control MOSFET current using PWM

Discussion in 'The Projects Forum' started by jeannie, Dec 10, 2009.

  1. jeannie

    Thread Starter New Member

    Dec 10, 2009
    6
    0
    i need a circuit to control the intensity of IR LEDs...

    for this i was planning to make a constant current source using MOSFET...and then control the current using PWM...i want to vary the current between 200mA to 750mA...

    is there a better way to do this???

    can u plz suggest me some circuit or can u suggest me a MOSFET thet is best suited for this...

    thanx a lot.....
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    What will be the application of these IR Leds?
     
  3. jeannie

    Thread Starter New Member

    Dec 10, 2009
    6
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    they will be used to check the blood glucose level
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I do not know how this is done.

    Does it matter if the Led pulses (direct PWM as to obtain an average current) or you want the Led to be constantly on (continuous current)?

    Will you control the current level by hand (e.g potentiometer) or a microcontroller will do that?
     
  5. jeannie

    Thread Starter New Member

    Dec 10, 2009
    6
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    it would be better if it is continuously on....

    and as of now the current will be controlled using a potentiometer and later on i might use a microcontroller...i am yet not sure whether i'l use a microcontroller or not....
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    How precision does this have to be, and how stable? IMO, analog might be the best.

    Does it have to be variable, or would it work with two levels?
     
  7. CaliusOptimus

    Well-Known Member

    Aug 14, 2005
    59
    0
    you could use a transistor or mosfet biased to its linear state as a kind of variable resistor. it would waste more power than a PWM, but it would be much less complicated. if you intend to draw 750ma im guessing it wont be battery powered.

    heres something that might work for you

    [​IMG]

    all resistor values are all dependent on supply voltage and your choice for Q1. a darlington or mosfet would be my choice, to keep the value for VR1 sensible. R1 is going to be a big resistor.. with 12v supply it will be a 10w. of course this circuit wont be super stable....the LED current will change as Q1 heats up
     
  8. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    A constant-current sink is the way I would do this. If you scroll down to the
    "Electronic Load" section at http://tinyurl.com/6cbn6h you will see the schematic
    for a single channel constant-current sink.

    I just prototyped a four channel constant-current sink. I am using it to control some 1W LEDs (see http://tinyurl.com/yzg9kd7). The sink will operate to 30A if you
    can keep the FETs cool. For an LED application the practical limit is probably around
    2A.

    (* jcl *)
     
  9. jeannie

    Thread Starter New Member

    Dec 10, 2009
    6
    0
    hey thanx for the circuit...
     
  10. jeannie

    Thread Starter New Member

    Dec 10, 2009
    6
    0
    could you plz suggest a MOSFET i can use in this circuit?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you really want to have to build this circuit?

    Or would a pre-built and fairly inexpensive unit do?

    There are ready-made regulators called BuckPucks, they come with various options.
    Here is a page on them:
    http://www.ledsupply.com/buckpuck.php
    You would need a BuckPuck with an external pot and an on-board trim.
    A 3021-A-I-700 runs on AC, and can be adjusted for a max output of 750mA.
    A 3021-D-I-700 runs on DC, and can also be adjusted for a max output of 750mA.

    These will be far more efficient than a linear regulator. They are switching "buck" supplies that use high frequency PWM and an inductor to keep the current flow through your LED(s) very constant, while generating very little waste heat.

    They are also small in size.

    However, if you really want to build a circuit, that can still be done.

    By the way, where in the world are you? It helps a great deal if you put your country and state (if applicable) in your profile, as unless we have an idea where you are, we won't know where you might be able to find the parts you need.
     
    Last edited: Dec 14, 2009
  12. CaliusOptimus

    Well-Known Member

    Aug 14, 2005
    59
    0
    +1 on the buckpucks

    if you still want to DIY, an IRF540 will work fine at 12v, and will be easy to find. if you use a mosfet, you can omit R2
     
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