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LED PSU board, wondering what it is..

Discussion in 'Power Electronics' started by flippineck, Jan 28, 2017.

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  1. flippineck

    flippineck

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    Sep 8, 2013
    I have a small circuit board out of a cheap chinese LED light. Live and neutral arrive at the board from a 13A 240V UK AC mains plug. Live then passes through a fuse before arriving on one side of a 0.1uF capacitor. Neutral tracks directly across the board to the other side of the capacitor.

    The capacitor has what looks like a green VDR connected across it labelled on the PCB as MOV1.

    Tracks then lead from each side of the capacitor & VDR, across to two terminals of what looks like a surface mount bridge rectifier labelled as BD1.

    The remaining two terminals of BD1 have another VDR connected across them, labelled as MOV2. On the PCB these points are labelled V+ and V-.

    Two flying leads then exit the board and head off to the LED PCB, which is well heatsinked and has an eight-legged IC, three resistors and a capacitor on it along with an array of 15 LEDs.

    Powering up the light, I measured something in the region of the mid 200's of volts across the small PCB's V+ and V- (output) points.

    I had thought that the small PCB was some form of capacitor dropper power supply, and was hoping to find a low voltage output feeding the LED board. The plan was to dispense entirely with the mains power supply, just snip it out, and then power the lamps from a 12V car battery if possible somehow. Either directly or via some kind of simple replacement mini-PSU

    So.. I was suprised, and as soon as I saw 200-something on the DMM display I switched everything off. Just glad I'd used my basic electrician training and started at the top voltage range on the meter and assumed from the start there was always a chance I'd encounter mains or higher where I didn't expect it!

    Modifying these lamps for 12V would make economic sense to me since these lights are extremely cheap and I need loads. They're on sale at a well known high street UK chain store so I doubted they would suffer from the 'live case' problems a lot of cheap LEDs seem to.

    What is the circuit on the small PCB I described? I'm figuring it's some kind of filter / rectifier / surge arrestor but not a dropper PSU. What would be the normal output from a circuit such as this, < 240V, 240V, or >240V?

    I'm figuring because of the encapsulated nature of the LED board and the lack of any markings on it's eight legged chip, if it requires a high DC voltage for operation I would need to modify these units by replacing the small PCB with some kind of small boost converter with a sufficient power handling capacity, and also ensure the heatsinking were sufficient for the modified configuration.. a lot of work so I hope I can just find something on ebay that works off 12V from the get-go!

    All the same, any info would be interesting..
     
  2. flippineck

    flippineck

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    Sep 8, 2013
    Yes.. to answer my own question, I should buy these or similar http://www.ebay.co.uk/itm/281606153831

    But, any illumination on what the capacitor-across-the-mains board is would still be interesting!
     
  3. Arouse1973

    Arouse1973 Adam

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    1,087
    Dec 18, 2013
    Can you take a picture so we can see?
    Cheers
    Adam
     
  4. flippineck

    flippineck

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    Sep 8, 2013
    The black pen on the LED board is where I was trying to follow the interconnections of the LED chain. Seems to be a long mainly series chain involving all 15, with 2 'taps' going off to IC pins.

    The white wires were originally whole - I snipped them whilst investigating applying low voltage DC to the LED board (0 thru 30V DC didn't make any LEDs even glow)

    This unit was labelled up in the shop at a reduced price as "ONLY HALF WORKS" lol.. one of the LEDs does seem to be blown. Even with the unit intact and mains applied, the LEDs all just about glimmered, very unevenly)

    Thought at the low price it was ideal for the purpose of investigation :)

    snapshot.jpg snapshot(2).jpg snapshot(1).jpg
     
  5. Arouse1973

    Arouse1973 Adam

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    1,087
    Dec 18, 2013
    Um it could be a transformer less power supply.
    Adam
     
    davenn likes this.
  6. flippineck

    flippineck

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    Sep 8, 2013
    Only because I saw 200+ volts coming out of it.. I figure not? It rectifies I guess.. and I suppose the VDR's are for transient suppression or something.. but what's the cap for?

    Power Factor Correction? Wild guess, an animal I've heard of but never really understood?
     
  7. BobK

    BobK

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    Jan 5, 2010
    The capacitor in series limits current for AC just like a resistor would for DC.

    Bob
     
    davenn likes this.
  8. flippineck

    flippineck

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    Sep 8, 2013
    Thanks Bob - just double checked though and in this instance the capacitor's definitely connected across the incoming mains supply, rather than in series with it
     
  9. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    Then the capacitor must be for HF noise suppression. Conducted emissions, immunity or both.
    Adam
     
  10. flippineck

    flippineck

    322
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    Sep 8, 2013
    Thanks. I'll google those animals :)
     
    Arouse1973 likes this.
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