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

In the era of fixed line telephones, each household's telephone was powered by the telecommunications bureau. Due to the long distance between the telecommunications office and the telephone in the home, 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 was used.

Comparing some small switch devices, due to the close distance between the switch and the telephone, a -24V power supply is generally used instead of a -48V power supply, which makes it clear. With the continuous advancement of technology, telephones have evolved from hand cranked magnet type to program-controlled telephones. In the process of development, in order to be compatible with early equipment generation by generation and reduce costs, the -48V power supply has been used until now.

The difference between 48V and -48V

1. The inheritance of history. Many years ago, when using electronic tubes and PNP germanium tubes, grounding the positive electrode of the circuit was intuitive, simple, and convenient. The anti-interference performance of negative power supply is better, but this is due to a long time ago. Nowadays, digital technology no longer requires high requirements for this, so positive power supply is also used in equipment, but considering the general applicability, it is still mostly -48V.

2. If the working power supply is+48V, the iron core of equipment such as relays is connected to the frame, and the coils are energized, the coils with very thin wire diameters will be corroded, and the failure rate will be relatively high; The positive pole of the power supply is grounded (rack), and the working power supply of the coil is negative voltage. The corroded iron core will have a large volume, and the failure rate will be greatly reduced. So communication equipment adopts a positive grounding method and uses a negative voltage power supply. Therefore, grounding the positive pole of the power system can reduce the corrosion phenomenon of the positive pole of the battery.

3. Reduce system noise and minimize interference.

4. Early communication could use the earth as a circuit to save wires.

5. To protect the cable from corrosion due to battery reactions, the cable must be negative.

6. The environment of telephone lines is complex. Before the invention of all plastic cables, the insulation of the lines was often poor, and the use of negative power sources was less affected by ground noise than positive power sources. This habit has been used till today.