Constant Current Source

Discussion in 'The Projects Forum' started by Broncos, Feb 2, 2012.

  1. Broncos

    Thread Starter Member

    Feb 2, 2012
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  2. crutschow

    Expert

    Mar 14, 2008
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    What is the voltage requirement of the LED?
    What is your power supply voltage?
     
  3. Broncos

    Thread Starter Member

    Feb 2, 2012
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    7.4 volts from the battery.
    2.8 to 3.5 drop across led depending on current, etc
     
  4. JDT

    Well-Known Member

    Feb 12, 2009
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    This is quite a high current even for a power LED. Usually they are wired in series strings running at 350mA or 700mA.

    To reduce wasted power they are usually driven from a switching circuit. The current is controlled on a cycle-by-cycle basis.

    See the attachment
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    To answer your question, the value of R3 determines the current. In operation, R3 is equal to the base-emitter forward voltage of Q1. Figure that voltage will be about 0.6 volts. The actual voltage depends on Q1 and R1.

    R = 0.6/I = 0.6/3 = 2 ohms.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have the specsheet / datasheet of the leds to use?
    3 Amp is very unlikely for leds.
    The highest current for high power leds I have seen is 1.4 Amp.

    Bertus
     
  7. Broncos

    Thread Starter Member

    Feb 2, 2012
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    It's a Cree t6 led. I don't have easy acsess to finding data sheets right now on a mobile, but it should be a top google hit for the datasheet.
     
  8. Broncos

    Thread Starter Member

    Feb 2, 2012
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    Doesn't that equal .2 ohms?
    Is there a value of r1 or specific q1 that could help create lower tolerance for error? Like how does r1 change the voltage/ what value r1 should I use?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you read the datasheet?

    The 3 Amp is the maximum that may be used.

    Bertus
     
  10. Broncos

    Thread Starter Member

    Feb 2, 2012
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    I don't always want to drive it at 3 amps, but I want a mode where I can show off its muscle!
    I was thinking using the circuit I linked to I would have a switch where I would lower the resistance down to where it would drive it at 3amps(or 2.8ish to be safe), normally I would drive it at 1 amp to 2.
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Showing off its muscle can very well mean showing how quick you can burn it out with LED's.
     
  12. Broncos

    Thread Starter Member

    Feb 2, 2012
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    I just want a momentary mode where I can have the current near max, is driving it at 2.8-2.6 amps going to burn it out? or cut its life soo much from its advertised 25000 hour life at 2amps?

    Really I'm just curious about building the circuit to drive it, its all I can think about the last few days, I just want to hear some positive feedback that the circuit will work, I can switch R3 resistors by connecting another resistor in parallel with a switch ( to change brightness), some theory, and how exactly to calculate the values I would need.
     
  13. castley

    Member

    Jul 17, 2011
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    One transistor, a reference diode at the base and an emitter resistor should work.
    The resistor vaue at the desired current should have a voltage accross it about
    .65 V. lower than the reference diode connected between the base and the current output point.
     
  14. DickCappels

    Moderator

    Aug 21, 2008
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    To answer Broncos, yes, you are correct, I left off the decimal point. The resistor should be 0.2 ohms. And yes, the exact current depends on Q1.

    The circuit is not terribly stable, as the current will increase at about +0.33%/°C. But I think you will find the LED's output drifts about that much in the opposite direction.

    This is not a particularly good regulator, but fine for an LED used for lighting. For laboratory instrumentation, you might want better. What I like about your original circuit is that the excess power is dissipated by a MOSFET, and they are pretty hard to kill with power (compared with a bipolar).

    A note about operating current of the LED: Take note of the chart "Thermal Design" in the datasheet. Learn to calculate power dissipation and temperature rise (good things to understand for any electronic design) and then observe that chart.
     
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