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5W voltage-controller LED Current Driver

Discussion in 'Electronic Projects' started by JBrock, Jun 27, 2009.

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  1. JBrock

    JBrock Member

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    Here is a voltage-controlled LED current driver for those who are not afraid to burn some POWER! Forget those wussy switching things trying to achieve, what?, 85% efficiency maybe? What a hassle.
    This driver is for a one-LED project. The LED is a 5-Watt royal-blue Luxeon Star. It cost $40 a couple years ago but you can get good Chinese counterfeits on eBay now for about ten. This device is a classified as a Class 2 laser - "Do not look directly at The Star!"
    What can you do with one LED? The range of output is so great, that this LED is operated in analog mode. A voltage between 1 and 10 VDC is applied to the input of the driver to get a proportional percentage of The Star's output.
    A controller generates eight different waveforms which run continuously. Lines from a pseudo-random linear feedback shift register choose which waveform is selected by the 4051 analog switch.
    The current waveforms are shown, but their schematics are not revealed - yet. Only a hint - the waveforms are all generated using only 555 timers, 324 op-amps, or 3900 Norton amplifiers. I would like to see some good guesses as to how these waveforms may have been generated. If the results are worthy, I will post the solutions that were actually built and used to animate The Star of Captain Brock's BlueLight Box.
     

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    Last edited: Jun 27, 2009
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  2. tytower

    tytower Banned

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    It looks to me like they were generated by the pencil and paper method. No need for a circuit then.You can make it anything you like.
    What is the purpose of this device ?
    Could I use it to warn incoming cars that I have cattle on the road ahead?
    Could I drive say six of them to do the same?
    What would you like me to guess at in a little more detail if you will?
     
  3. Hero999

    Hero999 Banned

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    You can build an 85% efficient SMPs with the same amount of parts.
     
  4. dave

    Dave New Member

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  5. JBrock

    JBrock Member

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    :DThe circuit is built and tested, as shown in the third picture, and as required by the rules of this forum!

    :DThe purpose is to control current through the massive 5-Watt royal blue Luxeon Star LED in linear mode in response to the voltage waveforms. (Read original post again.)

    :D No. Get a MagLite with an LED upgrade.

    :D At 5 amp collector current rating, the TIP31 would be marginal. Get a nice beefy TIP3055 in a TO-247 case. The transistor is also operating in linear mode. But behold The Star! There is no need!

    :D As stated, How might one generate the eight voltage control patterns graphed on page 2? I used 14 IC's to come up with all eight waveforms even though many share the same oscillators. Surely someone can do better. (No fair using microcontrollers!)

    :D That is interesting. This controller can go from a few milliamps to 750mA. Does your switchmode have that kind of range? Remember, this is a voltage-controlled CURRENT regulator. Please share. I could be convinced.
     
  6. Hero999

    Hero999 Banned

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    It's pretty simple, all you need is a comparator and a transistor (or a couple of transistors at most).

    Read the following threads.

    http://www.electro-tech-online.com/threads/high-power-led-controller.95832/
    http://www.electro-tech-online.com/threads/led-switching-regulator.41375/
     
  7. tytower

    tytower Banned

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    Just following on

     
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  8. JBrock

    JBrock Member

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    Check Out MAX16823 and Others

    Ty, your application is so simple, all you need is a 555 timer and go with current-limiting resistors. (Ya, Hero will scorn that.) I would like to know your LED selection. You probably want red or yellow. There are some monsters available now, up to 12 watts. You will need to have a heatsink.

    The point of my circuit is to run across the entire range of current from nearly zero up to ~700mA, controlled in response to an analog voltage. The star already becomes very bright at a few milliamps.

    Here's a great blog on driving LEDs:
    Constant-Current LED String Driver
    Check out the milled circuit board!
     
  9. Hero999

    Hero999 Banned

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    Did you check out the threads I linked to?

    A PWM 555 can also be configured as an unregulated SMPS, simply by adding an inductor, a Schottky diode and a power MOSFET.

    I'll post a spreadsheet which calculates the component values if you're interested.
     

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  10. JBrock

    JBrock Member

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    Pwm?

    Hero, I am intrigued... Since the project is completed, I don't plan on making any changes, but I am theoretically interested in the idea.

    What frequency are you suggesting for the PWM?

    How linear would the response be for this circuit?

    What is the control range? For the BlueLight Star application it would need to go from 6mA to 750mA. That is greater than 100:1.

    How much current ripple could be expected through the LED? (Probably not an issue at switching frequency.)

    What is the response time? (Only need about 1ms or less.)

    Yes, I did read the references. Many of these circuits are not intended for voltage-controlled response. It would be easy to convert voltage to PWM to use your idea.

    Thanks for the input. Nice to meet a switching guy! -Brock
     
  11. Hero999

    Hero999 Banned

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    The frequency will depend on the inductor size and ripple current. Higher frequencies will give less ripple but higher switched losses and larger inductors will reduce the ripple but are larger more lossy and expensive.

    The circuit I previously posted won't be perfect, as it's unregulated.

    Some of the circuits discussed in the other threads are more linear.

    Anyway, even if the controller were perfectly linear, I'm not convinced that the over all device would be because LEDs certainly aren't linear. The voltage drop increase with current so the power consumption and power output won't have a linear relationship to the current.

    The circuit previously posted can be controlled from 0mA to full power.

    It's a trade off between inductor size and switching frequency, see above.

    Ripple current isn't a problem with LEDs, providing the switching frequency is over about 500Hz.

    That depends on the switching frequency.

    Many of them can easily be voltage controlled.

    Here's an example:
    [​IMG]
    Just vary the value of Vref to alter the current.

    Vref can be a voltage reference, such as a zener, with a potential divider in the output.
     
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