230V LED driver: 230V -> 460V -> 14V DC?

Discussion in 'General Electronics Chat' started by JMD, Dec 10, 2009.

  1. JMD

    Thread Starter Member

    Dec 9, 2009
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    I got my hands a traffic light, in which there is a driver for the LEDs. Here's a pic: http://i46.tinypic.com/1z19vgw.jpg
    I looks very much like a switch mode powersupply. At the lower part of the board, there's an IRF820 - a quick search: 2.5A, 500V, 3.000 Ohm, N-Channel Power MOSFET
    I meassured the voltage at that component, and.. 460V AC.. wauw.


    My question is: Why turn 230V into 460V - whats the big idea behind that?
    Also.. when i disconnected the load, and turned it on again, something went up in smoke after just a few seconds :/


    (For those interested, heres the board with the red LEDs: http://i50.tinypic.com/24o1478.jpg )
     
    Last edited: Dec 10, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
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    It could have been used as form of rectifier. As I understand it a lot of power supplies run on during the right side of the AC cycle, they don't have any diode drop nor do they get hot. Just a possibility.

    A MOSFET is not a rectifier, but timing the on cycle they can become a virtual diode.
     
  3. JMD

    Thread Starter Member

    Dec 9, 2009
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    But couldnt you just match the LEDs with a capacitor and a resistor? Its widely used in home LED bulb-replacements, so why not here?
     
  4. Wendy

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    Mar 24, 2008
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    You could, but there are ways to limit current that don't require a resistor. A resistor generates heat, which is waste. If you use capacitive or inductive reactance you can do the same thing with no wasted electricity. This is one option. Another is PWM, which can accomplish something similar if designed correctly.

    Understand, all of what I say is speculation, not facts.
     
  5. JMD

    Thread Starter Member

    Dec 9, 2009
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    I understand that there are heat wasted in a resistor - but look at the powersupply, it got two large resistors.. and they are not that big because they dont get hot ;) The 390 Ohms resistor gets QUITE hot actually.

    Im studying electrical engineering, so naturally i asks questions if i dont understand something 100% :) And for me, this seems a bit odd. Even more if the powersupply kills itself if there's no load attached...
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Actually, that is normal for switchers. Most computer power supplies have problems with no loads.
     
  7. JMD

    Thread Starter Member

    Dec 9, 2009
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    But it doesnt make them go up in smoke ;)
     
    Last edited: Dec 10, 2009
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