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open-collector configuration

Discussion in 'Embedded' started by roopashree, Jun 8, 2006.

  1. roopashree

    roopashree Guest

    hi
    im using an optocoupler which has an open-collector
    transistor at the output. how should i calculate the output at the
    collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
    method. The emitter is connected directly to the ground

    What is an open-collector configuration and where and
    when exactly has it to be used?

    Thanks
    Roopa
    roopashree, Jun 8, 2006
    #1
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  2. roopashree

    Ian Bell Guest

    roopashree wrote:

    > hi
    > im using an optocoupler which has an open-collector
    > transistor at the output. how should i calculate the output at the
    > collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
    > method. The emitter is connected directly to the ground
    >


    You could calculate it that way but since you don't know Ic it is a but
    hard. The optopcoupler is essentially a switch so its transistor is either
    hard on (saturated) or off. The choice of resistor value depends on the
    circuit the opto coupler is driving. It needs to be small enough to source
    sufficient current when the transistor is off and high enough to ensure
    the is not excessive dissipation in the opto coupler when its transistor is
    hard on.

    Ian
    Ian Bell, Jun 8, 2006
    #2
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  3. roopashree

    Paul Burke Guest

    roopashree wrote:
    > hi
    > im using an optocoupler which has an open-collector
    > transistor at the output. how should i calculate the output at the
    > collector.


    You'll find a parameter in the data sheet called CTR (current transfer
    ratio). That basically tells you how the output current relates to the
    drive. So calculate your diode current, multiply by the CTR to get the
    collector current, and set the resistor such that the output (the
    collector voltage) is low enough for your logic (+noise margin) when the
    diode is driven.

    > What is an open-collector configuration and where and
    > when exactly has it to be used?


    You weren't listening at school! It's just the bare collector of a
    transistor, with the emitter grounded. You use it when you have to (as
    in your optoisolator, or say driving an interrupt line), and it gives
    the advantage that the outputs of several devices connected together,
    with one shared pullup resistor, are ANDed (positive logic) or ORed
    (negative logic- this is often used for shared interrupts, arbitration
    schemes etc). Usually called "wired-OR".

    Paul Burke
    Paul Burke, Jun 8, 2006
    #3
  4. On 8 Jun 2006 00:05:11 -0700, the renowned "roopashree"
    <> wrote:

    >hi
    > im using an optocoupler which has an open-collector
    >transistor at the output. how should i calculate the output at the
    >collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
    >method. The emitter is connected directly to the ground
    >
    > What is an open-collector configuration and where and
    >when exactly has it to be used?
    >
    >Thanks
    >Roopa


    In the linear region, the static current is Ic = If * CTR, so the
    output voltage is Vdd - (If * CTR * Rc), but that falls apart when the
    Vce gets low. Also CTR is variable from unit-to-unit, with
    temperature, and it decreases with time.

    Most applications also have to consider speed- the higher the load
    resistor, the slower the optocoupler. Most are spec'd with an
    unrealistically low collector resistance (eg. 100R) and will be far
    slower in a practical circuit.


    Best regards,
    Spehro Pefhany
    --
    "it's the network..." "The Journey is the reward"
    Info for manufacturers: http://www.trexon.com
    Embedded software/hardware/analog Info for designers: http://www.speff.com
    Spehro Pefhany, Jun 8, 2006
    #4
  5. roopashree

    CBFalconer Guest

    Ian Bell wrote:
    > roopashree wrote:
    >
    >> im using an optocoupler which has an open-collector
    >> transistor at the output. how should i calculate the output at
    >> the collector. is it Vcc-IcRc=Vo or has it to be calculated by
    >> some other method. The emitter is connected directly to the ground

    >
    > You could calculate it that way but since you don't know Ic it is
    > a but hard. The optopcoupler is essentially a switch so its
    > transistor is either hard on (saturated) or off. The choice of
    > resistor value depends on the circuit the opto coupler is driving.
    > It needs to be small enough to source sufficient current when the
    > transistor is off and high enough to ensure the is not excessive
    > dissipation in the opto coupler when its transistor is hard on.


    No, an opto-coupler is just another transistor that gets its base
    drive from the incident light. It does not have to be saturated,
    that depends on the actual light and the collector circuit.
    (Saturation is when the collector voltage falls below the base
    voltage) So you have to be careful with the collector circuit to
    ensure that collector dissipation ratings can never be exceeded.
    Collector dissipation will be maximum when one half the collector
    supply is dissipated in the collector resistor (and the other half
    in the collector junction).

    --
    "Our enemies are innovative and resourceful, and so are we.
    They never stop thinking about new ways to harm our country
    and our people, and neither do we." -- G. W. Bush.
    "The people can always be brought to the bidding of the
    leaders. All you have to do is tell them they are being
    attacked and denounce the pacifists for lack of patriotism
    and exposing the country to danger. It works the same way
    in any country." --Hermann Goering.
    CBFalconer, Jun 8, 2006
    #5
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