constant 5v from telephone line

Discussion in 'The Projects Forum' started by damienwhite, Nov 9, 2009.

  1. damienwhite

    Thread Starter New Member

    Nov 9, 2009
    Hi, I am currently putting together a project which i will be connecting to a telephone line. I have everything sorted except for the fact that i need a constant 5 VDC to power the device. The problem i have is that there are 3 different voltages that exist on a phone line and they vary between AC and DC. I need about 50ma

    Does anyone know how I will be able to get constant 5VDC from the line?

    Thanks in advance
  2. mik3

    Senior Member

    Feb 4, 2008
    You will need a kind of voltage regulator (SMPS is preferred in this case this a telephone line does provide enough power) but I don't know what is the maximum output current of a telephone line.

    Anyway, this threat will be closed by a moderator since it is not legal to do that. :p
  3. SgtWookie


    Jul 17, 2007
    Your telephone company will likely disconnect your service not long after you connect your project, and won't re-connect it until after you have removed your project from their lines - and there will be charges for the reconnection of the service.

    Telephone companies are not in the business of supplying power; just telecommunications. If they were in the power business they would not be called telephone companies; they would be called power companies.
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
    A 5V zener across the telco line will provide what you want. Observe its polarity or use it after a quad diode steering bridge.
  5. damienwhite

    Thread Starter New Member

    Nov 9, 2009
    Hi Guys,

    We will be getting this tested by the relevant testing authorities. This is a prototype of a commercial product which i am trying to build so it will certainly not be illegal. I just dont have the financial resources to hire engineers to design this part of the circuit.

    I want to draw current the same way any regular telephone or caller ID unit wants to. My problem however is i need constant power where most telephone devices do not need power all the time.

    Thanks guys
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    A Caller Id uses a tiny bit of power; like <10uA. That is within what the Telco allows; 50mA is definitely NOT!!!
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Think of a Telco switch with 50,000 users, if every home were drawing 50mA, the phone company would need to deliver 2,500 amps (120kW) continually.

    Isn't that a bit much to ask of them when they are only supplying power for voice and ringer?

    The actual power delivered is a throwback to the carbon microphone days of the phone system, when a handset had no amplifier.
  8. Audioguru


    Dec 20, 2007
    A 5V zener diode will waste all the current from the phone line (maybe as high as 30mA if the central office is nearby) making heat. There will not be any current to power your circuit.

    My speakerphone has its low volume audio amplifier powered from the phone line. It uses 20V at about 10mA.

    If your circuit draws power all the time then the phone line will be off-hook all the time and the phone company will disconnect the "faulty" phone line.
  9. BMorse

    AAC Fanatic!

    Sep 26, 2009

    Caller IDs have Batteries in them that supply power (show me a phone/called ID that does not have it s own power source....) , I dont care what kind of commercial product you are making, the phone company will still not let you draw any power from the line AT ALL!!! I have designed many products that deal with phone lines and not one is powered off of the phone line...

    But I have used a way to derive some power from the line and store it for powering something off of it, this can be done without disturbing the phone line....

    My .02
    Last edited: Nov 10, 2009