Sanity check please: current limiter using LED

Discussion in 'The Projects Forum' started by Robartes, Oct 26, 2014.

  1. Robartes

    Thread Starter Member

    Oct 1, 2014
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    Hi,

    as I'm just starting out in electronics and I had an old CPU power supply available, I thought to create everyone's favourite first project: an adjustable lab supply. As part of the lab supply, I want to build in a current limiter circuit. I came up with the below:

    Current_limiter.jpg

    I've tested this on a breadboard and it seems to work - with the value of R1 as stated (2.2 ohm, actually 3*6.8 ohm 1/4W in parallel) the current is limited to around 550mA when I short circuit the load. Of course, the transistor and the resistors get hot (although the power through them is still within spec for both) when they need to pass this current.

    I know the classic version of this circuit uses two diodes instead of the single LED, but when I use the LED I get a nice indicator of the circuit pulling too much current.

    Now to my question - does anyone see any glaring mistakes or caveats in this circuit? Thanks!
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    How about the Vcc, Load, Led type V/I?
     
  3. Robartes

    Thread Starter Member

    Oct 1, 2014
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    Vcc will be +12V maximum, the load can be anything (basically what's connected to the lab supply, so abstract it as say a 10K resistor). For the LED I have no detailed V/I curves -- it's a standard (for what that's worth) red LED which in my calculations I assumed drops 1.8V.

    Edited to add: to measure is to know, as we say (trust me, it sounds snappier than that in my native language :) ) I measured the voltage drop across the LED when the transistor on, ie when the load is a short circuit - the LED drops exactly 2V in that condition, with Vcc being +12V.
     
    Last edited: Oct 26, 2014
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No obvious mistake, and you're clearly aware of the power handling constraints. LEDs are commonly used as crude voltage references.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    There was something wrong with current what I calculated in the attached circuit .

    Current_limiter_Robartes_ScottWang.gif
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    When Q1 collector/emitter current is ~500mA the Vbe won't be as low as 0.7V. It will probably be nearer 0.9V (depending on the transistor).
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Yes, Vbe even could be 1.1V, but the Ib still has a long distance error.
     
  8. #12

    Expert

    Nov 30, 2010
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    It's a perfectly valid concept that will work as long as you don't go around exceeding maximum voltages or power limits.
    Devil in the details and all that...
     
  9. crutschow

    Expert

    Mar 14, 2008
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    How did you get such a high base current? The typical beta of that transistor, which is operating in the linear mode, should be in the neighborhood of 100, giving a few mA of base current.
     
    Johann likes this.
  10. ScottWang

    Moderator

    Aug 23, 2012
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    Only the Ib=Ie-Ic.
     
  11. Johann

    Senior Member

    Nov 27, 2006
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    And Ir2=Ib+Iz
     
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