, and the supply voltage of the cameras. Based on this, we can assume two shortcomings in such a network for video surveillance systems: close location of conductors twisted together, which greatly affects the quality of the image as a whole; small cross-section of the wire, which imposes significant restrictions on the length of the line. Problems with voltage drop when turning on IR illumination or heating/cooling modules of video cameras are also likely. This type of cable products should be used in video surveillance systems with special care, since when organizing power supply it is necessary to take into account the voltage drop in the networks depending on the length of the lines. For a standard video output designed for a load of 75 Ohm, the use of twisted pair is unacceptable.
Regardless of the type of cable, the main condition for selection is the correct assessment of the complexity of the object, assessment of the interference component of the network and calculation by cross-section. With high safety requirements, do not forget about additional shielding of wires and protection of lines from external influences using various boxes.
Switching and distribution circuits.
As practice has shown, soldering and clamp connection are considered the most reliable. The wires must be well stripped and tinned, otherwise the copper base oxidizes over time, and interruptions in the operation of the video surveillance system occur. The higher the degree of oxidation, the greater the voltage drop we get at this switching point, and at high load currents, sparking, short circuit, heating of the wire begins, and as a result — a fire.
Many manufacturers use not a clamp, but a male-female connector as a power connector. Indoors, such connections will work without any problems, but they are completely unsuitable for the external environment. Therefore, it is advisable to agree in advance with the manufacturer to maintain the warranty in case of cutting this type of connection. And use soldering or clamp.
In conclusion, I would like to note the presence on the market of a huge amount of material for organizing the power supply of the video surveillance system. But often a beautiful shell hides a lower copper content in the conductor cores, and the golden contacts oxidize at the first excess of the percentage of humidity. Be careful!