Voltage limiting for Bicycle lighting

Discussion in 'Electrical' started by SirJakesus, Dec 19, 2008.

  1. SirJakesus

    SirJakesus Guest

    Since theres no section on MBC devoted to bicycle electrical systems (tsk tsk!) I'm starting this thread here to get the greatest possible exposure.
    I have almost the whole lighting system for the NEXT wired up and ready to go but I need to limit the current for my Element flashlights with Luxeon K2 LED's so they don't burn out. I'm using a Tenergy lithium polymer 14.8v battery rated at 6.8ah with overcharge and discharge PCB. Both the flashlights run on 3 AAA batteries amounting to about 4.5v, supposedly they are 4 watt led's. I know I can keep them direct driven like they are right now from 4-4.5v safely and with tons of output. I just need to know how many ohms the inline resistor needs to be OR if I use an MA controlled driver like a buckpuck how many MA should I pump through these things before they burn out. I'm liking the resistor idea a bit more because that will allow me to retain the clicky switched on the backs of the lights and use only one for daytime visibility and lower power consumption or both for nighttime running.
    I know most of the people here aren't electrical geniuses but if I could be turned in the right direction it'll help keep me from burning out a light or two using the trial and error method.
    This will also help if I want to upgrade to one (or two :) of those 900 lumen cree flashlights that seem to run off 4.5-9v.

    Thanks in advance. I won't be around too much until I get my power back... still a week and counting since the ice storm kicked the carp out of this area.

  2. ZnsaneRyder

    ZnsaneRyder Member

    That's a very powerful LED. It's hard to take the 12V down to 4V without wasting a lot of power. I know from experience, that the LED's actually run on LESS than 4.5V (3.6V-4V) because AAA cells don't have much current, and have high internal resistance, so their voltage actually drops down to the voltage of the LED and the LED is only bright for about 50% of the battery life. When I used 3-AAA LED flashlights from a 3.7V Li-Ion battery (4.2V max charged), I have to use a 2.2ohm resistor to limit the current, because the Li-Ion has MUCH more mA than the AAA cells and burns LED's out over time without one.

    1. Since your 4W LED takes 4V of power - that's a 1 Amp, or 1000mA LED :shock: If you tried to use a 10 ohm resistor to go from your 14.8V down to 4V, that's over 10V @ 1 Amp (10W) wasted and makes a very HOT resistor! More battery power is used to heat the resistor than to actually run your LED! Together they use 14W, which still isn't too bad. However, this works very well, and Radioshack carries these resistors.

    2. Another way to do it is make a traditional current-limited 4V voltage regulator, but again, the same power is wasted, but with a transistor and heatsink instead. (that's how my bike battery charger works)

    3. If you are into electronics, check out Pulse Width Modulation (PWM). With PWM it would take your 14V, deliver it in very short pulses into a coil and capacitor, which filters and averages it to a lower voltage. The longer the pulses, the higher the voltage, and vice versa. With PWM, your LED would use less than 300mA from the 14V battery, (4W) while the LED still gets 4V @ 1Amp, (4W) and your battery will last much longer.

    I'm in the process of designing such a PWM circuit, as of now I'm charging a 3.7V battery, from a 12V battery, and I waste power with my regulator right now, and PWM would fix that. It's a bit complex however.

    So to keep it simple, use a 10 ohm, 10W resistor, or voltage regulator.
    Go PWM if you are concerned with your battery life.
  3. skyl4rk

    skyl4rk Guest

    The Arduino microcontroller and its clones have pulse width management included in their capabilities. I was thinking of trying to run an arduino as a brushed hubmotor controller (connected to a bank of mosfets). It should work for LED's as well.

  4. mmelvis

    mmelvis New Member

  5. ZnsaneRyder

    ZnsaneRyder Member

    Good find mmelvis!!!! I'm designing a PWM with TL594, but the LM2676 would be much easier to work with, especially with less external components.
  6. mmelvis

    mmelvis New Member

    ZnsaneRyder: Glad to be able to help
  7. SirJakesus

    SirJakesus Guest

    Wow, great information. Thank you all very much!
    Considering the potential for wasted power in all but ideal conditions I may just string 4 of these lights in series. My lithium at full charge reads 15.5v so I would be under-volting them which is good for heat and longevity issues. If I'm going to use more power stepping the voltage and current down I might as well just get more output for the same draw. This was the first way I was going to go about the problem but I figured if there was a way to stretch the battery further I might as well try. I'll keep y'all posted.
    Thanks again!
    Last edited by a moderator: Dec 29, 2008
  8. arceeguy

    arceeguy Active Member

    Instead of lowering the voltage with a SMPS, could you just get a single LiPo or Li-Ion cell and run the LED's? The brightness might be a little less with the reduced voltage, but LED life would go up. You could make up the difference in light with additional LED's.
  9. SirJakesus

    SirJakesus Guest

    Well, I have a good supply of high capacity NIMH rechargeables so I can easily run the element flashlights off of those without creating unnecessary waste or expense. However it's somewhat more annoying to run them that way given that I have to charge them separately from the main battery. To reduce overall bicycle weight and improve headlight run-time and intensity consistency (longer dimming) it's just more logical to run the whole system off the same battery. I have a switch for lights on my handlebar switch cluster anyhow so it would be easier for on-the-fly signaling as well. I also figure that the LiPo was a fairly expensive battery, might as well use it to it's full potential.
    Once it's all hooked up I expect the battery run time to far outlast any amount of time I'd want to be cruising around at night anyway. However, once I have it I'll probably be much more apt to take her out for long summer night rides.... At that point in time I'll be very tempted to run monkey-lectrics on the wheels to complete the ridiculous nature of the machine.
    Last edited by a moderator: Dec 30, 2008
  10. arceeguy

    arceeguy Active Member

  11. loquin

    loquin Active Member

    Philips Electronics, the makers of the Luxeon K2 LED, have published a suggested regulator circuit in this PDF. It uses the SP6641BEK-5.0 regulator IC.

    When dealing with LEDs, they should be driven by a current regulated source, rather than a voltage regulated source.

    There's a canned LED driver called the buckpuck that should fit the bill... It costs about $15

    Or, you can build your own for less than $10. (ref image)

    Attached Files:

    Last edited: Jan 20, 2009
  12. sparky

    sparky Active Member

    Hmm... I'm definitely gonna have to watch this thread.