Communications industry equipment generally uses -48V DC power supply, and the positive pole is grounded. Why is this?
Historical reasons
In the era of fixed-line telephones, the telephones in every household were powered by the telecommunications bureau.
Due to the long distance between the telecommunications bureau and the telephones in the home, in order to ensure a certain service range, the power supply voltage of the bureau-side switch cannot be too low, so a -48V power supply is used.
Comparing some small switch equipment, due to the close distance between the switch and the telephone, -24V power supply is generally used instead of -48V power supply, which makes sense.
With the continuous advancement of technology, telephones have developed from hand-cranked magnets to program-controlled telephones. In the process of development, in order to be compatible with early equipment from generation to generation and reduce costs, -48V power supply has been used until now.
Why is it positive grounding?
Why is positive grounding required? First of all, the power supply must be grounded, which can effectively avoid noise and other interference. Choosing positive grounding can minimize the corrosion of the grounding device.
The positive pole grounding is mainly to prevent the corrosion of the electrode. The telephone company battery pack -48V or -24V is positively grounded. The reason is to reduce the electrolytic corrosion caused by poor insulation of the metal sheath of the relay or cable, which damages the metal sheath of the relay and the cable.
Because during electrolytic corrosion, metal ions move from the positive pole to the negative pole under chemical reaction. If the insulation between the relay coil and the iron core is poor, a small current will flow through. When the negative pole of the battery pack is grounded, the wire of the coil may be corroded.
On the contrary, if the positive pole of the battery pack is grounded, although the iron core will also be corroded, the wire of the coil will not be corroded. The mass of the iron core is large and there will be no noticeable consequences. Positive grounding can also protect the core wire of the external cable from corrosion when the insulation is poor.
Safety and cost considerations
36V is a safe voltage, and it is unsafe to exceed it too much.
In addition, the voltage cannot be too low. If it is too low, the current on the line of the same power load is too large, and a thicker power cord should be selected, which will cost a lot of money and cause a large line voltage drop loss.
For example, for 5G base stations, due to the increase in AAU power consumption, if -48V is still used, the current will be larger, the line loss will be greater, and the transmission distance will be shorter. It cannot meet some AAU remote scenarios. It can only replace cables with larger wire diameters, but this will increase the cost and the load of the tower. For this reason, the voltage will be appropriately increased, such as -57V, to reduce construction costs and speed up the pace of construction.
The difference between 220V, 48V, and -48V
So what is the difference between 48V and -48V power supplies?
+48V and -48V have the same voltage, but the current flows in different directions. +48V flows to 0V, and V0 flows to -48V.
The -48V power supply system is only the communication power supply standard adopted by my country and most countries. Not all countries use this standard. For example, Russia uses a -60V power supply system, and some countries use a -24V power supply system. If the product is to be sold in these regions, these different standards must be taken into account.
The lighting circuit is 220V, the power circuit is 380V, the electric vehicle voltage is 48v, and there are other places where it is needed. 48, -48V is direct current. Because the international definition of DC voltage 36V is a safe voltage, weak current is basically operated with integer multiples of 6, 6V/12V/24V, 36V, etc.
48V is generally used in the field of DC drive. It has a safe voltage, but can provide a larger capacity. Therefore, electric bicycles are generally driven by 36V and 48V.
Why choose -48V as the power supply voltage instead of +48V?
1. Historical inheritance. n years ago, when using electron tubes and PNP germanium tubes, it was intuitive, simple and convenient to ground the positive pole of the circuit. The anti-interference ability of the negative power supply is better, but this is a long time ago. The current digital technology does not require this, so the current equipment also uses positive power supply, but considering the habit of commonality, most of them are still -48V.
2. If the working power supply is +48V, the core of the relay and other equipment is connected to the frame, and the coil is energized, the coil with a very thin wire diameter will be corroded, and the failure rate will be relatively high; if the positive pole of the power supply is grounded (frame), the coil working power supply is negative voltage, and the core with a large volume will be corroded, and the failure rate will be greatly reduced. Therefore, communication equipment uses positive grounding and negative voltage power supply. Therefore, positive grounding of the power supply system can reduce the corrosion of the positive pole of the battery.
3. Reduce system noise and interference.
4. In early communications, the earth can be used as a loop to save wires.
5. In order to protect the cable from being corroded by battery reaction, the cable must be negative.
6. The telephone line environment is complex. Before the invention of all-plastic cables, the line insulation was often poor. The use of negative power supply is less affected by ground noise than positive power supply. This habit has been used to this day.
Why choose -48V?
In the past, 24V and 60V were mostly used in communications, but later they were gradually unified to -48V. Voltages higher than 48V may cause harm to people, and voltages lower than 48V will cause too much current on the line for loads of the same power. Thicker power lines must be selected, which require large investments and cause large line voltage drop losses.
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