A little help please with a opamp LED driver please

Discussion in 'The Projects Forum' started by Viking, May 29, 2013.

  1. Viking

    Thread Starter New Member

    May 21, 2013
    4
    0
    Hi all,
    I'm looking to build a opamp pwm to drive a 1w single Led.
    Looking around this forum and other sources I've gone for a design which I've seen in a few places and then as luck would have it I found a circuit on here posted by Stgwookie for the exact opamp's I have (TL071).
    Ok so I've built this up and attached a normal low power led with a small resister to the pwm output and everything works. I put in a bigger cap on the frequency side and have introduced flicker in the led to prove the frequency adjustment works. The duty cuycle adjustment also seems good with the led dimming and turning off as well as going to full brightness.
    I get issues though when I try to hook up a high power led.
    Using a transistor to power the led results in the led not turning off it goes dim but still has a sight glow to it. I've tried different transistors in different ways including a darlington setup, but always the same result. Now I've no experence with transistors so fumbling in the dark here!
    What I want is to fully turn the led off (dimmer) and also to incorpora contant current for the Led. I have a few transistors but would like to hear recommendations on the best ones for the job.
    Source is up to 12v and I plan later to create multiple sets of leds controlled from this circuit. At the moment I'm planning to provide just enough voltage from the pwm to to power the led minimising waste.
    Led uses 700mA (peak 1000mA) and has between 3.2-3.8 voltage drop.
    Any ideas please.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    Posting your schematic would be a big help. It sounds like the opamp does not have enough current handling capability to turn the transistor off, but it is hard to tell without the actual circuit. Do us a favor -- won't you?
     
  3. Viking

    Thread Starter New Member

    May 21, 2013
    4
    0
    Yes I've been looking for it, didn't have it bookmarked!
    Ok this is the link: http://forum.allaboutcircuits.com/attachment.php?attachmentid=27222&d=1296977753


    I lowered the value for R8 to make it work with the low power led (can't remember the value right now) and left that as it was. Also R9 & R10 are 10k ohm. C2 and the audio input are not included. C1 is now 2.7uf.
    Ron h had an almost indentical circuit for lm393's which had vcc feeds to both opamp outputs. Would these have added the extra power to the circuit?
    I'm open to ideas on the transistor side.
    Thanks
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    I am skeptical about the use of a PWM circuit designed for audio being used to drive an LED. You said that you would post a circuit diagram and this isn't it--where is the transistor and the LED? You seem to favor the well-established Allaboutcircuits tradition of asking for help while regarding your design as the most precious secret! One thing I'm seeing is that the output of SgtWookie's amplifier (Vpwm) never goes below 2V, which suggests that any NPN transistor base driven from there will never turn off. But not knowing how the transistor is installed, it's impossible to be sure.
     
  5. Viking

    Thread Starter New Member

    May 21, 2013
    4
    0
    Thank you for your input John. Your observation of the output not going below 2v does sound like the issue.
    As for the lack of transistor circuit as I mentioned earlier I'm not at experienced with these at all and I'm trying to make the transistor work on top of a circuit with the above issue, no wonder i'm having a hard time! I have 2n2222's and hope I can use theses.
    Originally I was just planning to follow most 555 circuit setups (led plus resistor into collector, base to the pwm output maybe with resistor and emitter to ground). I've had this working on a 555 setup, but want the 0-100% duty cycle. Hopefully if I can get this basic circuit working move up to one of Bill_marsdens array setups in his 555 notes.
    Please bare with me here I'm on a steep learning curve.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    All right, I will attempt not to be grumpy any more. I've actually installed a number of 1W LEDs recently, to provide lighting for a model railroad. See here, and note that the regulator is just visible next to the LEDs in the top of the picture:
    http://tmrc.mit.edu/progress/reports/2013/01/IMG_3007.JPG

    However, unless you want a learning exercise, I'd suggest buying a driver for the LEDs rather than building one. You can get them very cheaply direct from China via eBay, like this one (which is similar to the one I used):
    http://www.ebay.com/itm/1w-LED-Driv...793?pt=LH_DefaultDomain_0&hash=item58a08ba6b9

    One point is that if you want a device like this that uses power efficiently, you must use an inductor, as the commercial device does. You might build a setup that burns power in a transistor or in a resistor, but otherwise there simply isn't any alternative. Look up "switching current regulator" and you'll see that every circuit diagram includes an inductor.
     
  7. Viking

    Thread Starter New Member

    May 21, 2013
    4
    0
    It's definitely for the learning John.
    I'm aware of the driver's available, but I'd much rather try and get my head around the problem and have a go.
    Thanks

    P.s. nice model setup.
     
  8. Colin Wonfor

    New Member

    May 29, 2013
    3
    0
    Dear Viking,

    The TL071 will not go rail to rail this will cause any transistor to stay in conduction , use a voltage divider or a MOS FET.

    Best Col
     
Loading...