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Please help understand this circuit .

Discussion in 'General Electronics Discussion' started by Elecbegginner, Jun 30, 2013.

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  1. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    Hello everyone in this vry helpful forum .

    Am trying to understand this circuit that belongs to the entry part of an electronic card .; the output goes to a lamp .
    Actually i have two identic cards , but one is not working ; So when i measure the number 2 (yellow one) capacitor on the working card i get 220 v , but none on the other , so i tried to change that , but same thing no light at output .

    How could i identify the faulty parts ??
    Here's the shematic and the card itself :
    [​IMG] [​IMG]
     
    Last edited: Jun 30, 2013
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    it looks like a switchmode PSU

    but I can almost garantee you have drawn C1,C3 incorrectly
    they WONT be in series with the supply rail

    check out your tracing of the circuit

    Dave
     
  3. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    Thanks sir for the feedback , so lets skip the c1 and c3 part ( i don't have the card right now)
    The output of the card is connected to a lamp , but a highly lightened lamp , because there's is 3 N CHANNEL ENHANCEMENT MODE MOSFETS right there that supplies the necessary current , if am not wrong .
    So what about the rest , i guess there's a DC/DC transformer , , and why the capacitors and bridge rectifier are before the transformer and not after like most power supplies ?
     
    Last edited: Jun 30, 2013
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    Cuz as I said in my first post, it looks like a switchmode PSU
    the mains voltage comes in (120/220/240VAC depending on your country) it gets filtered with inductors and capacitors, then it gets rectified to hi voltage DC and smoothed with that large electrolytic cap you see 1/2 way along the board then fed to a switching transistor/MOSFET connected to the primary of a transformer.
    you get low voltage AC off the secondary which is rectified and smoothed and then sent to the output pins.
    There will also likely be a feedback loop from the secondary side to the primary side to control the switching transistor so as to regulate the output voltage

    Dave
     
  5. duke37

    duke37

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    769
    Jan 9, 2011
    Has the big fat capacitor got a domed top?
    What is its capacitance and voltage?
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    There are two almost identical transformers. I suspect this means there's a power factor correction stage in there too. An in-focus close-up of the underside, with the IC part numbers visible, would confirm this.

    Also it looks like there may be a daughter board. A close-up of that would help as well.

    If you want us to diagnose the fault, you need to give us a lot more information. Photos would be a start, and a complete (and correct) schematic diagram would be VERY helpful.
     
  7. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    No the cap looks pretty good , that's the problem i don't see any physical damage on the parts , but as i already said , in the working card i see 220 v on the yellow cap , but not in the non functional card. and i checked it and its working fine , any ways the problem is just in that marked area , a just cant figure it out .
    How do i check that 1st transformer and is is a DC to DC trans ??
     
  8. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    could it be from the C4 and C5 caps ??
     
  9. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    Ok thanks ,
    Indeed there is a power factor correction , also there is a daughter board .(8 pins board)

    The difficulty for me is that the card is a RECTO-VERSO card .
    But i'll try anyways .
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    have you checked the fuse ?
    its that little brown component lower left corner just by the left corner of the socket ?

    Dave
     
  11. Elecbegginner

    Elecbegginner

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    Mar 24, 2013
    The fuse is good .
    How do i check those ceramic AC caps with DMM ?
     
  12. gacman

    gacman

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    Jun 27, 2013
    Davenn is right!

    Basically, it's a rectifier. It converts AC to DC. The transformer steps the voltage down...not only that, it serves as load isolation (from the mains).
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009

    you cant...
    you would have to remove them and test with a capacitance meter


    Dave
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    You seem to be guessing. You'll probably fix it quicker if you're a bit more methodical.

    You haven't posted proper pictures yet. I would like to see the underside, with IC markings clearly visible, and a closeup of the daughter board, as well as top-side views from different angles. (Some components are hard to see from some angles.)

    All photos need to be well-lit - outdoor light, but not direct sunlight, is best - and in-focus.

    The big electrolytic half way along the board is a good place to start. It should have around 400V DC across it if your mains voltage is 230V, and either 200V DC or 400V DC if your mains voltage is 110V.

    What is your mains voltage?

    What are the markings on this capacitor? (Duke asked you this earlier but you didn't answer.)

    If there is the right voltage on the big electrolytic, the problem is in the switching converter stage, which is at the top end of the board (as shown in post #1). Otherwise the problem could be almost anywhere.

    You've mentioned a yellow capacitor but I don't see a yellow capacitor on the board. There's a pale-coloured one next to the bridge rectifier... is that the one you mean?

    To help further, we really need some very clear pictures of both sides of the board, and ideally a complete and correct schematic diagram.
     
  15. Elecbegginner

    Elecbegginner

    103
    0
    Mar 24, 2013
    Thanks sir for the time , actually the guy who gave me the job does not want to repair them anymore , and have took the cards with him , he wanted a quick fix , and am not actually qualified to do that .



    But now i have another card and i have the time to fix it and share it with you .
    This is a speed variator (from an agitator) with 8pins switch with a pulse function . the speed variation is 7 speeds .
    I managed to make the full schematic witch wasn't easy to do , should we continue on this topic , or create a new one ?
     
  16. davenn

    davenn Moderator

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    Sep 5, 2009
    create a new topic
     
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