LASER PWM Driver

Discussion in 'The Projects Forum' started by rueffy, Nov 17, 2013.

  1. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Hi,

    Below is a driver circuit for a LASER module I purchased recently, the LASER will be receiving a PWM Audio signal from the output of the comparator, that signal will switch the gate of the MOSFET on/off effectively shorting and unshorting the LASER. There will be an inversion in that the on pulses will become the off pulses - but I don't think this will introduce any problems with the demodulation of the audio as the information is still present.

    My question is, is this the correct way to drive a LASER? Does anyone have any experience on the subject? I've attempt a couple of other configurations with minor success (i.e. LASER very dim, or noise introduced).

    The LASER should also be suitable for PWM, i.e. no supporting circuitry to introduce capacitance.

    Thanks,
    rueffy.

    [​IMG]
     
  2. Shagas

    Active Member

    May 13, 2013
    802
    74
    Alot of the laser modules from what I heard (especially the cheaper ones) won't handle fast switching . You should find a laser module with a TTL input which can handle the frequencies that you need.
    What you might try is feeding a square wave to your module and monitoring the output of the laser with a photodiode or something , taking note of the rise/fall times and seeing how far you can push it .
    If you find out that your module can't handle it then try looking for one with ttl.
     
    rueffy likes this.
  3. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    I can confirm that the LASER is suitable for PWM applications. I made sure of this when purchasing the module.

    I've had the LASER transmitting information successfully with good reproduction at the receiver. Just not getting enough power from the LASER, need to get that voltage/current combination perfect.
     
    Last edited: Nov 18, 2013
  4. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    I had no real success with that configuration, it worked but the LASER was too dim, the voltage/current was not correct for operation.

    I've sat down tonight and come up with an alternative circuit, just need verification that it would actually work!

    My LASER diode requirements are:

    Input Voltage: 2.4V
    Input Current: 40mA

    By incorporating a LM317, I can provide the fixed voltage input to the LASER diode of 2.4V, I would then have a MOSFET open and close the earth connection in sync with the PWM voltage applied to the gate. The current limiting resistor would be in series with the MOSFET. Would this actually work?

    [​IMG]
     
    Last edited: Nov 18, 2013
  5. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    I prefer the circuit in #1. Does your laser have a rating for continuous current? I would choose R11 to adjust the current to, say, 80% of that level. You might need to determine the proper value by experimentation using an ammeter in series with your laser. Lower the resistance by small increments until the current approaches the rating.

    Of course you can get more brightness with more current, but not without shortening the life of the laser.

    You want to be sure that MOSFET is a logic level. I don't think the peak voltage on its gate will fully turn it on otherwise. That 9V isn't a battery, is it?
     
  6. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Yes your right about the current adjustment method in circuit 1, but I cannot adjust the voltage, and the diode won't preform well unless its around its preferred operating voltage. What I'm trying to do is get the voltage and current both correct. Harder then I thought.

    Yes the 9V is a battery, the gate voltage is 9V / 0V accordingly.

    SPECIFICATIONS
    Threshold current: 24mA
    Operating voltage: 2.4V
    Operating current: 40mA
    Optical power: 10mW
    LD reverse voltage: 2V
    PD reverse voltage: 30V
    Size: TO-18 (5.6mm dia)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try it like this instead:

    [​IMG]

    The LM111/LM311 is capable of sinking up to 50mA; so you won't need a MOSFET. If you want to try using a different comparator, you must ensure that it's capable of sinking enough current. An LM339 quad comparator won't work.

    The LM317 is wired as a constant current supply. The formula for determining R1 for the desired output current is roughly 1.26/desired_current, where:
    1.5 >= desired_current >= 10mA.

    Since you want 40mA, 1.26/40mA is 31.5 Ohms; the closest standard E24 value of resistance is 33. Besides, you really don't want to cut it too close; the fastest way to kill your laser is to exceed the current rating. You MUST regulate the CURRENT through your laser diode; as the VOLTAGE across the diode will vary with temperature. Attempting to regulate the voltage across it can result in "thermal runaway"; as the temp of the laser increases, it's forward voltage decreases, which would cause the current flow to increase, which generates more heat, etc. until the laser fried.

    You didn't mention what you want it modulated with (or I just didn't see it) - so I arbitrarily chose R2 and R3 to establish a 1V reference level at "Ref", the noninverting (+) input to the comparator, and the input signal, a 1V PP sine wave with a 1v offset on the inverting (-) input. As you can see, current flows through the laser when the sine wave voltage is greater than Ref.
     
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  8. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    You need to adjust your thinking; you cannot control both at the same time. The laser (and many other electrical devices) will show a voltage across itself as a function of the current passing through it. You could also say the current is a function of the applied voltage, but that's a poor way to actually control the laser as Sarge has noted.

    A resistor does a decent job of limiting current. A constant-current regulator does a better - more precise - job.

    Sarge may correct me but I believe you could replace the comparator in his circuit with a single NPN transistor if you have enough signal to drive its base. The comparator gives you more flexibility.
     
    Last edited: Nov 18, 2013
    rueffy likes this.
  9. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thank you for your informative replies gents.

    I'll admit I've been having a hard time getting my head around the driver requirements, when the datasheet for the diode says 2.4V and 40mA I assume both of these requirements must be met, but as you say Wayne it's been practically impossible for me to establish both of these conditions, with one exception, when I connected the laser diode directly to my explorer boards V+ and E and programmed in 2.4V and 40mA current limiting, it generated plenty of light. I've only be able to get dull light using the other configurations I've tried - not enough to get my receivers photodiode above the light threshold for reception.

    So I thought I'd post my entire TX circuit diagram for you to get an appreciation of my requirements, as you can see an oscillator generates a triangle wave which gets compared with an amplified audio signal, the comparator generates PWM and that PWM turns the power supply of a laser diode on and off. I can confirm this circuit...with the exception of the laser driver works well, I've connected my TX to RX directly and successfully demodulated audio of good quality.

    Wookie, I see you've established a constant current source with the LM317, the comparator is comparing a sine wave and 1V reference. When sine wave dominates, output driven to 0, therefore diode cathode goes negative and conducts, visa versa for if the reference dominates. Would C1 have any effect on the capacity of the circuit to operate at around 50kHz, the switching frequency of my PWM.

    To make this circuit suitable for my application I would just connect the output of my comparator as is to the laser diode cathode?

    So to summarise my thoughts, you guys are basically saying forget the idea of trying to control the voltage, give the laser diode a constant current source of 40mA and let it do its thing?

    [​IMG]
     
    Last edited: Nov 19, 2013
  10. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Wookie,

    I've had a go at putting your circuit together and it seems to do the job nicely, I 'had' the circuit receiving and demodulating well, I've noticed an interesting waveform at the output of the comparator however, rather then the comparator output returning back to zero when the comparator is off, it holds at around 4V, this must be keeping the LASER in partial conduction as its not enough to get the receiver back below the threshold - because of this the receiver cannot demodulate, perhaps just tinkering with the receiver threshold will fix this but is that waveform on the output of the comparator normal?

    When I disconnect the LASER and monitor the comparator output it returns to normal, 9V / 0

    [​IMG]
     
    Last edited: Nov 19, 2013
  11. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    Is it possible your MOSFET is installed backwards? The source pin needs to connect to ground. It's very easy to get the pinout confused. Been there.
     
  12. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    I found by reincorporating the MOSFET I could eliminate the problem. Still not receiving a clean signal however, it appears as if the diode is constantly on despite the gate voltage.. investigating.

    [​IMG]

    [​IMG]
     
    Last edited: Nov 19, 2013
  13. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    MOSFET is installed correctly, I've got a negative on the source and positive on the drain.
     
  14. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thank you to all those that helped contribute to the development of my circuit. The project is now fully functional and I've made a video as part of my presentation to a class, the video shows the system in operation with a full description of signal flow etc.

    http://www.youtube.com/watch?v=CCAkpJ-jG6A

    rueffy.
     
    Last edited: Nov 25, 2013
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