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Led driver of constant current - is it really constant ?

Discussion in 'Power Electronics' started by kellogs, Jul 25, 2020.

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  1. kellogs

    kellogs

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    Jan 7, 2014
    That's quite an old school piece of work! Unfortunately it won't easily fit into my head. For the second circuit:

    Something tells me that if I replaced that battery with an alkaline AA cell, the LED will light up from brighter to dimmer as the battery depletes (1.6 -> 0.8 V). Is this the case ? What sort of dimming percentage should I be looking at for the whole battery life ?
     
  2. Sunnysky

    Sunnysky

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    121
    Jul 15, 2016
    Constant Current source using 200nA diode @ 376mV from Vbe=Vce driving an open-collector Comparator with an 18 Ohm Current sensor for 20mA = 376mV/18Ω . Unregulated R limited LED compared on the right runs from 28 to 12 mA while Vbat ranges from 3.2 to 2.2V... My Simulation here http://tinyurl.com/y683znyl with Vbat slider on the right.

    upload_2020-8-7_22-14-21.png

    LED current drops to 10mA when this circuit Vbat drops to 2V for a typical high bright Red or Yellow LED.
     
    Cirkit and kellogs like this.
  3. kellogs

    kellogs

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    Jan 7, 2014
    Thanks a lot Tony!
    Relearned a few things today (if I ever learned them from school, that is :) )... just one thing I can not grasp on the circuit - what does the 470 pF do ? In my simulation, removing it doesn't cause much difference.
    And one more - can I replace the first transistor with a diode of any sort and tweak the resistors ?
     
    Last edited: Aug 8, 2020
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,567
    2,356
    Nov 17, 2011
    @kellogs : Note that "open collector here refers to the assembly of opamp, 470 Ω base resistor and transistor. Using a true open collector opamp (or comparator) IC will not work as this could not deliver the base drive for the transistor.

    The BJT is used in a configuration called "diode connected transistor". This setup has a much steeper I-V characteristic than a diode. Depending on your stability requirements a diode may work, too.

    The capacitor is probably there to suppress noise. The circuit will possibly work without the 470 pF, but be a bit more noisy.
    But: as the capcitor is in th
    e feedback loop of the comparator it will influence the frequency dependent loop gain and therefore may be required to suppress oscillations. A thorough analysis would be in order taking into account the characteristics of the comparator to get a definitive answer. Or build the circuit on a breadboard and change the capacitor's value to see how the circuit behaves.
     
    kellogs likes this.
  5. Sunnysky

    Sunnysky

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    Jul 15, 2016
    My faux pas for calling it an OC comparator, rather it is any Op Amp with Q2 as a common emitter current sensed (voltage controlled) current sink. Rb/Re=30~50 for ~any Q2. Any silicon diode will work instead of Q1 , C to reduce startup current of diode voltage source. Otherwise no change in steady state.

    Q1 or diode has a NTC thermal coefficient. -4mV/‘C (?)
     
    Last edited: Sep 7, 2020
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