LED Brightness Control

Discussion in 'The Projects Forum' started by phoebe_, Jul 1, 2012.

  1. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    I'm trying to control the light intensity of an HPLED without using PWM. Would a simple circuit using LDR in series with the HPLED and a resistor work?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Do you have more detailed information on the led, like a datasheet?
    A LDR is probably not capable of dimming the led directly.

    Bertus
     
    phoebe_ likes this.
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Welcome to AAC.

    Part of the reason PWM is used is efficiency. You might as well use an incandescent bulb if you are going to waste the energy you are talking about.

    LEDs, 555s, Flashers, and Light Chasers
     
    phoebe_ likes this.
  4. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    The LED is 1W Hexagon high power LED. It looks like this.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I understand, have one that looks very similar.

    There are devices called buck pucks. They are SMPS (switching mode power supply) constant current regulators.

    I have one build I have yet to test out.

    Say you want 0.7A for the LED (typical for 1W). If you use one of these devices with 12VDC it will draw 0.3A, give or take. This is part of what I mean about efficiency.

    The LED will get very hot, heat sinking is a must. If you use a resistor it will likely have to be a high wattage. A buck puck does not get hot.
     
    phoebe_ likes this.
  6. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    I just want to make it as simple as possible. I've tried using a potentiometer but it just wouldn't work.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    That led probably needs 350 mA to light up.
    The voltage may be between 3 and 4 Volts.
    A LDR will never manage the 350 mA.
    As Bill said PWM will be the correct method to dimm the led.

    Bertus
     
    phoebe_ likes this.
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm wondering what your aversion to PWM is all about?

    High-power LEDs really need a buck or buck-boost type of current regulator instead, as other methods are inefficient (such as purely resistive). You'd need a physically very large resistor, and it would make a good room heater.

    Have a look at BuckPucks:
    http://www.ledsupply.com/buckpuck.php?gclid=CO7Hncve-LACFU2htgod7hXzlQ
    Not particularly cheap, but compact, reliable, and relatively easy to wire up.
     
    phoebe_ likes this.
  9. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    Thank you very much! I'll start reading about PWM and hopefully I can make my own circuit.
     
  10. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    Thanks a lot for the quick responses! Sorry for my ignorance, I'm a newbie to electronics and I just want to make things convenient for me. Anyway, this seems to be a great community.
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    If you need help with the circuit we can help, or I can show you how to do it with a LM317 (which will get very hot, as it is the same as a resistor).

    Wookie caught me out on one detail, his 350ma figure is correct. I have a 3W LED.

    What is the power supply voltage you are powering this with? If it is not stable (such as a car would be) we need to know.

    Most of these circuits are pretty simple. Don't sweat it.

    Since I brought it up automotive is not allowed. The exception is the cigarette lighter. That is never my first assumption though.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    An LM317 has a rather large dropout when used as a series current limiter (~3v), and since the LED is probably dropping ~3.2v-3.6v across itself, you're just about guaranteed to waste at least half as much power across the LM317 & resistor as you would in the LED itself.

    It was Bertus who mentioned 350mA, and I agree that Bertus is most likely correct.
     
  13. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    I would really appreciate it if you could teach me how to use a current regulator. I'm using a power supply with voltages up to ~5V.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If your power supply is only 5v, then you are just about out of luck.
    That won't leave enough "headroom" for either the BuckPuck or the LM317.

    If your LED drops 3.6v, you will need at least 2v more; 3.6v+2v=5.6v.
    With an LM317, you would need at least 6.6v.
     
  15. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    Oh, what I meant was I was only using around 5V for this project. I actually have a 15V power supply.
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Perhaps a bit odd use of a powerfet, but it will drive the led.

    [​IMG]

    The current through the led will be about 0.7 / Rbe = 0.7 / 2.2 = 0.318 A.

    Both the led and fet must be cooled.

    Of course the others may comment on my circuit.

    Bertus
     
    Last edited: Jul 2, 2012
  17. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    That would work.

    Where is this 5V coming from? I ask because we need to know how regulated it is.

    At that low voltage a buck puck or LM317 won't work. If you could use the 15V more options open up.

    I suspect I know the answer on this one, have you ever soldered electronics before?

    We'll probably have to adjust our responses to your skill level.
     
  18. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    I tried getting the characteristics of the LED by creating a simple circuit with the LED, a resistor, and the voltage supply. The peak voltage across the LED is ~3V. The current on that circuit was ~370mA.
     
  19. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Shoot, if the power supply is stable, like a computer PSU, you are there. I would consider 370ma close enough. Does the resistor get hot?
     
  20. phoebe_

    Thread Starter New Member

    Jul 1, 2012
    13
    0
    I don't really remember the resistor getting hot. The HPLED was quite hot thought.
     
Loading...