Pull-up resistor value

Discussion in 'General Electronics Chat' started by jaygatsby, Dec 26, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    I have 8 CMOS chips running from 5VDC. Normally I would use a 4.7k ohm resistor for a pull-up. However in this configuration these 8 chips all have their not LT pins tied together. I am going to run these to a pull-up connected to the positive rail, and to a button connected to the negative rail. Since there are 8 chips, I'm not sure what to do about the pull-up resistor. For pins like LT, I don't think the datasheet states current for operation. So where do I begin?

    Thank you
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It may help to state exactly what devices these are, and what the LT function does.

    Most CMOS inputs take very little continuous current, but putting many in parallel will add capacitance, slowing the rise time. Does switching speed matter?
     
  3. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    ok - they are 7 segment latch/decoder/driver chips and the LT is lamp test. Switching speed doesn't matter to me, I just want it to stay high until the lamp test button is pushed, then I want them to go low.

    Thank you
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Unless they have internal pull up or pull down resistor, each input is likely to be a standard CMOS load. The datasheet should mention any such resistors if they exist.
     
  5. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Thank you for that and I'll look to see if the data sheet mentions this. In the mean time, would you have time to explain what a standard CMOS load is? And would I multiply that load by the number of chips (8 in my case) when determining the resistor?

    Thanks
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    They draw only a microamp or so, normally less. Here is a data sheet for a CMOS 7-segment driver, not necessarily the one you have.

    http://www.onsemi.com/pub_link/Collateral/MC14511B-D.PDF

    Normally you do add up the load current and check that a pull resistor will take the input to a good potential. 4k7 ought to be OK.
     
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  7. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    So 4.7k, what I would use for one, should be fine for several at once?
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Does the switch or relay need a "wetting current"? If not, 1k would seem quite low enough. 4k7 could give you 40mV error maybe, but would it matter?
     
  9. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    I'm asking in the pursuit of such knowledge sir.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    The input currents of CMOS inputs are so low that it doesn't matter what you choose. You need to calculate how much current you wish the switch to sink when the switch is closed. For a 5V supply, 1K would supply 5mA, and 5K would be 1mA.

    In practice, anything from 1K to 100K will work fine. It does not matter how many inputs are tied to the one pullup because the input currents are so small. This is not true for TTL gates.
     
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  11. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Thank you MrChips
     
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