12v blue led trees. Resistor help

Discussion in 'Lights - Store Bought and Home Made' started by mborg10, Nov 26, 2012.

  1. mborg10

    mborg10 Michael Borg

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    I purchased second hand blue led trees. 18 in total that stand approx 1500mm high. Great for a chase sequence. They have a 12v 1000mA power pack which I'm going to cut off and control using a LOR DC board. I have a 12v 300w power supply from ray wu. I cut one of the power packs off and connected directly to the power supply. Lights worked. I Connected it thru the dc controller and it wouldn't work. So I tried another tree and it worked.

    Would I have fried the tree by connecting it directly to the 300w power supply.

    Will it still fry by going thru the board.

    If so is this what the inline resistors are for. If it is I have to figure out what resistors I have to solder in. And of course stop touching the hot end of the soldering iron and burning my fingers.
     

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  2. David_AVD

    David_AVD Good news, everyone!

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    LED strings are polarity sensitive, so could the working / not working be as simple as not connecting the wires the same way each time?

    Details of the original power supply and controller (if there was one) would help too. :)
     
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    mborg10

    mborg10 Michael Borg

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    Yeah I tried wiring both ways and nothing. I know the power packs are not original but work fine. I bought them off a Canadian family in frenchs forest who after 10 years were not doing displays anymore.

    I did notice when they were connected directly to power source. When I switched off power they stayed on for about 2 seconds then faded out. I just put this down to the energy displaying from the transformer.
     

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  4. AAH

    AAH I love blinky lights :) Community Project Designer

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    The lights staying on after being unplugged basically says that there is a capacitor in the plugpack and it is likely an unregulated plugpack.
    On some led sets there are no reistors inline with the leds at all. They rely on the losses within the plugpack to limit the current flowing into the leds.
    The 12V plugpack could be supplying anything from 12V to 18V depending on how good the plugpack is and what sort of load is on it.
     
  5. Slite

    Slite Full Time Elf

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    Correct me if i am wrong, but that wall-wart says AC-AC, and if I'm not misstaken you are trying to hook it up to a DC switched power supply...

    That is a bad idea if I'm not misstaken.
     
  6. David_AVD

    David_AVD Good news, everyone!

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    Is there a small lump in the low voltage cable?
     
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    mborg10

    mborg10 Michael Borg

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    The plug pack was removed when I tested the lights and they stayed on, but I guess that's minor compared to the fact I overlooked the 12vac not dc on the pack. Wasn't aware that ac was common for 12v so it was overlooked.

    Just thinking out loud but I thought LEDs were one way and need a dc supply. An ac supply would alternate the current and not make the LEDs work. Would this mean there might be some other electrical components inside the tree. I know the tees use to sit in a base which supplied power but these broke and the previous owner put on the power packs.
     
  8. AAH

    AAH I love blinky lights :) Community Project Designer

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    A lot of the cheapie lights run off ac-ac plugpacks especially if they have multifunction controllers. The 12V AC plugpack could be putting out a peak voltage of over 24VDC (12V x 1.41 x 40% regulation common with plugpacks). Strings that are running off 24V AC packs often need 31V DC when running off DC.

    The AC-AC plugpack can run DC lights no problem as the leds themselves will only conduct for half the time. Some strings will have a rectifier block in them so that they are running on full wave rectified power.

    If you are blowing up your lights running from 12V DC then it's reasonably likely there are no resistors inline with them. Alternatively they might be 2 or 3 leds in series at a time and they are working on the half wave rectified power which has the equivalent voltage of 6V although it's a lot more complicated than that.
     
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    mborg10

    mborg10 Michael Borg

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    Yeah not sure if it has blown up. No heat, smell, or smoke. Just no lights. I guess I'm a little lost as to what to do now. Can I still use my 12v dc power supplies and do some mods to the strings. Does 12v mean they won't be as bright. Or do I need to get a multi meter onto it and possibly up grade my power source.
     
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    mborg10

    mborg10 Michael Borg

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    David, not sure if it has a lump in the low voltage cable. The trees would sit in a base which was connected to power. The base connection has broken so the previous owners added the plug packs and the connection where the base would connect is all taped up. I think the one that's not working might have to be butchered open just to find out what's in the guts of the tree.

    I have 40 sets of 10m blue led strings that have been delivered to my other office. Fingers crossed they don't prove to be as difficult.
     
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    mborg10

    mborg10 Michael Borg

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    After hacking into the base of the light (I think this one wont be coming back. The wiring looks pretty basic. Just a whole bunch of wires soldered to the end of the power cable. No resistors or anything. From the amount of wires connected to the power supply wire it looks as if each branch which has approx 6 LEDS has its own positive and negative wire.
     

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  12. David_AVD

    David_AVD Good news, everyone!

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    Can't view the image in that last post.

    There will possibly be a series resistor somewhere in the wiring of each branch.
     
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    mborg10

    mborg10 Michael Borg

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    The photo was just showing an abundance of wires.

    I stripped back one of the branches. Just wires nothing else. So it looks like it was just a plug pack 12vac 1000mA wired directly to LEDs

    Any thoughts on what I should do, if anything.
     
  14. David_AVD

    David_AVD Good news, everyone!

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    If you can't see any resistors anywhere in the wiring, maybe they relied on the relatively high output impedance of the transformer to limit the LED current.

    You can always add your own resistor(s) for use with a different power supply and/or a DC controller.
     
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    mborg10

    mborg10 Michael Borg

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    Found my box of tricks from my TAFE days. Tested some resistors. Don't ask me what rating, I'm colour blind and my multi meter was dead. I just picked the brightest ones.

    I will have to test again in case the length of cable adds more resistance and dulls the light more but I think I'm headed in the right direction.

    I have 16 of these trees.
     

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