Optocoupler question

Discussion in 'General Electronics Chat' started by bobbie, Jul 30, 2013.

  1. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
    0
    Hi guys,
    I am trying to drive a 2n3055 with a CNY75B optocoupler. The output from the opto on the spec sheet suggests 50ma current. The minimum current to drive the 2n3055 seems to be 100ma. The problem I am seeing is a "flaky" output from the 2n3055 - erratic. Will I need to add a coupling stage to control the 2n3055, or could I upgrade to a high current output opto?

    Pretty basic I know, just trying to work it out.

    Thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    The optocoupler is a current sink, not a current source. The source current is determined by the value of the pullup resistor you choose.

    Show your circuit diagram.
     
  3. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
    0
    Hi there Mr Chips,
    Thank you for your reply. I suppose I should have not assumed my description was adequate. Here is a sketch of my learning project. A simple pulsed DC Motor, with a later option of 6v or 12 volt operation. What is happening is that I am not getting sufficient current through the batteries to drive the motor. I habe taken this to mean that I am not turning the 2n3055 on properly. The base emitter resistor is 2kohms and the second battery is also a 6 volt battery giving a rail voltage when operating of 12 volts
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    You have a number of problems with that circuit.
    Why are the two 6V batteries separated?
    The 2N3055 is a poor choice because the current gain is very low.
    Use something like a TIP31 or a MOSFET such as IRLZ44.

    Here is a circuit diagram taken off the internet:

    [​IMG]
     
    Last edited: Jul 30, 2013
  5. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
    0
    Hi there Mr Chips,
    Thank you for the circuit. I had a specific intent with my circuit to discover what happens by joining two batteries together. First was to try and understand the curent movement between the batteries and also learning how to switch from a single battery to a series battery. The dc motor I am using as a constant current source. Later, I shall try a few variations. At the moment, I was hoping to understand what I am doing wrong as theoretically it should work. Perhaps, your point about the poor gain I should investigate next.
    Once again, thank you for your reply

    Edit: I have just looked at the TIP31 that you recommended and I cannot see how this would help me. The hfe is comparable to the 2n3055 and the 2n3055 will conduct more current. Perhaps, my lack of understanding has missed the point of your choice. Thank you
     
    Last edited: Jul 31, 2013
  6. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    The way you have drawn the circuit the transistor will not turn on.
    Wire the two 6V batteries in series to provide 12V power.
    Interchange the motor and the 6V battery in your drawing.
    Connect the base resistor as shown in my diagram.

    (The current limiting resistor on the photo emitter diode can go on either side of the diode.)
     
  7. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
    0
    Hi there Mr Chips,
    No wonder it will not switch! Takes an expert to spot the errors.

    Many thanks for your assistance
     
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