LED Strip - Current Control good enough.

Discussion in 'The Projects Forum' started by mike8675309, Oct 3, 2010.

  1. mike8675309

    Thread Starter New Member

    Sep 25, 2010
    7
    0
    I have been working on a LED project for a few years now. Over time the project has evolved mostly due to the increased use of LED's in general, and thus the improved technology available.

    My project is for a LED string. It started as a fix to an existing LED string that failed. When I took this existing string apart, I found the reason for the failure. It was using only bias resistors to maintain current across the LED's. It appears that when one LED in the string failed, excess current caused damage both to other LED's as well as the circuit board that the LED's were mounted to. (visible in the attached picture).

    Currently I have a circuit and a functioning prototype that uses a LM334 (constant current source) a transistor, and a resistor to run up to 5 LED's per string. The full string consists of 20 LED (4 Strings) interleaved so that if a single string were to fail, the LED strip wouldn't have a blank spot but instead would just have some light drop out.

    The current circuit is able to maintain regulation from about 8v up to 16v.

    Right now, intensity is controlled by adjusting the current through the lamps.

    I'd like to make improvements, so the question I'm posing here is should I just leave it alone, or are there some simple improvements I could make? Package size is a limitation, though I've already determined the LED's will go in the small package (singe line of led's) and the controls can sit outside the container the LED's go in.

    Some ideas I've thought of. Implement PWM to handle brightness and possibly decrease LED heating to increase their life.
    Switch to some sort of microcontroler.
    Switch to some sort of IC (recently interested in the MAX16807 http://www.maxim-ic.com/datasheet/index.mvp/id/5257) but not sure how to implement.

    the problem is, since I haven't really finished this... I keep trying to mess with it and improve things. Do you think I've got it good enough?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,751
    760
    Best option is if u know programming, go for micro controller
     
  3. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    I wouldn't recommend an LM334.

    The reason for this is that it is temperature dependent without a diode to compensate, and it is limited in current without an external transistor.

    It is actually possible to make a constant current source using just a transistor, two resistors and a zener. It's not as good as a perfectly regulated constant current supply (current varies by about 5mA from 8V-12V) but it is small and simple. It is temperature dependant, but not as much as the LM334. And it can scale up to any amount of current by changing the emitter resistor.

    See attached screenshot. In this simulation it is driving 4 LEDs @ 21mA each. The zener is 5.6 volts.

    Paste this into Falstad's circuit simulator: http://www.falstad.com/circuit/

    Code ( (Unknown Language)):
    1.  
    2. $ 1 5.0E-6 10.20027730826997 45 5.0 50
    3. t 288 224 320 224 0 1 -1.591607041931848 0.6872788133136813 100.0
    4. r 256 224 256 160 0 1000.0
    5. r 320 320 320 240 0 56.0
    6. w 320 320 256 320 0
    7. w 288 224 256 224 0
    8. w 320 160 256 160 1
    9. R 256 160 256 128 0 0 40.0 9.0 0.0 0.0 0.5
    10. 162 320 160 320 208 1 2.1024259 1.0 0.0 0.0
    11. 162 352 160 352 208 1 2.1024259 1.0 0.0 0.0
    12. 162 384 160 384 208 1 2.1024259 1.0 0.0 0.0
    13. 162 416 160 416 208 1 2.1024259 1.0 0.0 0.0
    14. w 416 208 384 208 0
    15. w 384 208 352 208 0
    16. w 352 208 320 208 0
    17. w 320 160 352 160 0
    18. w 352 160 384 160 0
    19. w 384 160 416 160 0
    20. g 256 320 256 336 0
    21. z 256 320 256 224 1 0.805904783 5.6
    22.  
     
  4. mike8675309

    Thread Starter New Member

    Sep 25, 2010
    7
    0
    I see what you are saying with the LM334. Any thoughts on the MAX16807 ? Looks like a handy package to get PWM LED control and I could run up to 8 5 led strings if I'm reading the specs properly.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,751
    760
    It seems that this chip is quite sufficient for ur app.
    I did not read the whole data sheet yet. so I like to know if u make the use of this chip, that is it possible to Vary the brightness from 0 to max. Like a typical light dimmer would work.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    A LM317 works well with a single resistor. So do transistors.

    Just an example of what I've come up with over time.

    [​IMG][​IMG]
    [​IMG][​IMG]
    [​IMG]

    A quick side note, anytime the LM317 is used as a voltage regulator you need capacitors on on the input and output per the data sheet, but not with the current regulator mode.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers
     
  7. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    For constant current sources, I have used the Supertex CL2N8-G (20mA) and CL6K4-G (100mA) chips. When driving from a micro, the TLC5916 (8 channel, 20mA) works nicely. As with any linear constant current source, try to keep your power supply voltage close to the LED voltage drop plus the source's voltage drop, to minimize heating.
    /mike
     
  8. mike8675309

    Thread Starter New Member

    Sep 25, 2010
    7
    0
    The idea of the constant current control is to deal with voltage swings that could go as low as 11v and as high as 14v (granted, in failure modes of other things, not normal operational swings).

    the idea of the lm334, was being able to keep component count low and support long operational life. I.e. it was easy to drive a 5 led string with my current circuit.

    I have read the link from Bill, and plan to sit down with my various parts and play with all the ideas presented. I had a feeling my initial circuit had some deficiencies but without any significant experience, I just didn't see them. I appreciate the input. The MAX16807 just caught my eye as when i started this project, all LED drivers were really setup to just run one LED at a time. This was one of the first families I found where they supported numerous LED outputs, with high output voltage, thus giving me the option to drive multiple 5 LED strings all from a single IC. I'll probably play with one, the price is reasonable. I just am not sure how to implement it yet.
     
  9. mike8675309

    Thread Starter New Member

    Sep 25, 2010
    7
    0
    A quick question on the LM317t that I didn't find covered in the LED link above. How to do you determine the set resistor for a current source say in your figure 2.1? For example, if my minimum input voltage is 11.8vdc, max is 13.5vdc and my LED Forward Voltage is rated at 1.7 typical, 2.4max and their max brightness is at 20ma. I'm thinking of having 4 LED's in series.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    The spec for a LM317 is it outputs 1.25V ±0.05V. Figure the resistor needed to set the programing current. All that is really important is the 20ma, 1.25V÷0.02A, or 62.5Ω. A standard resistor is 62Ω, which will work well.

    LM317 has 10ma as a minimum spec, as well as a 40V max limit. Since it is a regulator it is self adjusting, figure its insertion voltage is around 2V, it needs that much voltage between the input and output pins to power the chip.
     
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