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Bandwidth of neon xfmr

Discussion in 'Electronic Basics' started by Kevin Tate, Jan 27, 2005.

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  1. Kevin Tate

    Kevin Tate Guest

    I know neon transformers are designed to operate from 60Hz. But what
    range of input frequencies can they handle and still produce enough
    juice to light a small neon tube?

  2. For frequencies below 60 Hz: Input voltage must be reduced
    proportionately with frequency once you go only a few percent below 60 Hz.
    Output short circuit current should not change much in such a case but
    will decrease more significantly once the frequency gets below about 20-30
    Hz or so, and will decrease nearly proportionately with input voltage
    (proportionate with frequency) once the frequency is low enough for output
    current limitation to be due mostly to winding resistance rather than
    "leakage inductance" - and I guesstimate this point to be roughly 15 Hz or
    somewhere in that ballpark.
    But more importantly I suspect that when using a transformer recommended
    for a specific length or set of tubing, 1/3 to 1/2 voltage may be
    insufficient to spark through at all - which means a low frequency limit
    of around 20-30 Hz to get any results at all unless the input vopltage is
    kore than the transformer should get at the frequency in question.

    As for frequencies higher than the transformer is designed for:

    As frequency increases, there is a trend for the maximum permissible
    input voltage to increase, but less than proportionately since one
    significant loss mechanism producing heat is eddy current loss, which does
    not decrease much when frequency deviates significantly upward from
    "proper" and "proper" input voltage is maintained.
    But do expect the "leakage inductance" to cause output current to
    decrease as frequency increases!

    Operate a neon sign transformer rated for 60 Hz at 400 Hz, and I would
    guesstimate it would have its usual temperature rise when the input RMS
    voltage is about 150-160 volts. Output current to a neon tubing load at
    400 Hz instead of 60 with normal input voltage I guesstimate to be about
    16-20% of "normal", and somewhat more than proportionately more (possibly
    30% of "normal") with input voltage of 160 volts.

    Open circuit output voltage at 400 Hz: With 120V input, most likely
    slightly higher than normal, but possibly very different and even possibly
    much more. Please have a load connected when operating a neon sign
    transformer at frequency much above rated!


    Neon sign transformers have a resonance mode at some higher frequency,
    where the "leakage inductance" and the stray/interlayer capacitance of the
    secondary form a series resonant circuit. In my limited experience with a
    small sample size this has happened at a frequency around 1-1.5 KHz as
    best as I can remember.
    Such a resonance mode can be a serious problem with no load - output
    voltage can be a few times that otherwise expected, and can be a major
    stress on or even causing outright quick failure of the insulation within
    the transformer! Please have your "neon" tubing connected!

    Once frequency increases to much above that of the "resonance mode",
    expect open circuit output voltage to be a little to somewhat more than
    "expected" times the square of the ratio of "resonant frequency" to
    applied frequency. As in mostly below usual and decreasing bigtime as
    frequency increases. Additional "resonance modes" may at some frequencies
    cause some significant deviations from this, but expect mostly output
    voltage awfully low once the frequency exceeds a few KHz.
    As for output current - mostly keeps going down as frequency increases.
    (Exceptions possible but not the "norm".)


    This is for the "traditional" iron core transformers as opposed to the
    newer "electronic ballast" type devices. The latter should have
    performance varying less with input frequency and hardly changing at all
    when input frequency makes a moderate change, although probably have
    nowhere to go but down in any performance perameter (including
    reliability) if the input frequency is changed greatly.

    - Don Klipstein (, )
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