TELEPHONE EXCHANGE

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A Simple Telephone

Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the 1920s, you could connect it to the wall jack in your house and it would work fine!
The very simplest working telephone would look like this inside:


power supply:

-48 V dc is used in telephone exchange:-

A rectifier converts the AC at the power mains to DC, and the DC output from the rectifier is used to charge a 48V lead-acid battery. The battery powers the user telephones through the telephone line. The battery polarities are connected so that the line voltage is negative (-48V). A negative line voltage helps to reduce the corrosion from electrochemical reactions occurring on a wet telephone line. A telecom network also uses several DC-DC converters to derive intermediate power-supply rails from the -48V DC input. The intermediate power supply rails power the switches, radios, routers, ATX computers, and other electronic equipment in the telephone exchange. A current-sense amplifier oversees the system health by monitoring the -48V power-supply current

Why 48V voltage is used in telephone systems ?
The -48V voltage was selected because it was enough to get through kilometers of thin telephone wire and still low enough to be safe (electrical safety regulations in many countries consider DC voltages lower than 50V to be safe low voltage circuits).
48V voltage is also easy to generate from normal lead acid batteries (4 x

12V car battery in series). Batteries are needed in telephone central to make sure that it operates also when mains voltage is cut and they also give very stable output voltage which is needed for reliable operation of all the circuit in the central office. Typically the CO actually runs off of the battery chargers with the batteries in parallel getting a floating charge.
The line feeding voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful.
When the wires are at negative potential compared to the ground the metal ions go form the ground to the wire instead of the situation where positive voltage would cause metal from the wire to leave which causes quick corrosion.
1. For Safety purpose (protection from Shock) .
2. To reduce Losses which occurs in field Transmission


COLOR CODE:
The 25-pair color code is a color code used to identify individual conductors in a kind of electrical telecommunication wiring for indoor use, known as twisted pair cables.The colors are applied to the insulation that covers each conductor.
The first color is chosen from one group of five colors and the other from a second group of five colors, giving 25 combinations of two colors.
• The first group of colors is, in order: white, red, black, yellow, violet. (Why Run Backwards, You'll Vomit)
• The second group of colors is, in order: blue, orange, green, brown, slate. (BSNL OFFICE GIVES BROADBAND SERVICE)