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AC Hum

Discussion in 'Electrical Engineering' started by Den, Sep 8, 2004.

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  1. Den

    Den Guest


    I have a cordless telephone. The base unit is powered by a wall-wart whose
    original spec was input 240Vac (frequency unspecified but as it was bought
    and operated in the UK it was de facto 50Hz) and output 9Vac/300mA.

    When I moved to the US I replaced the original wall-wart with a wall-wart
    who spec was input 120Vac/60Hz and output 9Vac/300mA.

    When I use the phone, I hear a hum. I assume that this is a AC hum - is
    this reasonable?

    Assuming that the hum is a AC hum. I assume that the phone circuitry was
    optimized to minimize a 50Hz hum. Are there any simple solutions for
    removing / reducing the 60Hz hum?


  2. Tim Perry

    Tim Perry Guest

    inside your phone there will be some sort of rectifier possibly followed by
    a voltage regulator. assuming there is no defect in you phone it is possible
    that the voltage is too low to be properly regulated. in this case the hum
    you hear is 120 Hz ripple of a full wave power supply.

    it could also be that the filter capacitor cap in the power supply has
    opened up

    it is also a possibility that the phone line itself has a problem. are
    other phones on the same line clean?
  3. Den

    Den Guest

    All of the other phones in the house (corded and cordless are fine),
    however, all of the handsets coupled to this specific coredless phone
    exhibit the problem. For this reason, I'm assuming it's an issue with the
    base unit? What to do?

  4. Some power supply units have " on board " regulation, others just supply an
    unregulated supply.

    If your phone had a regulated supply and you swapped it for an unregulated
    type this could be causing the problem...

    to test the supply,
    ( 1 ) measure the DC output voltage with a resistor of around 50 ohms across
    it, this should be around the 9V mark.
    ( 2 ) measure the AC voltage.. this is the ripple on the supply, it should
    be negligible for a regulated supply.
    ( 3 ) measure the voltage with a light load ( say a 50,000 ohm resister ) ,
    if the DC voltage is significantly larger than in ( 1 ) it's an unregulated


    Barnes's theorem; for every foolproof device
    there is a fool greater than the proof.

    To reply remove AT>
  5. Den

    Den Guest

    It's a 9V AC output not DC.


  6. Tim Perry

    Tim Perry Guest

    as I do not know your skill level in electronic servicing nor value of the
    equipment I would hesitate before recommending anything more then the

    1: take it to a qualified technician

    2: try a different wall wart (but sill the same value)

    3: place unit in a shoebox on the top shelf in you closet. obtain new unit
    at the Super Duper Wonder Bargain Made in Korea Store.

    going beyond these steps i feel would create a possable fire hazard.

    an additional thought.. as the unit is from the UK is it FCC type accepted?
    are you transmitting in an allowable band for the US?
  7. Both of mine work on DC....
    Are you sure your original supply was AC...

  8. Den

    Den Guest


    So get ready to hit me!

    The base unit was plugged into a surge protector. Take the surge protector
    away and the hum disappears!


  9. Tim Perry

    Tim Perry Guest

    i'd suspect a high resistance path causing low voltage and thence poor
    ripple regulation.

    wow first time i have ever used thence in a sentence. i hope my syntax is
  10. Den

    Den Guest

    Yup, *very* definitely AC.


  11. Den

    Den Guest

    Does that work for all applications, that a AC source can be replaced with a
    DC source. I assume not if there is a motor or a frequency dependant issue?

  12. Den

    Den Guest

    OK, so now getting well off-topic, but to continue my education.

    I have an battery powered inflator for an air-bed. The unit has a socket
    for a power supply from a wall wart that is market 6V. The wall-wart that
    came with the unit was specified as 7.5VDC/700mA. Does this mean that I
    could replace the wall wart with a 6V unit outputing the same power i.e.
    7.5V * 0.7A = 5.25W; 5.25W @ 6V = 0.875A

    Why would the wall-wart be apparently overvoltaged for the unit, or is this
    what you mean by the 20%-30 rule of thumb range (I had assumed that you
    meant current not voltage)?


  13. Den

    Den Guest


    Thanks for taking the time to explain. I appreciate it.


  14. Norm Dresner

    Norm Dresner Guest

    There are two kinds of wall warts, regulated and unregulated. A regulated
    power supply will put out a constant voltage up to and including its rated
    current limit (beyond that it may droop or it may shut down or it may
    self-destruct). An unregulated DC supply usually has a transformer and a
    diode or two. It may also have a capacitor to filter the hum and present an
    almost DC voltage for some current draw. The manufacturer may have measured
    the output of the wall wart they supplied and found that at the current the
    unit draws the voltage was close enough to the 6V required to be usable --
    and it was cheaper than a 6V regulated supply wall wart.

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