How do pull up resistors work

Just like the pull-up resistor in the first figure, the pull-down resistors in this circuit also ensures that the voltage between VCC and a microcontroller pin is actively controlled when the switch is open.

Unlike the pull-up resistor, the pull-down resistor pulls the pin to a low value instead of high value. The pull-down resistor which is connected to the ground or 0V sets the digital logic level pin to default or 0 until the switch is pressed and the logic level pin becomes high.

Therefore, the small amount of current flows from the 5-V source to the ground using the closed switch and pull-down resistor preventing the logic level pin from getting shorted with the 5-V source. When the button is pressed, the input pin is pulled low. The value of resistor near the supply controls how much current you want to flow from VCC through the button, and then to ground. High current will flow through the pull-up resistor if the resistance value is too low. It may cause unnecessary usage of power even when the switch is closed since the device will heat up.

This condition is called a strong pull-up and should always be avoided when low power consumption is a requirement. When the button is not pressed, the input pin is pulled high. The value of the pull-up resistor controls the voltage on the input pin. When the switch is open and a high pull-up resistance value is combined with a large leakage current from the input pin, the input voltage can become insufficient.

This is called having a weak pull-up. Based on the two conditions above, for pull-up resistors , you need to use a resistor that is at least 10 times smaller than the value of the input pin impedance. For pull-down resistors , it should always have a larger resistance than the impedance of the logic circuit.

Or else, it will pull the voltage down by too much and the input voltage at the pin would remain at a constant logical low value regardless of whether the switch is on or off.

Correctly, if the switch is open, is the voltage between the pin and vcc actively regulated, not Ground and Vcc? The pull-down resistor R1 which is connected with the ground or 0V. Thus making the digital logic level pin P0. In such case, the small amount of current flows from the 5V source to the ground using the closed switch and Pull-down resistor, hence preventing the logic level pin to getting shorted with the 5V source. So, for various logic level circuits, we can use Pull-up and Pull-down resistors.

It is most common in various embedded hardware, one wire protocol system, peripheral connections in a microchip, Raspberry Pi , Arduino and various embedded sectors as well as for the CMOS and TTL inputs. Now, as we know how to use the Pull-up and Pull-down resistor, the question is what will be the value of those resistors?

Although, in many digital logic level circuits we can see pull-up or pull-down resistors ranging from 2k to 4. But what will be the actual value?

To understand this, we need to know what is the logic voltage? How much voltage is referred as Logic low and How much is referred as Logic High? For various logic levels, various microcontrollers use a different range for the logic high and logic low.

If we consider a Transistor-Transistor Logic TTL Level input, below graph will show the minimum logic voltage for the Logic high determination and maximum logic voltage for detecting the logic as 0 or Low. As we can see, that for the TTL logic, the maximum voltage for logic 0 is 0. So, if we provide less than 0. On the other hand, if we provide more than 2V to the maximum 5. But at the 0. To determine the value, the formula is simple Ohms law. As per the ohms law, the formula is.

In case of the Pull-up resistor , the V will be the source voltage — minimum voltage accepted as High. Where V supply is the supply voltage, V H min is minimum accepted voltage as High, and I sink is the maximum current sinked by the digital pin. The same thing is applicable to the Pull-down resistor. But the formula has a slight change.

Another thing to point out is that the larger the resistance for the pull-up, the slower the pin is to respond to voltage changes.

This is because the system that feeds the input pin is essentially a capacitor coupled with the pull-up resistor, thus forming a RC filter, and RC filters take some time to charge and discharge. If you have a really fast changing signal like USB , a high value pull-up resistor can limit the speed at which the pin can reliably change state.

This is why you will often see 1k to 4. What resistor value should you use? It is easy to show how to calculate the pull-up resistor using Ohm's Law :. Remember to convert all of your units into volts, amps and Ohms before calculating e.

Now you should be familiar with what a pull-up resistor is and how it works. To learn more about electronic components and their applications, check out these other tutorials:.

Need Help? Mountain Time: Shopping Cart 0 items. Product Menu. Today's Deals Forum Desktop Site. All Categories. Development Single Board Comp. Contributors: Member Pull-ups are often used with buttons and switches.