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  1. #1
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    dc to ac coverter circuit not working

    hello guys, I ve designed a coverter from a premade schematic which by the way did work fine last time (1 year ago on project board ) now ,I reassembled it and can't make it work again

    I don't remember the arrangements for tip41/42 transistors but according to datasheets I think I'm doing it right, I'm using with ac to dc adaptor for input 12v (seems like when I connect it to circuit it short-circuits)
    any ideas? whats wrong? I need this so bad

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  2. #2
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    Re: dc to ac coverter circuit not working

    Hi,

    I'm understanding you mean the 555/BJTs circuit short-circuits.

    Does the AC/DC adapter have a barrel jack adapter, by any small chance? If the circuit is "exactly" the same as last year's version, I'd get a multimeter and check the polarity of the barrel jack connector terminals or look for the little picture on the adapter sticker that shows whether outer part is +V or 0V.

    It's quite common to fry things this way as unfortunately it's manufacturers' whim to use inner ring as +V or 0V - never understood why it isn't internationally obligatory for all devices to be +V inner ring and 0V outer ring on barrel jacks.



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  3. #3
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    Re: dc to ac coverter circuit not working

    That old circuit has many voltage losses in it.
    You did not describe the output current rating of the 12V adapter, the resistance of L1, and the transformer: primary and secondary voltage ratings, power rating and frequency rating.
    Does the circuit have a load connected? What resistance is the load?



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  4. #4
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    Re: dc to ac coverter circuit not working

    the 555 cannot really provide sufficient base current to drive the xtors ....



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    Re: dc to ac coverter circuit not working

    Quote Originally Posted by Easy peasy View Post
    the 555 cannot really provide sufficient base current to drive the xtors ....
    It depends on how much output power you want.
    1) The 555 output is rated at 200mA.
    2) The TIP41 and TIP42 have typical hFE of 25 at 4A.
    3) With a supply of 13V and an output of 160mA the output swing of the TIP transistor pair is 10.5V p-p and the 100 ohms resistor reduces it to ZERO so it will not work.
    4) Shorting the resistor and feeding the 10.5V p-p to the TIP transistors pair, their typical output at 4A will be 8V peak to peak which is 16W.
    5) The inductor filters the squarewave into a sort-of sinewave with an RMS voltage of 3V so the output transformer must be 3V to the supply voltage at 16W.
    6) If the transistors have minimum spec's then this project will not work.



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  6. #6
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    Re: dc to ac coverter circuit not working

    Hi,

    No new useful input: While it's generalising and a touch unfair to some legitimate people who share their ideas in the Interweb, I learnt to be very suspicious of coloured-in schematics (or those with a person's name plastered across them diagonally).



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    Re: dc to ac coverter circuit not working

    There is also the issue of reverse voltages across the transistor junctions being too high, particularly with an inductive load. Not a very clever design but with some additional driver components it could be made to work although it would never be very efficient.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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  8. #8
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    Re: dc to ac coverter circuit not working

    If there was full bridge drive, and the L and the C were resonant at 50Hz ( or 60Hz ) and the Lmag of the Tx was low, then the turn off current could be quite low and therefore efficient. But much better thought out gate drive needed - and large mosfets ... H bridge ...



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