Help with simple LED project?

Discussion in 'The Projects Forum' started by wrybread, Dec 15, 2015.

  1. wrybread

    Thread Starter New Member

    Jul 3, 2005
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    I'm working on a project that powers 6 LEDs, all 10mm super bright (.5 watts) red.

    I'm powering them with a single 18650 LiOn battery (3.7 volts).

    The LEDs are each separated by about 6 inches of wire.

    A few questions:

    - ideally I'd like for the LEDs to be dimmable. I know this should be done with PWM, but given the low voltage (and a desire for simplicity) can I just increase the resistor value?

    - and would it be possible to use a variable resistor to get variable dimming?

    - and can I use a single resistor (variable or not) for all the LEDs (assuming they're all identical)?

    - I'll be daisy chaining the LEDs together. So 2 wires will connect to each LED. Is there a neater way to connect the wire to them than soldering?

    Thanks for any help.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Need forward voltage and current specs on the LEDs..
    More than likely due to the low supply voltage you need to put each LED in parallel with its own current limiting resistor..
    That "could" be a variable resistor to offer dimming but you need to be very careful about resistor dissipation.. Typically variable resistors are low wattage and may burn up if not properly calculated.
    However since you need each in parallel that would mean 6 resistors (or 6 variable resistors to adjust)

    I think you should read a bit on how to properly size a resistor for a single LED and take what you learned there to complete your project..
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If Vf is about 2V then I will be about 300 mA each or 1.7 A total. As an approximation, use 5 ohm @ 1/2 W in series with each and drive all in parallel with an emitter follower power transistor or better, a darlington like D1326, 8A,60V, B 2000, isolated case, or TIP127 ?
     
  4. wrybread

    Thread Starter New Member

    Jul 3, 2005
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    Thanks so much for the input, comments below.

    > Need forward voltage and current specs on the LEDs..

    Oops sorry about that. Forward voltage is listed as "2.0-2.3v", current is "100-120mA".

    > how to properly size a resistor for a single LED

    Using the resistor calculator here (http://ledcalc.com), I should use a 15 ohm resistor for each LED. Or of course a 5ohm as Bernard suggested would be fine too.

    Is it possible to use a single resistor for all the LEDs? So I'd place the resistor just after the battery +, and run the LEDs in parallel from there? If so, any advice on what value to use? (Guessing 15 ohm * 6 = ~100 ohm?)

    > use 5 ohm @ 1/2 W in series with each and drive all in parallel with an emitter follower
    > power transistor or better, a darlington like D1326, 8A,60V, B 2000, isolated case, or TIP127 ?

    Could you elaborate a little on what the emitter follower power transistor or darlington would do?
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Does he have the voltage to lose across the transistor? Maybe use the transistor as the ballast resistor?

    How about something like this? (see attachment)
    The value of the resistor for dimming depends on the transistor you use. It doesn't have to be a Darlington I guess. What do you have on hand? Most any power transistor would probably do.
     
    Last edited: Dec 16, 2015
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    No.. LEDs in parallel without current limiting each string is a no-no.. They don't share current equally
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    What McGyvr says is good advice. That being said, my LED flashlight is just batteries and four LEDs in parallel. No resistors. :)
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  9. mcgyvr

    AAC Fanatic!

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    And you have to replace LEDs ~yearly due to that poor design... :p
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    :)
    An acceptable cost. Agreed, not a good design. Do you have an LED flashlight? What is it like?
     
  11. mcgyvr

    AAC Fanatic!

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    No flashlight.. But I've built numerous high powered (50 to 200+ Watt) LED fixtures for saltwater aquarium systems using everything from 3W Cree diodes to 100W clusters
    ALL feature full dimming and proper constant current control..

    My time is worth far more than a few resistors.. :p
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If you have a bag of LEDs you could select 6 that matched Vf to +- ..025 V & a 2.5 ohm @1W series resistor, otherwise use 15 ohm for each LED Or use transistor as amplifier as in thumbnail. The emitter follower will work but base would need about 5V for full output. NPN 00000.jpg
    Base V from dim to full output is about 1.54V to 1.7V.
     
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