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Thread: Converting WS2811 project to Teensy

  1. #1
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    Converting WS2811 project to Teensy

    Hello,
    I'd like to convert a project that someone else did to Teensy. The project currently uses overly complex custom made components that are expensive.
    It's a battery powered project, currently each unit is powered by 5x 7.4v Lithium ion batteries wired in parallel. The original concept used custom made leds that worked at 7.4v but was upgraded to use ws2811 strips. Currently we use 7805 to step down the voltage to 5v for the strips but I was wondering if there was a better option ? The final project would probably feature one teensy + microsd +xbee with around 350 leds which would all need to be powered from the 7.4v source. This also needs to fit in an aluminum tube that's about 3/4 of an inch in diameter...



    Thanks !

  2. #2
    Senior Member PaulStoffregen's Avatar
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    You could certainly do much better than a 7805 regulator. The 7805 has 2 main problems.

    #1: With 7.4V in and 5.0V out, a linear regulator is 68% efficient. Switching regulators are more complex, but efficiency tends to range from 75% to 90%.

    #2: A 7805 regulator has a high "drop out" voltage, which is the difference been input and output needed for it to maintain regulated 5V. In fact, it's about 2 to 2.5V, so 7 to 7.5V is needed from the battery. A converter that can maintain regulation with only 6.5 or 6.0V will let you keep a solid 5V as the batteries discharge.

    But beware "low dropout" linear regulators. Often they become relatively inefficient at higher loads. Check the specs.

    An efficient switcher that could run down to 6V would be ideal. Perhaps someone knows of one that will work and fit the size requirement?

    You also mentioned 350 LEDs, but using a 7805 which is rated for 1 amp. Usually WS2811 LEDs use about 1mA just for the controller chip, plus about 40 to 50 mA when on at maximum (fully white). I don't think anyone could really recommend a specific part until you get a realistic idea of the current rating you really need.

  3. #3
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    Thanks for the tips !
    Currently the 350 leds are divided in 20 sections each with a microcontroller and a 7805 so I assume the load should be ok. I'd like to simplify and have 2 independant strips of ~180 leds that I would power at both ends. I found the LM1084 which is rated at 5A but it seems to be of the low dropout type...

  4. #4
    Senior Member PaulStoffregen's Avatar
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    I just looked at the LM1084 datasheet. It's a hybrid design that doesn't suffer from the high base current problem. But the dropout voltage is 1.2 volts (or 1.5 worst case), which isn't really "low dropout" compared to the ones with dropout voltage of only 0.2 to 0.5 volts.

    Is 5 amps really enough? From only these 2 messages, I'm not getting a clear picture of what you're doing. Maybe someone else will see it?

    Would 180 LEDs need 9 amps?

    There are lots of great switching regulators out there. At these power levels, usually they involve a control chip and 1 or 2 mosfet transistors, an inductor, at least 2 high quality capacitors, and usually other parts to make it all work. That's quite a bit more complex, but especially with the synchronous rectification designs, they can be very efficient.

    Maybe someone makes a ready-to-use module that fits in your 0.75 inch diameter tube? I'm honestly not very familiar with the many different ready-made modules out there.

  5. #5
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    The TSR-1 DC/DC converters from Traco Power TSR-1 are designed to be direct, pin compatible replacements for the LM78xx regulators.
    When more amps are needed the TSR-3 could fit the bill. Still should be small enough to fit in tight spaces.

  6. #6
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    And if I connect both ends of the 180 led strip with an lM1084, would the 10a combined current be shared among the strip or is it a bad idea ?
    Thanks !

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