4017 switching transistor and AND gate

Discussion in 'General Electronics Chat' started by magnet18, Jun 12, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Hey, is there any reason that a 4017 wouldn't be able to both switch a transistor for my nixie clock and also run to two logic gate inputs?
    This wouldn't draw too much current, would it?

    I'm asking because these are the things that usually cause problems for me :p
     
  2. #12

    Expert

    Nov 30, 2010
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    C'mon magnet...you know how to get a datasheet, and you know we need one to answer the question. I expect the nixie driver is low current, unlike a 3055 transistor which can use over an amp for its base to emitter drive, but I don't know the limits for a 4017 chip. Help me out a little.
     
  3. #12

    Expert

    Nov 30, 2010
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    Here's the datasheet. Now you can look up your supply voltage and look at the graph to see how much current is available and the voltage drop in the chip.
     
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  4. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Sorry, I was planning on this 4017 - http://www.st.com/stonline/books/pdf/docs/2029.pdf, 10mA output drive current, and at worse case it will be driving an MPS-A42 (http://www.fairchildsemi.com/ds/MP/MPSA42.pdf), through a 33k Ohm resistor, collector emitter current of <2.5mA, as well as the input of a 4011 (http://www.nxp.com/documents/data_sheet/HEF4011B.pdf) and an input of a 4068 (http://ics.nxp.com/products/hef/datasheet/hef4068b.pdf) (through a rotary switch)

    Can it be done?
    I can't seem to find how much current the inputs for the logic gates need and I have no idea how much current is being put into the transistor...
     
  5. #12

    Expert

    Nov 30, 2010
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    I'm getting the idea that a cmos gate input only has leakage current, and that is in the 1na range.
    I think my datasheet is more helpful because it has graphs of voltage sag versus loading current. Still, current available is dependent on your Vcc.

    So, what Vcc through a 33k resistor? and look at the graph on the datasheet I provided.
     
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  6. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Ahh, that makes sense, yay :)

    Yea, I was writing my post when you posted that, and I posted mine because the chip has slightly different characteristics. (it's got more max current)

    FWIW, i have 12Vcc most of the time, but it might switch over to 9V.

    Sorry, but which graph?
     
  7. #12

    Expert

    Nov 30, 2010
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    The datasheet in post#3. page 3, right side. Voltage sag v. output current for various Vcc's.
     
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  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The transistor base current is not hard to estimate. Initially assume that the 4017 output gets to the full rail and that the base is at ~0.7V. That would make the base current (12V-0.7V)/33KΩ = 0.34mA.

    If you have figures for how much the 4017 output would sag for that current, you could use that to get a better value for the current into the transistor, but frankly it should make little difference unless 0.34mA is too much anyway.
     
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  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Well then, it looks like this should work no problem, thanks guys!
     
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