Parallel LED Dimming

Discussion in 'Analog & Mixed-Signal Design' started by Michael Murton, Aug 7, 2016.

  1. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    Hello everyone, I am new here but not to electronics.

    I have run afoul of something I am not finding enough information to come up with a solution for, I am hoping you wonderful folk will be able to help.

    I am working on the restoration of a 1965 Chevrolet C10 truck, this particular bit has to do with the dash illumination. The dash illumination will be mix of small bulbs and LEDs. They will all have a common ground and dimmed power feed, I am not opposed to having a separate power for the bulbs and LEDs, but all will be wired in parallel and the number of LEDs is not known. The driver must be able to handle changing numbers of LEDs without adjustment, although it is acceptable that the brightness of the LEDs be reduced slightly by the addition of more LEDs in the circuit.

    So I've done LED drivers before in design, but in terms of building them I have only ever used display drivers that handle a single LED per driver and have a common return back to the chip. I have made designs for buck drivers, but never built one yet. I do know that the best way to drive LEDs is in series, and that driving them in parallel has several drawbacks. I also know that a constant current buck converter will not be a safe option for this configuration because the load will not be balanced with the LEDs wired in parallel.

    Given these design criteria, I don't know how to drive them.
    The best theory I have at this time is to PWM a P-FET on the high side, low pass filter, and have individual current limit resistors at each LED. This does not seem like the best way to do it though.

    Any help is greatly appreciated,
    Mike
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Define "dimmed power feed" does this mean a variable voltage?

    The simplest way to drive LED's is just to use a single current limiting resistor of each LED.
    This way you can just keep adding more LED's anyplace you want- super easy and simple.
    Unless you are talking hundreds of LED's, the power level will be low, no need for a fancy driver circuit.
     
  3. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    I should have clarified that, the dimmed power feed can be whatever it needs to be and dimmed in whatever manner is best. Voltage in the vehicle will be 12V (14V when running) and I can step down from that to whatever is required.

    Not talking hundreds of LEDs, maximum I would say would be 50 but most likely less than that.
    Can I get decent dimming results with them each having their own limit resistor and then vary the voltage though?
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Assuming a white LED with Vfwd=3V and a series resistor of, say 1k, varying the drive voltage from 14V to 4V would vary the current from 11mA to 1mA.
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    Using a resistor on power leds isn't a great way to go. It does work for signal leds however.

    Pwm provides a good way of dimming here. Maintaining the color while dimming different leds is tricky, however.
     
  6. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    They aren't going to be power LEDs, just 3 or 5mm white and will all be the same type.

    What do you mean about maintaining colour?
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    With linear regulation (e.g. a current limiting resistor) some parallel LEDs might blink out at low settings - the Vf isn't quite identical for each and every LED. PWM would apply the full power to all the LEDs, but for a variable proportion of the time.

    With bulbs around - you could use combinations of series pairs of LEDs and 6V bulbs as current limiting resistors.
     
  8. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    I'm guessing the voltage you're talking about is under approx 2V or so? I doubt there would be any need to run them this low, but I will watch for this and add a safe lower limit in software to avoid this.

    This would only apply if I do not use an RC filter, no? This would seem to me to cause flickering, except at very high frequencies. How high a frequency would I need to run to avoid visible artifacts at low voltage?

    Series strings of any kind are not feasible, keeping track of them would become problematic.
     
  9. BobTPH

    Active Member

    Jun 5, 2013
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    LEDs will not appear to flicker at 100 Hz. That is not a "very high frequency". It you really want to be careful, use 1000Hz, which is still not a "very high frequency."

    Bob
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Definitely use 100Hz to 250Hz. Lower frequency can be distracting (60Hz) if you turn your head quickly, you will see a strobing tail as your eyes scan past the LEDs - especially at low Pwm duty cycle. You won't notice the flash of 60Hz looking right at it but you will notice it if it is in the edge of your peripheral vision or if you are scanning (moving eyes or head).

    I wouldn't go much more than 250Hz or you may start to loose Pwm resolution, depending on your Pwm control methods.
     
  11. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The basic idea is to have a PWM dimmer that drives the array of LED's, each with their own current limiting resistor.

    Since it's in an automobile, best to switch the high side (+12 V line) and have all the LED cathodes connected to chassis ground.

    I concur that you want a PWM frequency greater than 1 kHz for minimal flicker, some people don't mind/see this flicker but I find it annoying and "cheap" looking.

    Linear dimming would be more trouble than you imagine, it does cause slight color shift, white LED's look "colder" at low current levels.
    The effect of the LED Vf variations also makes it tricky to get them all even at low brightness, with PWM these issues are null.
     
  12. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    What do you mean by linear dimming being more trouble than its worth? Are you suggesting I program my microcontroller to dim like a log pot?
     
  13. Sensacell

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    Jun 19, 2012
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    Linear dimming means varying the current in an analog fashion, as opposed to PWM, which is a switching technique.

    The only time I ever use linear dimming is in super EMI sensitive situations, very rare.
     
  14. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    OK I wasn't sure if that's what you meant. This is a digital solution so that will not be the case.

    In regards to EMI though, if I am also driving bulbs on the same circuit the current through the resistor of an RC filter would be high. Would it be possible to use an inductor/choke in place of a resistor with similar results?
     
  15. dannyf

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    Sep 13, 2015
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    If those are singling leds, you may just try putting a resistor on them and then tie all of them to a power rheostat - just as what most cars do.

    This type of arrangement works best if the leds are 12v. If not, put a few of them in serial.
     
  16. Sensacell

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    Jun 19, 2012
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    Note that nobody uses any type of "filter" on PWM'd LEDs - totally unnecessary.
    Your eye and brain do the filtering.
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    The Vf of a white (or blue) LED is around 3.4V - the other colours are in the range 1.7 - 2.0V.
     
  18. BobTPH

    Active Member

    Jun 5, 2013
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    An LP filter after the PWM turns into an analog voltage. That is not what you want.

    Bob
     
  19. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    Right, because then it becomes linear dimming, duh.
    But that may present another problem. 1kHz PWM for LEDs is fine, but ideally I'd like to be able to use a single source to dim the bulbs and LEDs. 1kHz is audible and without an LP filter will likely cause ringing/buzzing/hum in the filament no?
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    Probably less than 50 or 60Hz AC.

    Just a point of interest - 500Hz was a common strobe rate for multiplexed LED displays.
     
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