Why is the communication power supply -48V?

Why is the communication power supply -48V? What is the difference between +48V and -48V?

Welcome to Get a Quote: Email: export@bestpluspower.com
Equipment in the communications industry generally uses -48V DC power supply and the positive pole is grounded. Why?
In the era of fixed-line telephones, the telephone set of each household was powered by the central office of the telecommunications bureau. Because the distance between the telecommunications bureau and the telephone set in the home is relatively long, in order to ensure a certain service range, the power supply voltage of the central office switch cannot be too low, so a -48V power supply is used.
Compare some small exchange equipment, because the distance between the exchange and the telephone is relatively short, generally use -24V power supply instead of -48V power supply, this will understand.
With the continuous advancement of technology, the telephone has developed from a hand-operated magnet type to a program-controlled telephone. In the development process, in order to be compatible with earlier devices from generation to generation and reduce costs, the -48V power supply has been used until now.
Why is the positive electrode grounded?
Why should the positive pole be grounded? First, the power supply must be grounded, which can effectively avoid noise and other interference. Choosing positive grounding can minimize the corrosion of grounding devices.
Welcome to Get a Quote: Email: export@bestpluspower.com
The positive grounding is mainly to prevent corrosion of the electrode. The battery pack -48V or -24V of the telephone office is grounded with the positive pole. The reason is to reduce the electric corrosion caused by the poor insulation of the relay or cable metal sheath, which will damage the relay and cable metal sheath.
Because in the case of electric corrosion, metal ions move from the positive electrode to the negative electrode under a chemical reaction. If the insulation between the relay coil and the iron core is poor, a small current will flow. When the negative pole of the battery pack is grounded, the wire of the coil may be broken.
On the contrary, if the positive electrode of the battery pack is grounded, although the iron core will also be subjected to galvanic corrosion, the wire of the coil will not be corroded. The iron core has a relatively large mass and will not cause perceptible consequences. The 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, too much is unsafe.
In addition, the voltage should not be too low. If it is too low, the current on the line of the load of the same power will be too large. It is necessary to choose a thicker power line, which requires a large investment and a large line voltage drop loss.
Welcome to Get a Quote: Email: export@bestpluspower.com

For example, in 5G base stations, due to the increased power consumption of AAU, if -48V is still used, the current will be larger, the line loss will be greater, and the transmission distance will be shorter. 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, -48V
So what is the difference between 48V and -48V power supply?
+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 system is only a communication power standard adopted by my country and most countries. Not all countries use this standard. For example, Russia uses -60V power systems, and some countries use -24V power systems. If products are to be sold in these regions, these different standards must be taken into consideration.
Lighting circuit 220V, power circuit 380V, 48v electric vehicle voltage, there are other places where it is needed, 48, -48V is direct current. Because the DC voltage 36V is defined as a safe voltage internationally, weak currents are basically operated at integer multiples of 6, 6V/12V/24V, 36V and so on.
48V is generally used in the field of DC drive. It has a safe voltage, but it can provide a larger capacity. Therefore, electric bicycles are generally driven by 36V and 48V.
Welcome to Get a Quote: Email: export@bestpluspower.com
Why choose -48V as the power supply voltage instead of +48V?
The inheritance of history. N years ago, when electron tubes and PNP-type germanium tubes were used, the positive grounding of the circuit was intuitive, simple and convenient. The anti-interference of the negative power supply is better, but this is the reason a long time ago, the current digital technology has not high requirements for this, so now the equipment also has a positive power supply, but considering the habitual versatility, most of it is still -48V.
If the working power supply is +48V, the iron core of the relay and other equipment is connected to the rack, and the coil is energized, and the coil with a very thin wire diameter will be corroded, and the failure rate will be relatively high; the positive pole of the power supply is grounded (frame), the coil works It is a negative voltage, and the corroded iron core will be very large, and the failure rate will be greatly reduced. Therefore, the communication equipment adopts the positive grounding method and the negative voltage power supply. Therefore, the positive grounding of the power system can reduce the corrosion of the positive battery of the battery.
Reduce system noise and interference.
Early communication can use the earth as a loop to save wires.
To protect the cable from corrosion due to battery reaction, the cable must be negative.
The telephone line environment is complicated. Before the invention of the all-plastic cable, the line insulation was often poor. The negative power supply was less affected by ground noise than the positive power supply. This habit is still in use today.
Why choose -48V?
Originally, 24V and 60V were used for communication. Later, they were gradually unified to -48V. For voltages higher than 48V, it may cause personal injury. If the voltage is lower than 48V, the current on the line of the load with the same power is too large. The thick power cord requires a large investment and high line voltage drop loss.
Welcome to Get a Quote: Email: export@bestpluspower.com
Best Plus Power is a manufacturer and provider of dependable high efficiency power electronics, energy conversion systems and power supplies. Best Plus Power solutions include a complete line of high-efficiency rectifiers, inverters and DC power systems for mission critical applications. With over 10-Years experiences supporting customers in more than 60 countries, Best Plus Power is focused on bringing reliable power solutions to customers across all applications and environments, which specialize in Battery Chargers, Power Inverters, Power Supplies / Rectifiers, DC / DC Converters, Frequency Converters, and other Power supplies.
Email: export@bestpluspower.com
Website: www.bestpluspower.com