# Telephone Circuits

#### Ahmed

Joined Mar 16, 2005
17
I need some tutorial about Telephone circuits

thankx for all

Joined Aug 22, 2005
23
Originally posted by Ahmed@Aug 24 2005, 03:44 PM
I need some tutorial about Telephone circuits

thankx for all
[post=9875]Quoted post[/post]​
can you be more specific on what it is you want to know?

#### Ahmed

Joined Mar 16, 2005
17
i need to know how we hear dial tone at off-hook case

and what the voltages levels allocated on telephone

i.e. at on-hook state the voltage on telephone from central is 48v - dc
and at off - hook its dropped to 9v - dc

can we replace the central by 48v-dc and it will work or not.

thankxxxxxxxxxx

#### thingmaker3

Joined May 16, 2005
5,083
Telephone systems are not created equal.

When the set is "on-hook," it presents an open circuit to the central office. (Note: "central office" is a very misleading term. "Central office" in American telephony can refer to a circuit card in the big green box down the street. This same term can refer to your PBX. If you are running V/IP, then the "central office" is your server.)

The CO detects a drop in impedence when the set goes "off hook" and then generates the dial tone.

Ring voltage can exceed ninety volts. This will most definately get the attention of the apprentice technician who is leaning agianst the 66-block!! (To be fair, I've seen at least one journeyman technician make this mistake as well.)

#### hgmjr

Joined Jan 28, 2005
9,027
ahmed,

Here is a link to the page on the HowStuffWorks website that provides a pretty simple explanation of the basic operation of a telephone.

How Stuff Works Website

hgmjr

#### Shams soomro

Joined Feb 6, 2008
2
Super Moderator
The central office sent called party a ring AC msignal of 90 v 20 HZ which is
super imposed on local -48 dc supply to telephone set through subscribers loop then how ring voltage exceeds 90 volts

#### Shams soomro

Joined Feb 6, 2008
2
Why there is need of transforming anolog signals into digital signals for transmission??????????

THANKS
SHAMS........

#### Audioguru

Joined Dec 20, 2007
11,248
The CO detects a drop in impedence when the set goes "off hook" and then generates the dial tone.
No.
The CO detects DC current drawn by an off-hook phone then generates the dial tone.

#### thingmaker3

Joined May 16, 2005
5,083
No.
The CO detects DC current drawn by an off-hook phone then generates the dial tone.
DC current increases as a result of the drop in impedance. Ohms Law subjected to algebra. Typically, less than 10mA will be drawn with the phone on-hook, and more than 15mA will be drawn with the phone of-hook. At 48Vdc, that means less than 3200 Ohms off-hook versus more than 4800 Ohms on-hook.