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Thread: Very basic red + white camping light DIY

  1. #1
    Join Date
    Feb 2018

    Default Very basic red + white camping light DIY

    Hi, Iím very new to this flashlight game although iv been using knockoff ďcreeĒ flashlights for a while and have dabbled in reef aquarium lighting. Id consider my electronics experience beginner to intermediate but I need to double check iv got things right for my current project.

    Iím currently building a light for camping/fishing which will be mounted on a flat aluminium heatsink and powered by a single or possibly double (in parallel) NCR18650Bs with a protection PCB giving me a max output of 4.2v.

    Ideally I want a separate red light and white light. Iv got a few 3w bridgelux red and daylight LEDs left over from old aquarium lighting projects but the drivers I have are incompatible with my project as they were powered from 230v AC.

    Bridgelux specs are as follows:

    Bright Red 630nm: Forward Voltage: 2.2V-2.7V Forward Current: 600-800mA Output: 85 lumens

    6500k Daylight White: Forward Voltage: 3.2V-3.6V Forward Current: 600-750mA Output: 240 lumens

    Am I right in thinking two AMC7135 chips per LED will be fine to drive them? Iím not really looking for any fancy features just simple on and off via a toggle switch.

    Iv also been playing around with some cheap eBay 3v 3w 60x8mm COB chips. I havenít actually powered them up yet to test them but also wondered if anyone has had any experience with these?
    Data seems to be limited and the sellers donít seem to have a whole lot of info when I ask them. The most I seem to have been able to get is this:

    White Forward voltage: 3-4v Max Current 1000ma
    Red 2-2.6v Max current 800ma

    Am I right in thinking that two AMC7135 chips would be fine to drive these as well?

  2. #2
    Flashaholic* DIWdiver's Avatar
    Join Date
    Jan 2010
    Connecticut, USA

    Default Re: Very basic red + white camping light DIY

    Hello, and welcome to the forum!

    Yes, two '7135s should be safe for either of those COBs, and probably for the Bridgelux devices given your application.

    I'd have to look at the Bridgelux datasheets to understand the implications of the current range given. I'm used to seeing just a max current. Perhaps they want to specify the max power, and are saying that at the max power the current and voltage will be in those ranges. If this is the case, two '7135s could overdrive either of them somewhat, but you wouldn't be able to know until you measured the actual units that you have. Either way, overdriving a good quality LED a little is not necessarily a bad thing. As long as you have a good heatsink the only thing you are likely to do is shorten the LED lifetime. For a camping light, it would probably take ten human lifetimes to achieve 50,000 hours on the LED, so who cares if you shorten its life from 50K hours to 1K? If you were talking about always-on emergency exit lighting, that would be a different story.

    Depending on your goals, you might be able to do a bit better than '7135 drivers.

    Given the large voltage difference between the red chips and the battery, using a linear regulator like the '7135 is not very efficient, dumping nearly a third of your battery power into the driver instead of the LED. A switching, or switchmode, driver could be substantially more efficient.

    And given that the white chip voltage range and the battery voltage range overlap, you could start seeing reduced output as the battery approaches half charge. You will see reduced output before the battery is fully discharged. The only question is when does it start and how bad does it get. This depends primarily on the actual Vf of the LED at actual drive current, which could see significant variability even with quality LEDs. Output reduction is not necessarily a bad thing, as it warns you the battery is getting low.

    If you want to maintain steady output until full discharge (not necessarily a good thing, so maybe you do, maybe you don't), then again a switching driver can be an advantage. Some (buck-boost type) can maintain a constant output voltage even as the input voltage drops from higher to lower than the output voltage. This is impossible for linear regulators and buck-type switching regulators, the most common and generally the most efficient type of switching regulator.

    I know that's a lot to consider. Share some more about what you want, and we can help guide you to a good solution.
    Last edited by DIWdiver; 02-14-2018 at 09:42 PM.

  3. #3
    Join Date
    Jan 2004
    Pleasanton (Bay Area), CA, USA

    Default Re: Very basic red + white camping light DIY

    Another possible path for a simple light like this is to just use resistors as the current limiting device.

    The efficiency is more or less the same as those AMC chips, and with the relatively flat discharge curve of a modern Li battery, the light output consistency will also be reasonable for a simple DIY light.

    If I correctly remember the math for this:

    V bat - Vf = approx 3.9 - 3.4 = 0.5 volts

    V = IR, so 0.5 = 0.7 amps x R. 0.5 / 0.7 = approximately 1 ohm (Within the accuracy of this type of build)

    Power consumed by the resistor = R x I x I = 1 x 1 x 1 = about 1 watt that needs to be dissipated (and the resistor needs to be rated at to work)

    So roughly 1 watt being wasted and 3- 4 watts going into running the LED. Moving from this 75 ish % efficiency to higher will take a driver that can run on this relatively small differential voltage.

    As noted above, it isn't as good as a true driver based setup, especially for the red LED.

    Depending on your goals, if you use 2 x 18650s in series, then at least in theory, a single buck driver could be used to run either LED (one at a time) As an example, a buckpuck is pretty simple and inexpensive. I have used them and they work fine.
    Last edited by HarryN; 02-15-2018 at 12:15 PM.
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