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voltage on telephone line

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ravi17

New Member
I look around on the net but still a concrete info on voltages on telephone line is not clearly given. is the following correct?

on-hook: 48V DC(or higher)
on-hook is understood,

what about the following?

ringing: 20Hz AC signal riding on 48V???
off-hook: speech signal 20-30mV on 48V???
 

MikeMl

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Most Helpful Member
I would say it a bit different.

On-hook line-to-line: <49.9Vdc

Off-hook line-to-line: 3V to ~ 20Vdc, depending on distance from central office (loop resistance).

Ringing: ~100Vac 20Hz riding on the on-hook DC level.

Off-hook speech level: 0.5V to 2Vac riding on the off-hook DC level. The speech source (microphone) inside a subscriber phone modulates the current on the line, so the voltage it develops across the line varies greatly depending on loop-resistance.
 

ravi17

New Member
thanks a lot mike, finally i get my answer. books and explanation on the net do not clearly state in this way.
 

canadaelk

Active Member
MM's specs are correct for North America. Other countries/continents may have slightly different specifications. E
 

audioguru

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The DC voltage at your phone drops when the phone is off hook because the telephone company and telephone line have resistance and an off hook phone draws DC current.
 

dr pepper

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Slightly diffrent here in the Uk, the exchange uses a 50v supply, so slightly less at the subscriber due to resistance, ringing is 75v ac, and to make the line go off hook a 600 ohm resistance is placed across it.
Also theres a max current for the ringing signal, they call it Ren, ringer extension number, which basically means if you pull more that 80ma while the line is ringing the phone company will think theres a short, and will come knocking.
Also here in the Uk 112 is the same as 999, or 911 in the Us, 112 is easily accidentally dialled when your installing an extension as I found out when the emergency services rang me asking why I'd called them.
 

ravi17

New Member
Slightly diffrent here in the Uk, the exchange uses a 50v supply, so slightly less at the subscriber due to resistance, ringing is 75v ac, and to make the line go off hook a 600 ohm resistance is placed across it.
Also theres a max current for the ringing signal, they call it Ren, ringer extension number, which basically means if you pull more that 80ma while the line is ringing the phone company will think theres a short, and will come knocking.
Also here in the Uk 112 is the same as 999, or 911 in the Us, 112 is easily accidentally dialled when your installing an extension as I found out when the emergency services rang me asking why I'd called them.
the ac ringing is not well understood, do you mean during ringing, 75VAC, 20Hz is riding on on-hook DC level(3V to ~ 20Vdc)?
 

dr pepper

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For the Uk the line is 50v dc, either polarity (in the old days the polarity used to switch to energise the call billing meter), when the phone is on hook, which drops to about 20v when the phone is off hook.
While ringing the line changes to ac 75v @ 50hz.
Btw the phone company doesnt like people using the line to charge batteries or power things, their computerised diagnostic system picks up on it and they send an engineer unnecessarily to repair a fault that doesnt exist (some 70s dtmf phones had recahargeable batteries).
 

audioguru

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the ac ringing is not well understood, do you mean during ringing, 75VAC, 20Hz is riding on on-hook DC level(3V to ~ 20Vdc)?
The DC on-hook voltage of the phone line is 50V. Ringing rides on it.
 
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