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Current Mirrors, help?

  1. Nov 12, 2013 #1
    Hi, Im working on a design and I am trying to re-work the horrible schematics left by a previous mechanical engineer, I am trying to make a current mirror that will mirror a reference current across 5 LEDs, AFAIK from my little knowledge in Electrical engineering (I am a Mechanical Engineer) this can be done using Transistors in the following way:

    http://img198.imageshack.us/img198/4972/waeg.PNG [Broken]

    I am using Multisim to do this circuit and the probes do not show the current being "mirrored" into the other lines, AFAIK they should all be 20 mA, but they are dropping down to 5 pA which doesnt turn on the LED.

    Can anyone help me please?

    Thank you!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 12, 2013 #2


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    Staff: Mentor

    LED1-5 are drawn upside-down -- their cathodes should be pointing down instead of up. :smile:
  4. Nov 12, 2013 #3


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    Staff: Mentor

    BTW, the mirrors will be more accurate if you add emitter resistors below the transistors, but for just driving LEDs, you probably don't need that level of matching accuracy.
  5. Nov 12, 2013 #4
    OMG i cant believe it was only that =,=
    silly diodes!

    Anyways thank you very much!
  6. Nov 12, 2013 #5


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    Science Advisor

    berkman has explained that little current flows because the LEDs are reverse biased.

    As drawn the 20mA reference current is being wasted because it does not flow through a LED.

    This circuit seems a little complex to light 5 identical LEDs. The heat generated in the collectors of the transistors might as well be dissipated in resistors. Unless you need to vary the current through all 5 LEDs together it would simpler to use 5 LEDs with 5 series resistors.
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