Last Updated on October 22, 2023 by Kevin Chen
Image source: circuit digest
Pull up and pull down resistor are two types of resistors designed to create a stable voltage on the signal line.
These resistors rectify or correct the input voltages of the digital signals and stop them from staggering randomly in cases where there is a no input bias.
We all know that in the field of electronics, the main function of a resistor is to resist the flow of electric current. Other than that, it can be used as a voltage divider whereby it regulates the supply.
The latter is the main reason why the resistors can be used as step up and step down resistors.
Are you building an application that will require signal regulation? You should consider buying pull up and pull down resistors. In this guide, you are going to learn everything about it.
Overview
Pull up and pull down resistors have been in use since 1950s. They were mainly used for preventing the electric circuits from floating.
But, what does floating mean?
In the field of electricity, floating occurs when there is zero connection between electronic components so the current tends to flow freely.
This occurrence can be a danger to the circuit. Apart from causing inaccurate readings, it can end up damaging some sensitive components in the circuit.
The pull up and pull down resistors tend to provide some form of connection between the electric components, hence preventing the current from floating.
By dividing the voltage and regulating the flow of current, you will be able to achieve stable voltage in a signal line.
The pull up and pull down resistors are mainly used on applications that have digital logic gates.
These gates are connected to the external circuits where the input and output functions tend to vary To bring harmonization, these two resistors become a necessity.
Any modern digital logic circuit should have logic gates which has the AND, NOR and NOT gates.
However, since they are digital, they only operate in two states, 0 and 1
To give you a clear understanding, we are going to look at the pull up and pull down resistors separately.
Pull up resistor
A pull up resistor in this context is a resistor that ensures that a particular digital signal in a circuit remains high unless it is driven low by another resistor.
A pull up resistor will provide a well-defined path to ground for the logic device.
Working principle
How doe a pull up resistor keep the signal high?
As we have defined, as long as there is nothing else driving the signal, the resistor will always keep the signal high.
The high voltage can be +3.3V or 5V. All this will depend on the input of the incoming signal and the voltage specification of the pull up resistor. The value of the resistor is always carefully chosen such that when the circuit is off, the output voltage will be the same as the voltage source.
When the electric circuit is in the active state, the output pin will always be driven to a high or low voltage level. Since this is a pull up resistor, the voltage will be driven to a high level.
Usually, the value of a pull up resistor is high enough to prevent any incidences of power dissipation. Also, this resistor value should allow the voltage to be within a limited range. This ensure that the voltage does not exceed the limit and damage the output pin from the resistor.
Key functions of a pull up resistor
Here are the main functions of a pull up resistor in an electric circuit:
-It increases the output high voltage in a general gate circuits
-The resistor increases the output o the TTL circuit, which is also used for driving the CMOS circuit. This is always necessary especially when the output of the TTL is lower than that of the CMOS circuit.
-Pull up resistors enhance the drive capacity of the ouput pins in microcontrollers.
-Used in the CMOS chips to minimize the input impedance hence protecting the chip from the static electricity which can cause some physical damage.
-Improve the noise tolerance of the input signal hence hence minimizing incidences of external interference.
Applications of pull up resistors
The most common application of the pull up resistors include:
-Switch debouncing whereby they prevent false signals on the mechanical switches
-Serial communication whereby the resistors enhance the communication protocols such as SPI, and I2C
-Open drain interfaces whereby the pull up resistors interface the open drain outputs with other circuits
-Logic level conversions whereby these resistors are used for converting signals between different logic levels
What are the disadvantages of the pull up resistors?
The main drawback of using pull up resistor is the high energy consumption. As we have seen from the working principle, a pull up will result to an increase in the voltage level. This will in turn consume more energy.
Pull down resistors
A pull down resistor is the opposite of the pull up resistor. Like the latter, the pull don ensures that the circuit does not remain floating or in an undefined state. It ensures that the circuit remains at a defined low voltage state.
These resistors are also used to ensure that the circuits have known starting states.
How does a pull down resistor work?
The working principle of a pull down resistor is pretty simple. The resistor with a known resistance is connected between the negative voltage(ground) and the signal line.
When the signal line is not in use, the pull-down resistor will ensure that the signal is to the lowest level. It can be as low as 0V. It does so to prevent the signal line from floating, a factor that can cause significant level of interference during the signal transmission.
However, when the signal line is active , the signal in the circuit will be high. At this state, the pull down resistor will do nothing on the signal since the signal is on a way high voltage than what the pull down resistor can keep up with.
We are simply implying that a pull resistor is active only when the signal line is inactive.
Like for the pull up resistor, the resistance of the pull down is chosen based on the amount of current that is needed to pull the signal line down.
Key functions of the pull down resistor
When it comes to pulling down the signal line, a pull down resistor does the following:
-Preventing time lag: A pull down resistor can cut down the duration that is experienced when the transistor is turned ON and OFF. This time lag is usually due to the residual charge depending on the state of the transistor. In such cases, the resistor is connected between the base and the emitter.
-Minimizing noise interference: This is the primary function of the pull down resistor. By pulling the signal line low, it ensures that there is no floating current and voltage. Here the the resistor is connected between the input and the output signal.
Helps in setting the Bias voltage: In most applications, a pull down resistor is added to a circuit so that it can set the bias voltage. This is a common feature in most AC sources where the bias voltage is likely to be affected by various external factors such as temperature.
What are the disadvantages of pull-down resistors?
The main drawback of the pull-down resistor is the tendency to draw a significant amount of current even when there is no active input signal. This can lead to some level of energy wastage.
You should be careful when choosing the resistor value to prevent such unwanted current drawn.
Pull up vs pull down comparison
Let’s compare the key features between these two resistors:
-Driving power: It is clear that a pull-up resistor has a higher driving power than the pull-down resistor. This also means that its power consumption.
Connection: There is also a difference in their connection points. Pull-up resistor is connected between the signal line and the voltage source. On the other hand, pull-downs are connected between the signal line and the ground.
Resistance value: Pull-up resistors have a way higher resistance value than the pull down resistors. This is because the current flow through the pul-up is always higher than in the pull-down.
Conclusion
Now you know the difference between the pull-up resistors and pull-down resistors. What remains is buying the right resistor and implementing it into your circuit.
Before you buy, ensure that you know its value. You don’t want to buy a resistor that does not meet the needs and requirements of the circuit.
Go through the data sheet to know all the technical specification of the resistor.
Other than the technical ensure that the resistor is of high quality. And the surest way of is by buying from reliable electronic components suppliers in China.
We at ICRFQ are dedicated to delivering pull up and pull-down resistors for sale.
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