Relating to the stability of the whole weak system power cord is how to choose? Wire How much is appropriate? Cable attenuation how calculated?
Here example video surveillance system computing power line
A monitor power cord damping calculating
1, the wires drop the following factors:
1, the specifications of the transmission line, i.e., diameter; diameter know to know the resistivity calculated!
2, from the transmission line;
3, movement of the front devices (cameras, PTZ, decoder) current rated power;
4, the control to know the rated voltage of the control voltage and the distal end of the load.
2, a detailed analysis
1. Wire calculated: wire and cable specifications are represented by cross-sectional area, such as 1.5mm2, 2.5mm2, usually diameter wire may be divided by two, and then squared, multiplied by 3.14. As only 1.5 square copper wire diameter 1.38mm, calculated (1.38 / 2) × (1.38 / 2) × 3.14 × 1 = 1.494954 square share, which is qualified GB diameter.
2. Calculating the resistance: Resistance = resistivity * length / cross-section
3. If the growth of a variety of materials 1 m, 1 mm2 cross-sectional area of the wire, measuring their resistance at 20 ℃ (this material is referred to as resistivity) and compared, the minimum resistivity of silver, followed by press copper, aluminum, tungsten, iron, copper, manganese, sequentially, the resistivity nichrome gradually increases.
The resistivity of the aluminum wire is 1.5 times more than copper wire, its resistivity p = 0.0294Ωmm2 / m, the copper resistivity p = 0.01851 Ω · mm2 / m, the resistivity changes with temperature have some differences.
If the copper wire 2 * 1.0 200m long as a power supply line, the electrical resistivity value = 0.01851 * 200/1 = 3.702Ω
4. Voltage drop circuit enables the: general requirements infrared bolt voltage is DC 12V, if 15V DC power supply, then the bolt, allowing the voltage difference is 3V.
5. Maximum current line * = number of device operating current devices, such as a bolt on a total of two lines, each bolt operating current of 500mA, then the maximum line current = 500mA * 2
6. = The line resistance of the wire to allow maximum current line voltage drop ÷
Can be deduced from the above equation, if the station is a required operating voltage of 12V, at a maximum current of 500ma supply bolt, for remote power feeding DC 15V, copper wire 2 * 1.0 power transmission is performed, the maximum line length Should be:
Resistance of the wire = (15-12) ÷ 0.5 = 6Ω
Resistance of the wire should be less than 6Ω
The resistance value = resistivity * length / cross-sectional length = cross-sectional resistance value * / Resistivity
Therefore, the maximum permissible wire length = 6 * / 0.01851 = 324M
So the use of copper wire 2 * 1.0, the wire length should be less than 324M, since the wire is bifilar, so if the copper wire 2 * 1.0, the length should be less than 324M / 2 = 162M.
As can be seen from the above calculation, in general, it should be the front end power supply, i.e., a bus using the 220V power line transformer to each camera, is economical and practical manner.
12V, 1A power by 300 m (300 m if the loop length so that the distance 600 m, according to calculation noodle 600) copper power supply line, the number of voltage decay obtained?
If the cable is 1 mm2 copper wire:
That was as follows: Attenuation circuit voltage U = 300 meters. 1A * Current * (1 mm2 copper conductor resistance of 0.018 ohms) / Cross section (if it is 1 square mm) = 5.4V (if it is 600 meters, then the pressure can drop down 10.8V)
Several metal conductor at 20 ℃ resistivity (Ω m)
(1) silver 1.65 × 10-8
(2) Cu 1.75 × 10-8
(3) Aluminum 2.83 × 10-8
(4) 5.48 × 10-8 W
(5) iron 9.78 × 10-8
(6) platinum, 2.22 × 10-7
(7) 4.4 × 10-7 manganin
(8) 9.6 × 10-7 mercury
(9) 5.0 × 10-7 constantan
(10) 1.0 × 10-6 nichrome
(11) Fe-Cr alloy 1.4 × 10-6
(12) aluminum-nickel-iron alloy 1.6 × 10-6
(13) Graphite (8 ~ 13) × 10-6
It can be seen that the smaller the resistivity of the metal, the resistivity of the alloy is larger, a larger number of non-metal and metal oxide, the resistance of the insulator great. Germanium, silicon, selenium, copper oxide, boron, and less than the resistivity larger than metal insulator semiconductor we named this material (semiconductors).
Second, the selection of monitoring the power supply line wire
A, Note:
1, if the centralized power supply used in too small diameter, and then using the power supply is wasted.
2, note that in the calculation of the power loss of 30% of the line to be calculated, and the left margin of 30%.
3, try not to use only one power supply, often need to be open when the system maintenance, turn off the power, turn the power on all instant camera starting current is particularly large, a great impact on the power supply, a serious burn power; when the power fails when the entire system to a standstill.
4, the outdoor environment is relatively poor, try to avoid direct sunlight and rain place, the actual use of the power supply should be 40% larger.
B, the use of inferior cables hazards
1, the power vacuity
This is the most common phenomenon. At present, many marked security monitoring 2A power supply on the market, the actual output is only about 1A or less! Security monitoring part because the use of low-quality power components or the like disassemble failed components, in use for some time, the output power generation significantly attenuated!
The impact to the camera: image camera jitter, no color or no image, poor night vision. Serious, can not even start the camera (especially at night when the infrared light is turned on). In this case the camera can be loaded in the case, the voltage value of the power measured at the interface, can be found in the actual output current value of the power supply.
2, lack of protective measures
Many low-quality security monitoring power supply, failure of normal power supply design, the use of overload over-temperature protection design, and even save fuse. Adverse effects: cause the camera to be burned, the fire appeared to smoking occur in extreme cases even the power supply itself.
3, the filter is not clean, serious interference image
Currently on the market to unreasonably low prices security monitoring power supply, which circuit boards are substantially no common mode inductance, or filter. Such profiteers in order to reduce costs, gain profits.
Adverse effect on the monitoring results: moire monitor screen appears, in poor lighting conditions (e.g. night) especially moire.
4, the input line, output line failure
(Cut line input and output, and count a few copper, facework are reluctant to spend money, let alone the inside) diameter of the input lines, output lines too small, even using poor quality materials such as steel clad.
Adverse effects: After a period of insufficient output. Cable sheath cracking, especially in the winter. O cable breakage. Impact of surveillance cameras working properly.
5, inferior plastic shell
Power supply housing is an important protection against damaging external power supply or electric shock. Part bad businesses produce cheap low-quality plastic housing security monitoring used, after use for a long time or in outdoor environments, discoloration occurs, shell cracking.
Hazard: If the water supply, ranging from damage to the power supply, while in burned surveillance cameras, and even electric shock accident occurred, causing injuries to personal safety.
Poor selection of power supply, there are a variety of potential unnecessary risk of failure. Part of the poor quality power supply does not appear in the early use of obvious problems, but after a period of use of various prone to failure, the more harm to the monitoring system, to buy those unnecessary economic loss or loss of reputation.