Analog phone line audio device ring protection

Discussion in 'The Projects Forum' started by Netwaves, May 6, 2015.

  1. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    1
    I'm working on a analog phone audio interface. I'm using a ceramic cap on the tip/ring side and it's only rated at 50V. Can I use two diodes to keep the voltage from destroying the ceramic caps? If so, any suggestions on diodes? Thanks.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Probably not -it depends on your circuit. If you shunt the ring voltage you might effectively be signaling that the phone has been taken off the hook. Best to use a capacitor with an appropriate voltage rating.

    As a teenager I discovered that the local phone company had low tolerance for youngsters connecting their own circuits directly to the lines.

    I know that in the United States there are federal regulations that strictly spell out the performance of an interface to the telephone lines. Safety of people and equipment is consideration, and of course protection against interference with the service and damaging phone company equipment is another.

    If you have not already, you might want to check out the regulations in your area, or maybe even contact your telephone company for advice.

    Certified telephone interface circuits that meet the safety and signaling requirements can be bought as well.
     
  3. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    1
    Good point on signaling off hook, I didn't think of that. Fortunately, in this instance, this is my own home phone multi line system and it is not connected directly to outside analog phone lines. The ring is generated via my hybrid Panasonic phone system. The analog line shouldn't ring at all unless someone dials the number by accident, and I was trying to protect against that. I guess I will just disable ringing via system software. Thanks.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Way back, I subscribed to a quasi-underground newsletter with tech details about the US phone system, blue boxes, etc. Thanks to that, I have a letter from the California Superior Court informing me that I am in possession of "illegal information". Good times...

    Search for things like "telephone hybrid circuit" and "telephone network interface schematic" to find lotsa examples of audio interface designs. Old vacuum tube audio amplifier output transformers are great for this (connected backwards with the phone line running through the secondary) because they can handle the DC static current without saturating, and have much better audio characteristics than the typical hybrid transformers made today.

    ak
     
    Last edited: May 6, 2015
    Netwaves likes this.
  5. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Yeah...I built a 'line tap' as a kid. My sister hated me!
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I bugged my sister's room - twice. And I'd say at least 50% of the electronics-type-persons I know who have sisters did the same. What is it about sisters?????

    ak
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    All of the sisters are on Facebook right now talking about all of the things they did to their brothers and asking, "what is it about brothers?"
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    Think about what I must have missed by not having a sister.
     
  9. Søren

    Senior Member

    Sep 2, 2006
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    Yeah, all said sisters hot girlfriends to pick and choose from :D
     
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