12V, 1 A offline powersupply

Discussion in 'General Electronics Chat' started by rogerthatpsr, Oct 14, 2009.

  1. rogerthatpsr

    Thread Starter Member

    Aug 22, 2006
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    this is a 12v, 1 A power supply using off-line switcher.... but M facing some problems... usually any one of rectifier diodes(D1,D2,D3 or D4 ) will burnout..or either D5 or the TNY... can anybody help me troubleshoot... wat is the purpose of R9 ? (i don think its required n will burn when 1A output is provided) n value of L1 should be more than 1 A...apart from this any other clues ?? is it because of back emf ??? .. i have attached pdf of schematic
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    R9 is a surge current limiting resistor. Limits the peak current when first plugged in. Also might be a special fuse-able resistor for safety.

    L1 is RFI suppression. Should be nowhere near 1A passing through this.

    This is only a 12W supply. So running off 230VAC the supply current will be less than 60mA.

    You might have a shorted turns in the transformer. Very hard to test this. Does the transformer look like it might have run hot? This sort of small off-line PSU is almost a throw-away item these days. Probably not worth spending too much time on it!
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    I have noticed that your schematics differ some from the suggested design in the data sheet. Se comments in pdf.
     
  4. Jasal

    New Member

    Oct 14, 2009
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    Just a quick reply, I haven't read the datasheet for the controller or anything, just some observations.

    When it fails, is it just one of those components, or is it always something in addition to some other stuff? Such as the controller always fails, and sometimes a diode.

    Just a quick look at your schematic, your RCD snubber looks kinda weird, never seen a resistor in series with the cap (R2 and C4). That kind of snubber is intended to clamp the inductive kick of the transformer right after the FET turns off...not entirely sure what effect that resistor (R2) would have. Also, I noticed you don't have an RC snubber...it wasn't needed I guess?

    Also, I'm not sure why the inductor is placed where it is. If it's for conducted EMI, then I think it should be on the other side of the diodes with a much more elaborate filtering scheme. If it's for generating the DC rail, I don't think it's needed. Maybe try just taking it out and see if it makes a difference, or scoping your ripple.

    Cheers! good luck
     
  5. rogerthatpsr

    Thread Starter Member

    Aug 22, 2006
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    @ t06afre thanks for the reply , u had told me to shift the position of the L1 to other side...even the suggested schematic says the same...... but how does it affect...its just on the other side of series connection ( sorry M new to the field )
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    It is only two components in the suggested schematics that are optional. I am not an expert in this chip so I can not give you any help. But start with following the suggested schematics to the letter, as you now this will work. Then things are working you may start to experiment with values. But it is obvious that swapping the inductor will give a new circuit.
     
  7. rogerthatpsr

    Thread Starter Member

    Aug 22, 2006
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    @ t06afre ok sir i'll try that first....
     
  8. rogerthatpsr

    Thread Starter Member

    Aug 22, 2006
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    If operated at 230v...can this ckt only support 60mA of current ?? can anybody suggest a better ckt for 12v, 1A with a smaller or no transformer??
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
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    Your circuit will deliver 1A current. I guess the 65mA is related to current drawn at the high voltage side
     
  10. rogerthatpsr

    Thread Starter Member

    Aug 22, 2006
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    one more fresh observation done as soon as the board is assembled n power is switched on.... one of the rectifier diodes(D1 thru D4, usually D4 80% of the time burns) and R9 wire wound resistor also burns....if the above 2 components r replaced by same the value the circuit will work properly for long time.....so its something related to turn on surge or transient.....its the first few milli secounds that's causing the problem....don no where n how to fix
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
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    Take a look at this site http://www.powerint.com/en/design-support
    Also are your circuit 100% equal to the suggested design. Do you have a diode in series with R1 (your schematics). This component is present in all design I have seen regarding TNY279. If you look at the data sheet suggested schematic The primary clamp (D5, VR1, C4 and R1) limits the maximum peak drain voltage to less than the 700 V BVDSS rating of the internal MOSFET. I recommend that you Google the term "inrush current limiter" You may pick up something. Also it could be that your RV1 is wrong type or not working
     
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