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Choke design

Discussion in 'Electronic Basics' started by bubbles123, Aug 2, 2007.

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  1. bubbles123

    bubbles123 Guest

    I need a choke to block out AC signals of approximately 1 MHz. It
    needs to work for 500mA for a +/- 5V supply. How do I choose a value
    for the choke? What factors need to be taken into account?
  2. Lord Garth

    Lord Garth Guest

    Xl=2*PI*F*L Calculate for a high value of Xl

    It's easier to find what values of L are available to you
    at a reasonable cost unless you intend to wind your own.
  3. ehsjr

    ehsjr Guest

    Factors: the choke current rating and DC resistance.
    Get the highest value of inductance that will handle
    the current for maximum attenuation of the ~ 1MHz signal.
    Check that the DC resistance of the choke will not impact
    circuit operation beyond specs.

    For example, a 2.2 mH choke from Mouser (542-5900-222-RC)
    will handle 500mA, but with 2 ohms DC resistance, you'll
    get a 1 volt drop across it at 500 mA. Depending on where
    that is in the circuit, it might be unacceptable.

  4. Phil Allison

    Phil Allison Guest


    ** The choke must have a high impedance value at 1 MHz and not affect the DC
    power delivery.

    This is not an easy double requirement to meet because all chokes have self
    capacitance - the larger the value the more capacitance it has -
    resulting in a self resonant frequency that may well be below 1 MHz and ruin
    the wanted blocking effect.

    Normal practice is to use a low value choke wired in series PLUS a
    capacitor wired in * parallel * to filter DC power. It is easy to find
    capacitors with very low impedances at and around 1 MHz - ie most
    electros of over 100uF.

    ....... Phil
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