Open Collector Output with the load on the Emitter

Discussion in 'General Electronics Chat' started by iang, Mar 4, 2008.

  1. iang

    Thread Starter New Member

    Jan 15, 2008
    3
    0
    I have attached the circuit that I need to use to switch the 28 Vdc to the load. Normally I would use the open collector output with a pull up resistor and the load on the collector. The issue is that we have to use DIP resistors and there is no product available that can dissipate enough power at the resistance we need to meet voltage threshold requirements.

    The design I was thinking of using is attached below. The circuit would actually have to be programmed different but that is no problem (active low). Does anyone see any issues with this design (ocwithload.jpg)? It is simply to switch 28VDC and this will happen very rarely if ever.
     
  2. Davidpostlethwaite

    Member

    Mar 2, 2008
    12
    0
    I don't understand what your asking? Why can't you just power your load using the open collector way? is your load a 6k Resistor?, that's only 4.7mA. Can you not power your load directly from your opto-coupler? why the need for transistor amplification circuitry?
    If you could explain in more detail your limitations with resistor DIP etc.. i may be able to help?
     
  3. iang

    Thread Starter New Member

    Jan 15, 2008
    3
    0
    The load is 6K. So there would be a 28V external voltage and a pull up resistor. We would need to have at least 20 V threshold voltage as a high. Solving for this we would need 2400 ohm resistor of less (voltage divider with 2400 and 6000 being in series). The problem with this then is the pullup resistor would need to dissipate 326 mW when the transistor is active (28^2/2400). The maximum DIP pull up resistors are rate for are 250 mW.

    The circuit we are using is from an input/output card so we have to use whats given. I guess the other option you are saying is to use it as a current sink and have the 28V attached to one end of the load and the other endattached to the open collector. This way then no pull up resistor is used.
     
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