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Telephone Volts?

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  • 08-09-2009 7:16pm
    #1
    JOHNPT
    Closed Accounts Posts: 517 ✭✭✭


    What is the voltage of telephone lines? The problem is when there is 3 pairs of telephone wires how to identify which pair is active. When i use multimeter to measure voltage it usually goes from 12 volt but then reduces to 0 volts. Any help would be greatly appreciated.


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,565 Mod ✭✭✭✭2011


    52 volts DC would be normal.
    It can vary quite a bit.


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    #3
    KoolKid
    Moderators, Home & Garden Moderators, Technology & Internet Moderators Posts: 24,789 Mod ✭✭✭✭KoolKid


    I presume you are referring to your home phone John.
    If this is on a phone system if can be compleatly different. Intermittent 12vdc would be possible there.


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    #4
    JOHNPT
    Closed Accounts Posts: 517 ✭✭✭JOHNPT


    Yes its a home phone. So what volts should be on the incoming line from eircom? And if i have extensions to phone line what volts should these pair be. When i measure volts of active pair is seems to measure about 20 volt but then reduces to 0 volts. Is there a correct way to measure telephone voltage.

    Thanks for replies in advance


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    #5
    DublinDilbert
    Registered Users Posts: 5,372 ✭✭✭DublinDilbert


    How long does it take to drop off?

    Is your meter set to measure AC or DC volts?


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    #6
    Randyleprechaun
    Registered Users Posts: 876 ✭✭✭Randyleprechaun


    Generally 50 volts approx., if you ring the line while checking voltage it should jump to around 100-120V


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    #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,565 Mod ✭✭✭✭2011


    I presume you are referring to your home phone John.
    I took it to be the incoming phone line.
    When i use multimeter to measure voltage it usually goes from 12 volt but then reduces to 0 volts.
    You may have the meter switched to AC or there may be a short between the live pair. Go to the point were the line is first terminated, disconnect and try there.

    A standard (PSTN) phone line will be in the region of 52 volts DC. This will vary.
    The problem is when there is 3 pairs of telephone
    What many people do is make up a "test phone". Buy a cheap phone (about €10 in Power City). Cut the RJ11 plug off the end of the cable that goes to the phone socket. You will see that it has 2 very thin wires. Solder two thin wires to these (cores form alarm cable are ideal). Strip the ind of these wires (some people put crocadile clips on the ends). You are now the proud owner of a teast phone! When you touch these wires off the correct pair you will haer a dial tone. This is a fool proof way for testing line quality, and finding which pair is the correct pair.

    Note:
    1) Phone lines are not polarity conscious.
    2) ISDN phone lines come in at over 100 volts and can give a little shock!


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