Running COB LEDs from batteries

Discussion in 'The Projects Forum' started by Doktor Jones, Jul 1, 2014.

  1. Doktor Jones

    Thread Starter Active Member

    Oct 5, 2011
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    I'm looking to run some COB LEDs (these to be specific) off a battery pack of 4-6 "C" or "D" alkalines. I'm shooting for maximum longevity -- having to change the batteries as little as possible. This is going to be built as an under-cabinet lighting system for a legally blind person, so the less he has to worry about changing batteries the better.

    What would be the best way to do this? Should I just go with 4x batteries and run the LEDs directly, or put something like a buck regulator in the middle and use 6x batteries so that, as the batteries drain, the LED remains lit at a consistent brightness?
     
  2. alexfreed

    Member

    Oct 8, 2012
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    LEDs need to be driven with constant current, not voltage. Adding resistors is very bad for efficiency. The best solution is a regulator with a feedback that maintains the constant current. Can even be a boost converter from lower voltage, but for longer operating time more batteries in series and a buck regulator is better.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    yes define "maximum longevity"
    1 day.. 3 days.. 1 week.. 1 month..1 year..?

    how much light do you require? lumens?

    There are plenty of under cabinet (both line and low voltage) solutions at your local "DIY" home depot.. why build?
     
  5. Doktor Jones

    Thread Starter Active Member

    Oct 5, 2011
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    60 hours would be fine; they will only be used for a minute or two at a time while he finds something under the cabinet, so that will likely get him through 2-3 months between battery changes. There are no outlets available near these cabinets, and he doesn't want me drilling holes and running wires all over the place, which is why we settled on a battery-powered solution. Most store-bought battery operated solutions just drive a couple of 5mm LEDs, which don't put out enough light for him to see by (being legally blind, he needs things to be fairly bright to see by) -- something in the vicinity of 200-400lm should do well. These COB strips seem to strike a pretty decent balance between battery life and light output (assuming I can buy or build a regulator to work with them).

    I thought a buck regulator with 6-8 batteries might be the way to go... can anyone recommend a good buck regulator schematic that actually provides the formula for calculating the current?

    I found this constant current regulator @ Instructables; however, it looks like a linear regulator rather than a buck regulator (and would thus be very wasteful when running on batteries). I found this one too from EDN.com but seemed to be missing a variable somewhere, and couldn't hash out the formula I needed to figure out the values I needed to set it @ 0.48A.
     
    Last edited: Jul 2, 2014
  6. Doktor Jones

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    Oct 5, 2011
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  7. ronv

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    Nov 12, 2008
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  8. NFA Fabrication

    Member

    Aug 12, 2012
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    It should be noted that a lot of commercially available COB Leds are pre-packaged to be hooked up directly to a power supply. A ton of these Ebay COB Leds are this way. I assume that they have built in drivers. many are targeted at direct hook up automotive use even.
     
  9. Doktor Jones

    Thread Starter Active Member

    Oct 5, 2011
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    Hmmm, the COB (chip-on-board) LEDs I've seen have just been one multi-chip LED module on a board with no driver circuitry. Are you perhaps thinking of the "corn cob" LED assemblies that cluster large numbers of SMD LED modules?
     
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