pull up pull down resistor

Discussion in 'General Electronics Chat' started by Nathan Hale, Dec 20, 2013.

  1. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
    Hello guys, hope all is well. Can someone explain to me how a pull up / pull down resistor works? Why does it do what it does?
    i do know that a pull up pull down resistor pulls up or pulls down the logic level when connected to an IC but i am looking for a more in depth answer. what is the AC/DC theory behind it?
    thank you.
  2. #12


    Nov 30, 2010
    Transistors, especially jfets and mosfets have so much input impedance that leakage currents can turn them on or off. I'm talking about pico amps over a period of seconds to minutes. If you do not lower the impedance of the input pins, leakage currents will make the gates wander all over the place.
  3. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    Can I get that in English please???
    #12 likes this.
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Pull-up and pull-down resistors are used to connect switches and transistors to logic pins and mosfet devices. They assure a high or low logic level - essentially assuring that an input pin is never disconnected from either high or low.
  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    That I can think of off hand, one of the most common uses for pull-up resistors is with open collector outputs on a logic chip, there can be various reasons for OC outputs such as wired-or connecting the outputs of several gates together, or if the output is needed to control something at a higher voltage than Vcc.

    Apart from discrete circuitry - pull-down resistors are more from the days of P-MOS & N-MOS chip.
  6. dougc314


    Dec 20, 2013
    Logic inputs are typically "undefined" that is they aren't connected to anything that is a logical level (1 or 0). This makes sense, as normally a logic device input gets connected to another logic device's output. Sometimes they don't though, usually when connecting to external sensors. A good example is a switch. A switch is either open or shorted. If the switch is connected between ground and a logic input, then when the switch is closed the input is connected to ground, or logic low (usually). When the switch is open however the logic input isn't connected to anything, and so its input logic value is undefined. A pullup resistor connected between the input and logic high (usually the pos voltage supply) solves this. The switch can still make the logic input go low when closed, and when open the pullup resistor makes the logic input go high.
    A pull down resistor (resistor connected between ground and the logic input) would do the same thing if the switch was connected between the input and the logic high level.

    The electrical reason why the logic input is undefined is that it is usually a very high impedance. (nowadys any how) That means that stray electrical fields for example can make the input wander around, and that make the logical value appear to fluctuate between high and low. Also even though the inputs are high impedance and if perfect require no current, in practice they may actually want to source or sink small amounts of current (called leakage current) These currents are small, but can make an input that is not connected appear to be high or low, depending on their value. The leakage values can be different from device to device (sign and magnitude) and over temperature, so their effect varies somewhat unpredictably.

    Some logic devices and some analog devices have "open outputs". Usually the collector or drain, sometimes the emitter or source. These open outputs need to be pulled up, or down, usually with a resistor. Many analog comparators have this feature as well as some logic gates. One use for such function is something called "wired and", and "wired or". Another use is converting from logic that operates on one voltage to another.
  7. ian field

    AAC Fanatic!

    Oct 27, 2012
    IIRC - TTL inputs assume logic 1 if left unconnected, I think CMOS can float pretty much anywhere.

    In a noisy environment (TTL) and for CMOS its usual to connect unused inputs to either Vss or Vdd - whichever doesn't interfere with the logic function controlled by the inputs that are used.

    There are sometimes exceptions with specialised chips where hard grounding or supply to/from an unused input would cause excess current to flow - this is usually made clear in the datasheet.

    Pull up or down resistors are usually used on a parallel data bus such as PC expansion slots that may or may not have a card inserted, the resistors bleed any static buildup when no cards are inserted.
  8. #12


    Nov 30, 2010
    That's why there are several of us here. Everybody thinks in a different way. We just shoot answers at the question until the person that needs an answer finds one he can understand.:p
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    And we have Loosewire for answers no one can understand.:):p:D
    gerty, tindel, absf and 1 other person like this.
  10. MrChips


    Oct 2, 2009
    Think of pull up/down resistors as load resistors.

    You can calculate the current through the resistor and hence the voltage across the resistor. Then ask yourself if the voltage at the active node of the resistor satisfies the condition required for that node.
    GopherT likes this.
  11. #12


    Nov 30, 2010
    Look up there. MrChips thinks in a way that I can not understand. Do I complain? No.
    Sometime he gets through to the customer, sometimes I do. We all make a team here.
  12. HenryTj

    New Member

    May 18, 2016
    Not likely technically precise, but might provide a mechanic means, or actually hydraulic, way to conceptualize. AT least it'n the one I use. Imagine you have a plumbing situation. A valve with a pressure sensor to turn it on or off. But the seals aren't perfect. Somehow a tiny bit of water leaks to the other side.of the valve. The water will slowly build up. So you put a drain that lets a tiny amount drain out, so only the proper surge of valve trigger flow will trip the sensor and turn the valve on. You don't want the water leaking in and building up to trip the sensor. That would be one way of thinking about a high ohm "pull down resistor." It puts a small drain on that part of the circuit so that voltage/current doesn't leak in and build up.

    A "pull up" resister is sort of the opposite where a small line lets a tiny trickle of water in to keep the maintain the pressure on the sensor, otherwise a leaking fitting will slowly let the pressurized water leak out and the sensor will trip when it's not supposed to.

    At least that's my way of conceptualizing it. There are interesting similarities between electronics and plumbing.
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
  14. Gibson486

    Active Member

    Jul 20, 2012
    The bigger you make the pull up or pull down, the weaker you make it. This becomes apparent when you you try to use a bjt or mosfet a switch. Put the resistor on the drain and the other end to ground. Measure Vout at the drain. make the resistor bigger. What do you notice?
  15. WBahn


    Mar 31, 2012
    The most basic answer, I think, is to consider the voltage drop across the resistor when the current is very small -- unless the resistance is REALLY big, the voltage drop will be very small, too. So connect such a resistor between the input of a device and the positive supply. The voltage at the input is the positive supply minus the small voltage dropped across the pullup resistor. Similarly, if the resistor is between the device and common, the voltage at the input is the common plus the small voltage dropped across the pulldown resistor.

    The next step up is to realize that, for most digital inputs, the input resistance is a very high value (for CMOS, it is an incredibly high value) to each of the supplies. When you put a pullup/pulldown resistor in the circuit you create a voltage divider in which the external resistor is very small relative to the internal resistors. Hence, like any such voltage divider, the voltage at the junction is very close the voltage that the other side of the external resistor is connected to.