Implement max current pot

Discussion in 'The Projects Forum' started by LaserAdct, Jan 27, 2011.

  1. LaserAdct

    Thread Starter New Member

    Jan 27, 2011
    3
    0
    Hello all.

    I have been searching for a solution to my problem for a long time and havent been able to come up with a solution. When I discovered this site I was thrilled because it seems that there is a huge amount of knowledge from the various users posting here. I am hoping a kind person is willing/able to help me out.

    I dont think my problem is a big issue, however my skill level does not allow me to come up with a solution myself.

    My problem is that I have a laser diode driver that has a pot which sets current from 0-500mA. Its a driver I use for 3 different diodes (RGB) and they all have different mA specs. I need to enhance the driver schematic so that an extra pot is implemented which can set the MAX mA that can be set. Meaning first pot sets a maximun, the second pot can then be adjusted from 0-MAX mA.

    My skill level at this point only allows me to build curcuits from schematics, measuring and stuff like that.

    The original schematic looks like this:
    [​IMG]

    The pot that sets current is R5(220), and it is for this that I need an extra pot which sets the MAX mA that R5 can be set to.

    Any help would be highy appreciated.

    Best regards

    Nicki
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    What would "MAX" be? A value between 1 and 499mA per Laser, or a value between 3-1500mA for all 3 Lasers to share?

    When the "MAX" is increased, would the current to all 3 Lasers be increased as well, oris "MAX" simply a hard limit to prevent burnout?

    Reason I ask, is if you explain the "big picture", some wizards here can come up with an overall simpler solution than trying to rework what you are looking at starting with, if that makes sense.
     
  3. LaserAdct

    Thread Starter New Member

    Jan 27, 2011
    3
    0
    Thanks for your reply.

    The max would be 1-499 since I have made 3 boards, 1 per laser. I was just told that implementing another pot might not be as straight forward as I first thoght.

    Lets say the value of the resistor R6 sets the maximum current to 500mA, then the pot as it is now will be able to adjust from 1-500. If it is possible to implement another pot this would ie. be set to an upper limit of 350mA, then the R5 would no longer span from 1-500, but from 1-350mA.

    Hope this clears things up.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    Increasing the voltage at the base of Q1 reduces current to the laser. So, changing the relationship of R6 to R5 will give you control over current. That much you know.

    What you need is to read about using pots in parallel and series. I saw a very good description of this somewhere. Sorry I can't recall, but I just found it by googling. If it helps, I think I was looking for information on the different tapers available in pots. Anyway, several articles described how you can set minimums, alter the range and so on, just by configuring the pots differently and including additional resistors.

    What you want to do should be quite easy. You just need standard pot circuits to choose from.
     
  5. LaserAdct

    Thread Starter New Member

    Jan 27, 2011
    3
    0
    I have been searching and decided not to make this improvement. I dont have the skills for it and reading about it only confuses me more. Thx anyway. Will try to find someone who actually know what to do elsewhere.
     
    Last edited: Jan 28, 2011
  6. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    How is your LM431 regulator connected? There should be a pot for adjusting its reference voltage. Turning up the reference voltage will turn down the LED current for a given setting of R5, so it could be used as a limiter. If there is no pot, then the zener is just being used at its reference voltage of 2.5V?

    Another way would be to place a pot in parallel to R4, across it. Full on (low ohms) would raise the voltage seen at the base of the transistor, reducing current. Full off would return the circuit to its present state. Something like a 10K pot might work. A 1K might work, but might also limit the max current too much even when you don't want to.

    Instead of a pot, you might also consider a switch and resistor in series, both in parallel to R4. You could then choose the resistance to switch from 500mA max to something less.
     
    Last edited: Jan 29, 2011
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