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Need help on building a filter for power supply
Last Post 10 Jan 2013 09:11 AM by jwd217. 3 Replies.
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einstein2nd
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Posts:3

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05 Jan 2013 05:59 PM
    I am building a power supply from scratch and need help on selecting the right size inductor and capacitor for an LC filter.

    The power supply consists of: transformer, full wave rectifiers, filters, and regulators.

    Transformer specs:
    Input: 110 VAC 60 Hz rated @ 15 amps;
    Output: 15 VAC 60 Hz rated @ 30 amps;

    Full Wave Rectifier specs:
    bridge rectifiers rated @ 40 amps;

    LC Filter specs:
    Input: 15 Volt Pulsating DC rated @ 40 amps;

    Regulator specs are currently unknown.

    I have avionics background but a little rusty on LC filter circuits.
    I am trying to flatten out the ripple as much as possible before sending it to the regulator.

    I am hoping someone can help me.
    einstein2nd
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    Posts:3

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    05 Jan 2013 06:50 PM

    Transformer specs:
    Input: 110 VAC 60 Hz rated @ 15 amps;
    Output: 15 VAC 60 Hz rated @ 30 amps;


    When I built the transformer as an 8:1 I didn't realize that when rectified the output voltage would be cut in half. So I will have to redesign the transformer to currect the output voltage. I will test the outputs of the transformer soon to see if my realization is correct then make changes accordingly.
    jwd217
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    Posts:35

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    10 Jan 2013 09:10 AM
    An LC filter is not normally used in a power supply. Normally the transformer output is connected to a bridge rectifier and then a relatively large capacitor is used to filter the pulsing DC to fairly smooth DC. The “L” inductor is not needed. Also when the output of a transformer is connected to a bridge rectifier, the DC voltage output increases about 30 to 40% more than the AC input, instead of being cut in half. After the capacitor you would then have any regulation circuitry needed.
    Something like this.

    Trans Rectifier
    -- -------
    AC )( _/\___________ DC Output
    )( | \/ _|_
    ) ------- _ _ Cap
    -- |_________|____ Common
    jwd217
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    Posts:35

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    10 Jan 2013 09:11 AM
    When my previous reply was posted it got reformated so the text based drawing I did got messed up. Sorry.
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