Bread here has two meanings:
(1) Why is the polarity a negative source (i.e., a positive ground)?
(2) Why is the voltage - 48V (- 36 ~ - 72V)?
Let's talk about the second problem first. The use of - 48V power supply is caused by historical reasons. The earliest communication network used is the telephone network, and the telephone is powered by the Telecommunications Bureau. 48V is selected to increase the distance from the user to the end office as much as possible under the conditions at that time (36V is the safe voltage, which exceeds too much insecurity) Later, in order to be compatible with early equipment and reduce costs, the office communication equipment still used - 48V power supply.
Similarly, with the negative power supply system, positive grounding is only a convention. There was a saying that there are a lot of negative charges in the air. According to electrochemical knowledge, positive grounding can adsorb negative ions in the air, so as to protect the shell of telecommunication equipment from corrosion. In fact, this statement is not very correct. Galvanic cell reaction and electrolytic reaction will lead to equipment rust, but because of them It exists in microcosmic form on the equipment and has little impact. For example, the networks of non communication systems are negatively grounded (such as the computer you are using), but they are not rusted. In addition, the - 48V is internally isolated by DC / DC, the DC / DC output is negatively grounded, and the corrosion and rust of the single board are not seen. Therefore, no matter which electrode is grounded, it is the same.
As for the shell grounding (PGND) of the equipment, this is for the purpose of protection. The accumulated charge on the equipment will be quickly discharged to the ground, so as not to damage the equipment and staff.
Our products basically use - 48V power supply system, and the measured actual voltage is - 53.5v. This is because for the sake of reliability, the communication equipment is equipped with a backup battery (- 48V). In order to ensure the reliable charging of the battery, the power supply voltage needs to be slightly higher than the battery voltage.
Through the media, you may also know that there are equipment using - 24V power supply system, which is used by some modern internal equipment for design convenience. Generally, the output voltage of the power supply is 26.8v.
Generally, the equipment is required to work normally within ± 20% of the voltage fluctuation range. For - 48V system equipment, the working voltage range is required to be - 38.4v ~ 57.6v, but we generally require the working range
–36V ~ -72V。 It is mainly considered that - 48V system equipment should be compatible with - 60V power supply system, which requires - 48 ~ - 72V. In this way, it is required that the working voltage range is about - 36V ~ - 72V
Incidentally, - 48V power supply system is only the communication power supply standard adopted by China and most countries. Not all countries use this standard. For example, Russia will use - 60V power supply system, and some countries use - 24V power supply system. If products are to be sold in these regions, these different standards must be taken into account.
The standards of mains power are also different all over the world. For example, 220V mains power system is adopted in China and Europe, and 110V mains power is adopted in the United States and Japan.
Why choose - 48V as the supply voltage instead of + 48V?
1. The voltage is relatively safe. For example, the human body is a 50K ohm resistance, the voltage is - 48V, 48 / 50000 = 0.00096a = 0.96ma, and the human body is in danger of life if it flows through 9ma.
2. Historical inheritance. N years ago, when using electron tubes and PNP germanium tubes, the positive grounding of the circuit was intuitive, simple and convenient. The anti-interference performance of negative power supply is better, but this is the reason a long time ago. The current digital technology does not have high requirements for this, so now the equipment can also use positive power supply, but considering the commonality, most of them are - 48v3. The positive grounding of power supply system can reduce the corrosion of positive electrode of storage tank. 4. Reduce system noise and reduce interference.
5. Early communication can use the earth as a loop to save wire
6. In order to protect the cable from corrosion due to battery reaction, the cable must be negative.
7. The voltage is higher than - 24V, which is conducive to power transmission and reduce loss
The telephone line environment is complex. Before the invention of all plastic cable, the line insulation is often poor. The negative power supply is less affected by ground gas noise than the positive power supply. This habit is still used today.
Let me explain for your reference: why should the positive pole be grounded? Firstly, the power supply should be grounded, which can effectively avoid noise and other interference. The selection of positive grounding can minimize the corrosion of grounding devices.
I personally understand why 48V is selected: 24V and 60V were originally used in communication, but later they were gradually unified to 48V. The voltage higher than 48V may cause personal injury. If it is lower than 48V, the current on the line of the load with the same power is too large. It is necessary to select a thicker power line. The investment is large, and the line voltage drop loss is large.
- 48 transmission equipment generally has two power transmission modes, single power transmission (3 power lines: - 48V, working place and protective place) and double power transmission (5 power lines: - 48V, 2 working places and 1 protective ground wire). Generally, single PDH equipment and 155m box SDH transmission equipment also use single power transmission mode, and other rack type equipment are double power transmission. The diameter of DC power line is generally 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185 and 240, with the unit of mm2. Wire diameter s = I × L/(K ×Δ 5) Where: S - conductor cross-sectional area (mm2), I - loop current (a), L - conductor loop length (m) Δ V - allowable DC voltage drop (V), K - conductor conductivity (M / Ω * mm2), in which copper conductor k = 57, aluminum conductor k = 34, the whole process voltage drop of general communication DC power supply system is less than 3.2V. Empirical formula: S = I * L * 2 / 57 / 0.5, the result is rounded up, the upper level wire diameter is - 48V and the working ground wire diameter is selected, and the protective ground wire diameter is reduced by one grade. The above are design experience values for reference only
