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Driving a transistor array

Discussion in 'Electronic Basics' started by Chris W, Mar 4, 2005.

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  1. Chris W

    Chris W Guest


  2. If you are demanding less than 500 mA sunk into each
    Darlington collector, plan to drive their bases with an
    HCT device, and don't expect saturation on the output
    (or what might be called saturation for a Darlington), you
    should be OK. Do you plan to use the logic output for
    anything else, at the same time?
     
  3. Chris W

    Chris W Guest

    I plan on driving either 1 to 4 LEDs in parallel, or a 12v around 2 or
    so amp relay, on each Darlington collector. The flip flops will be
    driving nothing else, just the Darlington array. I need 16 Darlingtons
    so with 7 in a chip that will leave 5 unused Darlingtons on the one of
    the 3 arrays or 1 on one chip and 2 on the others. I am still new to
    all this. What exactly does it mean to have saturation on the output?

    --
    Chris W

    Gift Giving Made Easy
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    give the gifts they want
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  4. Upon closer inspection of the datasheet, I would revise
    than collector current down to 350 mA.
    You had better be careful about burning up your
    array(s) if you put more than about 150 mA into each
    of the 7 devices in a package. The 2 Amps (or so) you
    mention cannot be switched by just one of those devices.

    Maybe you should be looking a power MOSFET rather
    than a Darlington transistor array.
    It means something a bit too involved to explain in all
    its glory. For your purposes, the issue is: What will
    be the voltage drop between the collector-emitter
    terminals when the device is supposed to be switched
    on? I urge you to consult the datasheet about that.
     
  5. James Beck

    James Beck Guest

    I think he means that the relay is reated for a 2A load NOT that it
    requires 2A to operate. Also, what in the datasheet leads you to
    believe that the HCT part can't drive the darlingtons to saturation?
     
  6. I imagine you are right. But I've seen and
    wired some pretty big relays.
    Well, saturation is kind of a funny thing for the
    Darlington transistor pair. The input transistor
    can be readily saturated but the output transistor
    is generally not saturated, having its base drive
    modified such that its base and collector voltages
    are about equal. So, using the term "saturation"
    loosely to mean "as on as can be", there is no
    reason the HCT device cannot cause that to
    occur in the Darlington. But for the more precise
    definition, requiring an excess of carriers in the
    base region and a forward biased collector-base
    junction, the output transistor certainly cannot be
    brought to that state.

    The reason I thought this might matter to the OP
    is that Darlingtons often present a dissipation issue
    in switching application due to their drop while in
    the on state and that drop can be significant with
    respect to the load voltage as well.
     
  7. Chris W

    Chris W Guest

    To clarify, yes I did mean a relay that is rated for a 2A load.

    I've been looking at that data sheet and I don't really understand a lot
    of what it is saying. On page 4 it has the chart with all the values
    and refers you to diagrams on page 5. Most of the diagrams say "Open"
    by the output of the clamping diode. Doesn't that mean the Darlington
    is off?

    One line says Vce and has 1V typical for a 200ma load. Does that mean
    that 1 volt will be dropped by the Darlington? If so I will need to
    make a significant change to my voltage to drive my high intensity
    LED's. For the 12V relay, that will probably have something more like
    13.8 volts, it probably won't make any difference though.

    I don't understand what the Vi(on) numbers are trying to tell me. For
    the SN75469 in the MAX column they vary from 5V to 8V depending on the
    collector current. What exactly does that mean? Is that the maximum
    voltage the input line can handle? What about the minimum voltage
    needed to turn the Darlington on?

    Then the Vf for the clamp diode is 1.7V .... does that mean there will
    be 1.7V drop from my supply?

    Ii(off) seems obvious and Ii seems to be well with in what my flip flop
    can provide. I assume Ir is the amount of revers current that will flow
    for inductive kick back from a relay?

    Finally what about Icex? Is that the minimum amount of current the
    collector needs to stay on?

    Thanks for everyones help. I really need to get a good book. The
    problem is most books about things I am interested in are either way too
    slow paced, or over my head. I learn this kind of thing very quickly
    and often find beginner type books very slow paced.


    --
    Chris W

    Gift Giving Made Easy
    Get the gifts you want &
    give the gifts they want
    http://thewishzone.com
     
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