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resonance: power supply vs. frying pan

Discussion in 'Electronic Basics' started by Dan Jacobson, Aug 17, 2005.

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  1. Dan Jacobson

    Dan Jacobson Guest

    My ham radio 110->13.8 V power supply interacts with my electric
    frying pan. No matter how far away in the house I plug them in, there
    is a harmonic that causes the power supply to make a loud hum.

    I must choose between rag chewing and food chewing. Help. What gizmo
    can I purchase to filter out the resonance between them? Actually, it
    only happens when the pan is switched to "low", not on "high".
  2. Dave Platt

    Dave Platt Guest

    My ham radio 110->13.8 V power supply interacts with my electric
    What these symptoms suggest to me is that the "low" setting of the pan
    is probably switching in either a half-wave rectifier diode, or some
    sort of triac-based switcher... the former seems more likely. This
    would cause the pan's load on the powerline mains to be very
    asymmetrical... it'd draw full current during one phase of the cycle
    and none during the opposite phase. This will have the effect of
    causing the mains voltage at all of the other outlets on the circuit
    to have a significant DC offset voltage. This will induce a current
    into the primary winding of other transformers, possibly saturating
    the core (a bigger problem with toroid transformers than with EI-core,
    I believe). The symptoms are usually buzzing and humming, and
    sometimes excessive heating in the windings and cores of affected

    Triac-based switchers, such as are found in many cheap light dimmers,
    tend to cause a slightly different set of problems. They don't
    generate a DC offset, but they do generate a lot of odd-order
    harmonics. They can cause transformer buzzing and the "singing" of
    light bulb filaments.

    There are a number of possible solutions, none of which are terribly

    - Replace the electric fry-pan with a different model that doesn't
    cause the problem. For example, a model which has multiple heating
    elements, and switches them into different combinations of series
    and parallel and disconnected states, can provide multiple power
    levels without having to use a rectifier or triac - it'll always
    present a well-behaved resistive load to the mains and won't cause
    the problem.

    - Put a high-amperage isolation transformer between the frypan and
    the mains. This will prevent the DC offset from travelling back
    onto the mains. The isolation transformer may hum or buzz when the
    pan is on its low-power setting, but the problem at your power
    supply should be greatly reduced or eliminated.

    - Put the isolation transformer between your ham supply and the
    mains. The isolation transformer may be less prone to hum or buzz
    than the power supply's transformer.

    - Try a ferroresonant constant-output-voltage transformer (Sola makes
    these). This will ensure that your power supply receives a fairly
    consistent AC waveform. [Unfortunately, these ferroresonant
    transformers are themselves somewhat problematic... they run hot,
    hum, and their output isn't a terribly good sinusoid even in the
    best of times.]

    - Change to a different type of power supply. A ham-rated
    switching-type supply might be much less affected by this problem.

    - Unplug your ham supply from the mains when the fry-pan is in use,
    and operate your station from a large battery during those times.
    Recharge the battery from the supply when the fry-pan is not in use.

    - To mis-quote the old saying, "Be cooking with gas!"
  3. ehsjr

    ehsjr Guest

    As Dave said, a battery. It will give you emergency power
    for the rig during field day or an outage, too. You can
    keep it charged by running it in parallel with the supply
    when the rig is on and the frying pan isn't.

  4. Steve Nosko

    Steve Nosko Guest

    Nice tutorial, Dave. Just what I would think, but it is clear that this
    power supply is VERY susceptible to whatever the pan is doing. One more

    Run the fry-pan on high, but use a real Variac to reduce the power... Not so
    $$$ a solution.

    73, Steve, K9DCI

    snippers on high...

    Not to mention RF noise @ HF.
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