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Open collector circuit

  1. Oct 5, 2008 #1
    I have his problem on open collector and I don't know how to deal with it:

    In the figure attached, the first transistor is driven by an output of the MCU, with the goal of turning the LEDs(several in parallel) on and off.
    Compute the values of R1,R2,R3 required to make the circuit work properly, if it can.

    Please can I have some suggestions meaning the step I need to do.
    My idea for the first resistor R1: If the output is low, The transistor inside the output of the MCU will sink current, letting T1 off I am right?

    However, I dont know how to solve the case where the output is 'high'. Please can someone help me? what the different currents do at the first node?

    Thank you

    Attached Files:

  2. jcsd
  3. Oct 5, 2008 #2


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    Science Advisor

    Open collector means that the output stage is an NPN BJT with an open collector. When the BJT is on, it (hopefully) saturates, and pulls the collector low to the V_ol voltage. However, when the BJT is off, the collector just floats. That's the reason you need an external pull-up resistor.

    The value of this resistor needs to be low enough to allow enough current to pass through to turn on your next stage (and put it in the right mode--linear or saturation), and needs to be high enough that your collector doesn't burn out from excessive current.
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