Why communication Use 48v power supply Know

1. Historical relationship:

The use of -48V power supply is due to historical reasons. The use of the earliest communications network is the telephone network, the phone is powered by the Office of Telecommunications, the selection of 48V is in the conditions of the user to the end of the distance as high as possible (36V is a safe voltage, more than too much is not safe). Later, in order to be compatible with the earlier equipment and to reduce costs, the communication equipment at the end of the bureau is still using -48V power supply.

Similarly, the use of negative power systems, positive grounding is just a convention. Originally, there is a saying that there is a large number of negative charges in the air, according to the knowledge of electrochemistry, positive grounding can be adsorbed negative ions in the air, thereby protecting the shell of telecommunications equipment from corrosion. In fact, this argument is not quite right. Primary cell reactions and electrolytic reactions do cause equipment to rust, but because they exist in microscopic form on the equipment, they have little effect. For example, the network of a non-communication system is negatively grounded (e.g., the computer you are using), but it is not rusty. And -48V is isolated internally by DC/DC, and the DC/DC output is negatively grounded, and there is no corrosion or rust on the veneer. So it's the same regardless of which pole is grounded.

As for the grounding of the equipment's enclosure (connected to PGND), this is for protective purposes, the equipment on the accumulated charge quickly cascade to earth, so as not to damage the equipment and staff.

Our products basically use a -48V power supply system, with the actual voltage measured at -53.5V. This is because for reliability reasons, communications equipment comes with a battery backup (-48V), and in order to ensure that the battery is charged reliably, the supply voltage needs to be slightly higher than the battery voltage. Through the media, you may also learn of equipment that uses a -24V power supply system, which is used for design convenience in some modern internal equipment. Generally measured to the output voltage of the power supply 26.8V.

2, voltage fluctuation range relationship:

Generally require the equipment in the voltage fluctuation range of ± 20% within the normal operation. For the -48V system equipment is required to work voltage range -38.4V ~ 57.6V, but we actually generally require the working range of

-36V ~ -72V. The main reason is to consider the -48V system equipment to be compatible with the -60V power supply system, which requires -48 ~ -72V. 48~-72V, so take the set is that is the requirement of the operating voltage range of about -36V ~ -72V.

3, the human body voltage safety relationship:

Voltage is safer, for example, the human body is 50K ohms resistance, voltage -48V, 48/50000=0.00096A=0.96mA, the human body flow through the 9mA on the life-threatening.

4, the early material design relationship:

History along. n years ago, the use of electronic tubes and PNP-type germanium tubes, the circuit positive grounding comes intuitively simple and convenient. Negative power supply anti-interference to be better, but this is a long time ago, now the digital technology of this requirement is not high, so now the equipment is also useful for positive power supply, but consider the habit of versatility most of them are also still -48V

The power supply system positive grounding can reduce the corrosion phenomenon of the positive terminal of the battery

Reducing the system murmur, reducing interference

Early communications could use the earth as a loop, saving wire

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

The voltage is higher than -24V, which is conducive to the transmission of power and reduce losses

Telephone lines are complex environments, before the invention of all-plastic cables, line insulation is often poor, the use of negative power supply by the noise of the ground to be smaller than the positive power supply, a habit that is in use to this day.

5, battery line relationship:

-48 transmission equipment power supply generally have two kinds of power supply, single-channel power supply (3 power lines: -48V, working ground and protective ground) and dual-channel power supply (5 power lines: -48V two, working ground 2, protective ground line 1), generally a single PDH equipment and 155M box-type SDH transmission equipment also use single power supply, while other rack-mounted equipment uses double power supply. DC power line diameter is generally used 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 kinds of units for square millimeters. Wire diameter S = I × L / (K × ΔV), where: S - conductor cross-sectional area (mm2) I - circuit current (A), L - conductor circuit length (m) ΔV - the permissible DC voltage drop (V), K - conductor conductivity (m / Ω * mm2), where the copper conductor K = 57, aluminum conductor K = 34,? General communications DC power supply system throughout the voltage drop <3.2 V. Experience formula: S = I * L * 2 / 57 / 0.5, the results rounded up, select the upper level of wire diameter -48V and working ground wire diameter, protective ground wire diameter is reduced by one grade.

6, battery voltage relationship:

Communication power system voltage is generally the charging voltage of the battery pack. Switching power supply does not control the charging of each battery individually, but to control the charging voltage of the whole group of batteries. Generally require a single float voltage of 2.25V, the communication power supply 24 battery pack, the whole battery voltage is set to: 24 × 2.25 = 54V; 48V is required for a single float voltage of 2V. 48V for the load alarm voltage. Specific settings according to the whole battery float voltage and power system to set.