telephone working?

[dereksohail]

Just wanna know the following ( iam not an engineer just a hobbist that;s why wanna to find out the electronics in the following):-

1) Why telephone rings even if it is on-hook ? i klnow its 60 volts Ac but how come it rings whe the telephone on-hook ?

2) When the telephone goes off-hook voltage drops why is that ?

regards,
Derek

 

[Nigel Goodwin]

Just wanna know the following ( iam not an engineer just a hobbist that;s why wanna to find out the electronics in the following):-

1) Why telephone rings even if it is on-hook ? i klnow its 60 volts Ac but how come it rings whe the telephone on-hook ?

Because there's a bell in the phone, connected via a capacitor (which passes AC but blocks DC). When the phone is on-hook this bell and capacitor are connected across the phone line.

2) When the telephone goes off-hook voltage drops why is that ?

Because there's a switch that disconnects the bell/capacitor, and connects the phone circuitry in it's place. The phone is designed to draw sufficient current to let the exchange know you have lifted the handset - as you draw current from the line the voltage drops (simple ohms law).

 

[goug]

1) Why telephone rings even if it is on-hook ? i klnow its 60 volts Ac but how come it rings whe the telephone on-hook ?

Telephone rings if it is on-hook; You can imagine it this way:
When it is on-hook, it's an open-circuit. You will be measuring the incoming voltage arround 48 to 52 Volts DC. When you off hook it, you'll have a closed circuit (through a 220 ohm resistance) and since the resistance consumes power, the voltage measured will drop to 9V (this is for your second question).

2) When the telephone goes off-hook voltage drops why is that ?

Telephone doesn't ring when it is off-hook! Nobody can call you when you lift the phone (unless you subscribe to some additional feature).

so, When the phone is On-hook, and someone calls, the ring will be sent as an AC voltage arround 70 to 100 Volts. In telephone different modules(=circuits) are responsible for different things...

Each of the following have it's own circuit or module, and all linked together to form what we called "Telephone":
- On/Off hook
- In use light indicator
- Ring detection
- Tone detection (DTMF receiver/decoder)
- DTMF generator/encoder

Good luck.