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noisy power adaptor/transformer in telephone intercom

Discussion in 'Electronic Basics' started by Simon Bartlett, Dec 21, 2003.

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  1. Hello group,

    I'm trying to create a phone intercom system, but am running into a
    problem. The circuit
    (http://www.epanorama.net/documents/telecom/telephone_intercom.html)
    calls for a 12v power supply. I've got two power adapters/transformers
    (the things you plug into the mains). One is 12v 1A, the other 12v
    860mA. With the 12v 1A unit, the phone system works perfectly, but with
    the 860mA unit, I get a loud buzzing on the line. If I use more
    resistors to get rid of the buzzing, no voice is able to get through
    either.

    I've also tried a 20v unit I had lying around, and it too makes the
    buzzing noise.

    Now I can't use the 12v 1A unit, as it belongs to my computer router :-(

    Does anyone have any idea? I blew up my multimeter, and am hoping to
    get a new one for Christmas, so haven't replaced it yet, so can't use
    for testing.

    I'm very new to all this, so please don't assume a heap of knowledge on
    my part in any response.

    Many thanks,

    Simon.
     
  2. Assuming you are using a DC wall wart it sounds like you need to apply some
    filtering and smoothing to it's output. Like a choke or resistor with some
    electrolytic capacitors of a few thou microfarads.
     
  3. tempus fugit

    tempus fugit Guest

    I'm guessing that your 860mA adaptor has insufficient filtering on it. When
    the AC from the wall gets converted to DC, there is a considerable amount of
    noise (known as ripple) that we hear as hum (maybe the buzzing you are
    hearing). To get rid of the ripple, large value capacitors are used after
    the rectifier stage in the adaptor. If the caps aren't big enough, there
    will be ripple that you will hear as noise. The other possible problem is
    that the adaptor is not regulated. If the DC voltage is regulated, it means
    that the voltage will remain constant regardless of how much current is
    drawn from it (within spec of course). Regulation also helps to reduce
    ripple.

    Maybe the 1A adaptor is regulated, maybe it just has better filtering.
     
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