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What Are Pull-Up and Pull Down Resistors?

8 March 2009 8 Comments
What Are Pull-Up and Pull Down Resistors?

The terms “pull up” and “pull down” resistors confused the heck out of me but I get it now.

A resistor limits current. In the water analogy, current is the speed of the flow of water. But I never knew what a pull up/pull down resistor was. Until a few weeks ago when I started building a set of buttons for a project I’m working on…

pull up resistorEssentially a button is a switch, when the switch is closed the current flows, when it’s open the current doesn’t flow. Now if your switch is open, without anything driving the circuit, the value on the line is going to float all over the place, sometimes high, sometimes low. Any electrical noise will cause a change in the circuit. This causes a problem as when reading a value off the circuit you might think the switch is closed when in fact it’s just floating high.

The circuit on the left uses a pull up resistor. If Vcc is +5Volts, that goes through the 10k resistor and has nowhere to go but to pin 1. So when the switch is open, pin 1 is showing +5Volts (but very low current due to the 10k). Pin 1 will be steady, it won’t float up or down since it’s got some power to it.

When you close the switch, the current flows to ground. It’s always going to go to ground if it can so Pin 1 will show 0Volts.

pull down resistorThe pic on the right is the pull down resistor. Same thing but the 10k resistor is between the pin and ground, causing it to “pull down” whilst the switch is open. Once it’s closed the majority of the current flows through the pin vs going to ground hence the pin reads +5V.

It’s making sense to me and now that I’ve actually built some stuff from scratch, I’ve actually experienced this voltage float and fixed it with a pull down. This blog will help me remember it in a year.

P.S. The Image is from “Resistance 2″ on the PS3. Pardon the pun.

8 Comments »

  • Jen said:

    I have no idea what they are, but that is one scary monster!

  • Francis (author) said:

    Don’t worry, it’s photoshopped

  • Justin said:

    I’m not quite sure that your explanation of pull down resistor is correct. The pull down resistor has two functions in your diagram.

    1)-sink the input impedance of the powered chip
    —-many chips with input pins will have non-zero voltages when the chip is powered on. If your input voltage is below the chip’s input impedance (which will vary with voltage VCC/VDD to the chip), you input isn’t going to signal the pin. If the input pin is grounded, it will read zero volts in the absence of input current. Any small voltage will switch the pin state from low to high.

    2)-sink the current from your input.
    —-your input current has one goal in life – go to ground.
    —-if it can’t get it ground, something will get hot, fast, then go sizzle, sizzle and finally pop (maybe with a little smoke, followed by a moment of regret).
    —-A pull down resistor provides that path to ground while retaining the signal.

    Remember, it is voltage, not current that your IC uses for a signal. Excess current >25mA is what makes your ic hot.

    As for the pull up resistor explanation…the resistance of a loop in a circuit does not ‘hold’ current, it the amount of current flowing. The current at any point in a single loop of a series circuit is the same. What the pull up resistor does do is stabilize the voltage at the pin, which is acting as an effective ground for VCC. Pin 1 should be at VDD if nothing is connected to it. With the Pull-Up connected, fluctuations in the Pin 1 voltage would be damped by the ration of the input impedance and the value of R1. The IC’s input impedance is usually on the order of 230 ohms.

  • Francis (author) said:

    Justin, I’m not sure I have it right either ;) I’m a computer major with some electronics from college but mostly self-taught. Your input definitely helps better my understanding so thanks for that.

  • Tommy said:

    Thank you! Great explanation.

  • Bjorne said:

    Thank’s for the info…simple and clear for a “newbe”(hm….i think):)

  • alan said:

    thankyou well explained

  • Robot Basics: Using an H Bridge to Move Your Bot Backwards - Science & Tech | Tech wikiHow said:

    […] hang unconnected. This will be our direction control button. You can use a real button and add a pul-up or pul-down resistor if you like, but the wire will suffice for this […]

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