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Electronics Help with Hi-Powered LED Resistors

Discussion in 'Modding' started by eleones, 15 Oct 2010.

  1. eleones

    eleones New Member

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    I have a project with two, separately wired high-power LEDs.

    LED #1 is a 1 watt LED rated at 3-4 Vdc at 350/700 mA
    LED #2 is a 3 watt LED rated at 2 Vdc at around 350/700 mA

    According to Resistor calculations, I get:

    LED #R1 = 4.7 Ohm, 2 Watt rating (for 700 mA)
    LED #R2 = 8.2 Ohm, 4 Watt rating (for 700 mA)

    The problem is I can't find any online vendor that sells common resistors at these specs. Am doing the calculations wrong to arrive to 2 and 4 watt rating or is there a vendor out there that sells these? Alternatively, is there a unique type of resistor that I can substitute for these ratings?

    Thanks for any info.
     
  2. mvagusta

    mvagusta Did a skid that went for two weeks.

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    You gotta take the supply voltage into account, try an led resistor calculator:

    http://led.linear1.org/1led.wiz

    And you gotta figure what current you are gonna run them at! Those ma you have listed don't make alot of sense to me :confused:

    A 1 watt 3.5v led, would run at 285ma = full power, so you'd want to check out what the mtbf is at that wattage, and check what the minimum heatsinking required would be.

    A 3watt 2v led, would run at 1500ma! Maybe 3watt is a short term maximum rating? Same deal here, consider the mtbf and heat at that power level.

    Got some links to the leds you're gonna use?
     
  3. capnPedro

    capnPedro Hacker. Maker. Engineer.

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    Don't use a resistor to drive them, get a constant current driver.
     
  4. eleones

    eleones New Member

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    Thanks for the replies. I'll check out a dedicated LED driver on eBay.

    Cheers,
    E
     
  5. Sea Shadow

    Sea Shadow aka "Panda"

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    Sounds to me like you are driving some CREEs, SSCs, or maybe even Luxeon LEDs.

    I would also agree with capnPedro about using a constant current driver. There are 2 main ways to go about it though, depending on your source voltage. If you are starting with a voltage lower than your LED's drive voltage (a single AA for example) you will need a boost driver. If you are starting with a voltage higher than the LED's drive voltage (usually the case) then you are going to need a buck driver.

    There are lots of instructions online for building fairly simple constant current LED drivers using an LM317 (here is one of many examples on instructables). Or you can buy a digital regulation circuit from one of many dealers online. I personally recommend dealextreme, don't expect to get your order anytime soon though. They are hammered with holiday shopping, and depending on what you get it could be anywhere from a couple weeks to several months before you see your parts. I have always had a good experience dealing with their customer service though. You just have to remember that they are a VERY popular site and that it will take a while to get your parts. If you go in anticipating the wait, then they are a great source.

    Personally I have always ordered the drivers from dealextreme, and I have always been pleased. Just be sure to read the reviews on a product before you buy so you can see how others have fared with the boards.
     
  6. eleones

    eleones New Member

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    Thanks Sea Shadow and all the other responses --

    One of the LEDs, the 3W IR version is from DealExtreme... you're right, they take a while to ship, especially from Asia. But the pricing and 0 shipping is excellent. The 3W unfortunately didn't have any specs on ratings, so the specs listed above are estimates based on other users who wrote reviews on the LED.

    I'm driving with 6 AA batteries, 7.2 Vdc altogether. So I plan to use the following for the 3W http://bit.ly/9mGpB0 however, all the drivers for 1W LEDs on eBay require 12 V. Does anyone think I can be okay driving both LEDs in series with the 3W driver from eBay?

    Thanks
     
  7. Sea Shadow

    Sea Shadow aka "Panda"

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    It looks like the driver in your link is capable of running on a voltage range of 3-12v. One thing to keep in mind though is the type of AA cell you are using. For example: I built a small headlamp that ran off 2xAA; I calibrated the driver to run from two alkaline cells at 3 watts (it was a driver that was easily adjustable). Later on, I unwittingly tossed some NI-MH cells in and forgot that they could supply much more current. The room lit up a blinding white and then the body of the headlamp started to glow bright orange, then faded out, leaving me in darkness. I opened it up to discover that several solder points had failed and the driver board as a whole had been reduced to a carbon plated PCB. All from switching my battery type. My driver had previously been wringing out the power form the Alkaline cells, effectively limiting the max current as the cells could only give so much power. When I was using the NI-MH though the cells could supply much more current and so my board went POOF as it was relatively unrestrained.

    With regards to using it for the 1w LED, I wouldn't be able to say for sure. It doesn't look like it is a digitally regulated driver, so I would be hesitant about hooking up any pricey LEDs to it. However that is just my opinion, perhaps others will be able to give better advice with regards to that question.
     
    Last edited: 19 Oct 2010
  8. ulfar

    ulfar holy s**t, i can change this?

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    those are the standard values following the e12 series, don't understand why you can't find them (those are the exact values). i find several on elfa.se, probably you cant get them on many other sites as well.

    (just saying in case he decides to skip the cc and go for resistors)
     
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