Features of power supply of video surveillance systems.
Chura Nikolay Iosifovich
FEATURES OF POWER SUPPLY OF VIDEO SURVEILLANCE SYSTEMS
Video cameras are the basis of any security video surveillance system.
The possibility of implementing their characteristics, stability and reliability of operation largely determine the parameters and operability of the entire system as a whole.
Moreover, video cameras are usually located in various, often unfavorable climatic conditions, at very different and quite large distances from other devices.
In this article, we will not consider cameras powered directly from the industrial network.
In such cameras, the quality of the supply voltage is ensured by the manufacturer of the camera itself. The mutual influence of cameras through the power supply circuits is minimized.
Only video signal currents flow in the connecting lines, and with proper wiring, losses and mutual interference are minimal.
With all the advantages of the above-mentioned structure, systems with direct current power supply for TV cameras are quite common.
Apparently, this is due to some savings due to the smaller number of power sources used, smaller dimensions and weight of TV cameras, electrical safety of the system, as well as the simplicity of providing unified control over the power supply of the entire system and its relative autonomy.
Power supply from the industrial network has no alternative for outdoor television cameras in sealed housings with heating with their high energy consumption and at distances of more than 100 — 150 meters.
However, due to the minimization of the size of modern television cameras and the reduction of energy costs for heating, as well as the increasing distribution of miniature sealed television cameras without special heating with an extended range of operating temperatures, the use of network power supply often becomes less than optimal.
A modern video camera is a fairly economical device. The most common video cameras today with a 1/3-inch CCD matrix have a typical consumption of no more than 100 — 120 mA at a voltage of 12 V DC.
Color or high-resolution video cameras have slightly higher current consumption, but not exceeding 200 — 250 mA.
The operating voltage range of cameras from different manufacturers is from ± 10 to ± 30% of the nominal value.
The presence of internal stabilizers and voltage converters ensures relatively low sensitivity of a modern TV camera to the stability of the supply voltage.
Despite this, one should avoid the temptation to use cheap “adapters” and rectifiers.
They use low-quality transformers with a small cross-section and mediocre quality of the magnetic circuit.
For reasons of economy, the number of turns of the primary winding is minimal.
This leads to core saturation, growth of the stray field, distortion of the sinusoidal shape of the current and voltage. The growth of internal resistance significantly increases the ratio of the output voltage at idle and at nominal load.
Instead of 1.4, this ratio can reach 2 — 2.5. If the load capacity is not matched to the consumption, the video camera may fail due to increased supply voltage.
For powering video cameras, it is most optimal to use stabilized power supplies.
In addition to constant voltage, even the simplest source based on a typical integrated stabilizer, due to its low output impedance, ensures, at a minimum, the absence of mutual influence of video cameras connected to it.
However, this only applies to a properly designed device.
Nowadays, with the entry into the market of a huge number of manufacturers who are not always responsible and competent enough, it is not uncommon to come across power supplies that are stabilized only by name.
Typical disadvantages of such products:
- overheating at nominal load and maximum network voltage;
- lack of stabilization at nominal load and minimum network voltage;
- parasitic high-frequency generation in the above modes.
The reasons for these disadvantages are:
- use of low-quality transformers;
- insufficient heat dissipation for the integrated stabilizer;
- insufficient voltage at the stabilizer input;
- lack of blocking RF capacitors at the stabilizer input and output;
- low capacity of the filter capacitors at the stabilizer input.
If in this system crosstalk from adjacent cameras or broadcast TV stations is visible in the video signals, and the image is constantly distorted, it is advisable to check the supply voltage with an oscilloscope.
The amplitude of pulsations and noise should not exceed 20 — 40 mV.
The noise path should not contain regular components other than the first and second harmonics of the network frequency.
Figure 1 shows the output voltage diagram of a power supply with insufficient filter capacity and a transformer operating in saturation mode.
In Fig. 2 – with insufficient filter capacity and low input voltage for the stabilizer.
Fig. 3 demonstrates the excitation of the stabilizer at a frequency of 1.8 MHz at the moments of reaching the stabilization voltage.
Fig. 1.
Fig. 2.
Fig. 3.
If oscillographic control is not possible, a qualitative assessment of the nature of the power supply noise can be made by ear.
For this, you can use headphones (earphones) from any household electronics.
The output voltage must be connected to the phones through a separating capacitor with a capacity of about 1 — 10 μF and an operating voltage of at least 25 V.
Problems with the stability of the stabilizer are indicated by the modulation nature of the noise, “itchy components”, whistling in the high-frequency region of the audio range, signals from broadcast TV and radio stations. In extreme cases, you can make a trial replacement with a source from another manufacturer.
The power supply design must provide for effective heat dissipation of power at least P = 10x Imax, assuming that the stabilizing element drops to 10 V.
If there is a metal case, none of the power supply poles should be connected to it, and a separate grounding terminal is required.
Currently, power supplies with pulse voltage converters are becoming increasingly widespread. As a rule, they use converters with a pulse repetition rate of up to 100 kHz, and in some models even higher. To regulate and stabilize the output voltage, the pulse duty cycle is changed at a constant or variable repetition rate.
These sources have a very wide range of input voltages, high efficiency and specific power.
At the same time, the operating principle of the converters causes the presence of broadband electromagnetic fields, current and voltage pulsations in the input and output circuits. The spectrum of this interference can extend to tens of megahertz.
Moreover, during regulation and in the process of voltage stabilization, the frequency and magnitude of the components of the noise and pulsation spectrum change.
The pulsations at the output of such sources have the form of very short (less than µs) pulses of large amplitude and variable duty cycle.
Moreover, the more efficient the converter, i.e. the higher the frequency and the shorter the switching time, the wider the spectrum of pulsations and noise.
Normalization of pulsations by the root-mean-square value, accepted in classic network power supplies in this case gives significantly underestimated values and is completely inapplicable.
With an unsuccessful design, poor shielding and insufficient filtering of the input and output voltages, a power supply with conversion can cause direct and cross-talk in a video surveillance system. A radical solution to these problems could be sources with resonant sinusoidal converters, but they are still not very common.
In a typical modern TV camera, the pulsation range of the video signal components in the power supply circuit does not exceed 50–80 μA. This allows using one stabilized source to power up to 5–7 video cameras and achieve significant cost savings.
A number of conditions must be met when laying out the power supply lines:
- each camera must be connected to the source with separate wires;
- it is unacceptable to use the braiding of the signal cable for power supply, especially for lines longer than 25 — 30 m;
- when using a common power bus, its cross-section must be increased proportionally to the total current consumption, and each camera must have at least the simplest RC filter with t і 10 ms;
- the connection of the negative pole power wire to the common terminal must be made only at the camera itself;
- the cross-section of the wires should be selected based on the calculation that, taking into account the voltage drop on them at the minimum ambient temperature, the supply voltage on the TV camera will not go beyond the operating range.
Naturally, the more extensive and branched the system is created, the more strictly these requirements must be met.
It is highly undesirable, and in some cases unacceptable, to connect the common output of a video camera to metal structures at the place of its installation, especially for extended systems. In addition, in many video cameras, and even in the design for outdoor installation, the common signal and power output is structurally connected to the body.
Such a connection can lead to the appearance of industrial frequency currents in the circuits of the video surveillance system, which can damage the equipment.
The cause of these currents is the potential difference between the equipment, when instead of high-quality grounding, “zeroing” or surrogate grounding on heating networks, water pipes and other metal structures is used.
In these cases, it is advisable to electrically isolate the camera from the mounting structures.
As a rule, there is some induced industrial frequency potential on the housings, chassis and common buses of equipment used in video surveillance systems and powered by an industrial network.
The reason for this is capacitive and resistive leaks in the power supplies of these devices, induced potentials from external electromagnetic fields, accidental contacts with engineering or building structures, etc.
To eliminate the possibility of breakdown of video signal inputs and outputs, before connecting devices into the system, it is advisable to check the equalizing currents between the common terminals of the devices included in the power supply network.
For measurements, you can use any industrial frequency current meter with an upper limit of 50 — 100 mA. The current value should not exceed 10 — 20 mA. Minimization of this current is ensured by phasing the network plugs.
A radical solution to this issue is good individual grounding of the equipment housings.
Figure 4 shows an approximate installation diagram of a 4-camera system with a common power source.
TV cameras of various designs are used: caseless, cased with a replaceable lens, and similar models in thermal boxes.
Fig. 4.
Considering the features of the layout of power and signal circuits in video surveillance systems, it is necessary to focus on the reliability of contact connections.
Unfortunately, very common twisted wires, insulated with adhesive tape and even screw or crimp fastening (without significant deformation and cold welding) can be considered a relatively reliable connection method only in dry rooms and moisture-proof spaces.
In other conditions, only soldering and welding can guarantee against spontaneous fluctuations in the signal or supply voltage after several months of system operation.
In conclusion, we can conclude that compliance with such simple and obvious at first glance recommendations for power supply of television cameras can become a decisive factor for the long and reliable operation of video surveillance systems.