Pull up resistor rating

Discussion in 'General Electronics Chat' started by phase ghost, Mar 9, 2010.

  1. phase ghost

    Thread Starter New Member

    Mar 3, 2010
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    I understand what pull up resistors do, but I'm confused on which resistors to use. Mostly, I see 10k and 20k resistors being recommended. I'm confused on why these are used though. Why couldn't I use a 200Ω resistor instead? The voltage will be from 0-5v in the circuit.

    Also, what if I used a resistor well over 20k as a pull up resistor? Would that be problematic?

    thanks
     
  2. Von

    Active Member

    Oct 29, 2008
    65
    0
    A higher resistance results in little current flow thus voltage drop insuring the pull-up voltage is high enough and less current drain.

    200 ohm may work in many instances but there is usually little cost benefit to a lower reisitance.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It really depends upon the application in question.

    For example, if you are using a transistorized 555 timer and want the output (pin 3) to go above Vcc-1.3v, you will have to use a pull-up resistor. 330 Ohms would work OK for 12v, but around 36mA current would flow through the resistor when the 555's output was low.

    For the old 74xx series TTL, the use of 330, 470 and 510 Ohm resistors for pull-ups was common.

    For 4000 series CMOS, 10k Ohm pull-ups are quite common. If one used a low value resistor such as 330 Ohms when Vdd were 15v, a 4000 series CMOS output would have to sink over 4.5mA to get the output voltage down to 0v; and that's not likely to occur.

    With microcontrollers, frequently one of the uC's input pins needs to be held either high or low while operating, and momentarily shorted to ground or Vdd to effect a clear/reset, or even driven higher than Vdd for programming. If a low-value resistor is used, excessive power consumption will result, and may even damage a programming device.

    The use of resistor values that are too high may also cause problems; such as slow rise/fall times or excessive noise on analog signals.

    Usually, 0.1mA to 2 mA current is enough to hold a CMOS input pin high or low. If you go a lot higher or lower, you may have problems or start wasting power as heat.
     
  4. phase ghost

    Thread Starter New Member

    Mar 3, 2010
    14
    0
    Thanks for the detailed replies. I'm using the arduino micro controller (will be using in a few days) for a project and was stuck on what do for pullup resistors. Although, I think I'm going to be using them as pulldown resistors. Not totally sure just yet.
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The important factors are the leakage current, the speed of operation and the capacitive loading.

    For a CMOS input within a self-contained circuit operating at low frequencies, 10K or even 100K may be fine.

    If you go to something like a microprocessor I/O bus operating at some MHz in a rack of plug-in cards, it may have something like a 220 ohm plus 330 ohm at each end of the rack, so it's pulled to 3V with the lowest possible impedance for a TTL logic chip to drive.

    One other point to watch out for is a pullup on a switch - many switches have a minimum 'wetting current' and won't operate reliably without a low value load, eg. you may have to use a 470 Ohm resistor to get the switch working reliably even though the CMOS input it's connected to would be fine with 100K.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    1,525
    Doesn't the choice of a pull-up resistor also need to be balanced with the amount of current the out-put pin can sink? I mean you wouldn't want to use a pull-up that would allow 500mA on a pin that could only sink 50mA.
     
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