Query regarding capacitor based transformers

Discussion in 'General Electronics Chat' started by mahanteshec28, Jul 22, 2013.

  1. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    Dear All,

    Recently I burnt a capacitor based transformer which I used to power LEDs. It was 12V 1Amp. I bought new one with same ratings but what is observe is -
    1. After turning ON, after 1-2 mins, transformer turns off by itself
    2. After being OFF for 10-20 seconds it'll automatically turn ON
    3. Again after being ON for 10-20 seconds it'll automatically turn OFF
    4. This process repeats

    Can anybody please tell me what I'm doing wrong?

    Thanks in advance!

    Regards,
    Mahantesh
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Show us how you have it wired up, a schematic would be nice.
    What do you mean by a "capacitor based transformer"?
    What is all of this connected to, what is your load?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sound like it could be a Switching type supply that is shutting down because of O.C. maybe?
    Max.
     
  4. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    Thanks for quick response, Gerty :)

    "capacitor based transformer" - light weight power transformers

    LEDs are connected in series and positive of 1st LED is connected to positive of power supply and negative of power supply to negative of last LED. Like this I have connected 10 LEDs in series. Hope you got the picture. I'm not able to draw pictures here and I cannot upload any file from my office computer.

    Regards,
    Mahantesh
     
  5. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    Thanks to you also, Max.

    Yes, it is switching type supply. Can you please explain more on O.C.

    Regards,
    Mahantesh
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    OC = Over current, if you have a smaller or no load, does it still shut down after being on for a while?
    Max.
     
  7. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    Thanks for qucik response, Max :)

    When it is no load, it won't turn OFF.

    Can you please suggest any solution for this.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What is enforcing current regulation for the LED's? (I'd guess nothing as thats what the problem is)
    They CANNOT just be attached to a regular constant voltage power supply. They MUST be powered by a constant current power supply or include series resistors to limit current.
     
  9. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    Thanks.

    But my doubt is why the earlier power supply used to work?

    Thanks & Regards,
    Mahantesh
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    It didn't.. You burnt it up too. :p
    Was it the same model/brand/type?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Why not pick up a bell transformer and a small bridge and maybe a 10μF capacitor and you should be good to go. Maybe have to add another LED or 2?
    Max.
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Buy a proper constant current power supply or include some form of current limiting or it will happen again and again.
     
  13. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    :)
    Actually it worked for more than 5 months.. it burnt coz, I spilled water on it :)

    Anyways, its not same brand. But both have 12V/1A output.
     
  14. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    @mcgyvr - I'm tired searching for the supply. I already returned 2 power supplied. Luckily I got full refund. Both of them behaved the same.

    @Max - Can I just add couple of LEDs and try? Coz to get all those stuffs I need to drive for 2 Hr in full traffic :(
     
  15. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Why don't you post a schematic of the LED circuit you are attaching to this power supply..
    Including the specifications of the LED's (forward voltage/current rating)

    You are ONLY having problems because you are NOT doing it right.
     
  16. mahanteshec28

    Thread Starter New Member

    Jul 22, 2013
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    @mcgyvr - It is a LED strip where LEDs are in series. I bought it on ebay. So other than LEDs I cannot see what is in there. I have break it open to see what else is present in there.

    Let me try connecting some other load and check. I'll update you soon. Meanwhile if you have any other solution, please let me know.
     
  17. Ramussons

    Active Member

    May 3, 2013
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    Maybe the supply has an inbuilt timer to flash the LED's being used as a decoration or signalling ....... ?

    Ramesh
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    If you have 10 LEDs in series, then you probably need 18V to 24V to actually get them to conduct. Many power suppllies like the one you describe will put out considerably more than their rated voltage when there is no load. If it will put out enough to overcome the turnon voltage, then current will start to flow. But as soon as current starts to flow it will start regulating, so the voltage will start to drop. But as soon as it drops very much, the LEDs will turn off and the current flow will stop and so the voltage will go back up. What you end up doing is operating at the point where the output voltage that is supplied exceeds the threshold voltage of the LED string by just enough to drive the corresponding current. That current is probably not going to be very much, but it is going to vary considerably from one power supply to the next. Your old supply reach equilibrium at a current that it could handle. The new one reaches equilibrium at a current that it can't, possibly because it is being forced to operate so far outside its designed range, and so it goes into thermal shutdown. As soon as it cools down a bit, it turns back on. Since it is already starting at a higher temperature than when first turned on, it doesn't take it as long to reach the shutdown temperature again.

    You need a supply that produces the correct amount of voltage for your LED string, AND you need to properly limit the current in the string.
     
    mahanteshec28 likes this.
  19. WBahn

    Moderator

    Mar 31, 2012
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    Given the times involved, that is highly unlikely.
     
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