Skip to content

The Differences Between Pull up and Pull-down Resistors

The use of resistors today is a common topic. Often electric circuits have resistors to restrict current flow, and there is more than one type of resistor. The difference between these is what we refer to as pull up and pull -own resistors. These devices are often helpful in power supplies or other circuits where an electric current flows for a specific period. The increased use of chips in today’s engineering means that resistors are more commonly helpful in circuits than ever before. These circuits would not work without resistors.

Each electrical device we carry around, including our plasma TV, needs a resistor. However, they operate under different conditions. Therefore, the type or value affects how they will operate.

Evolution of resistors

The first resistor was in the 19th century, and we call it a ‘grasshopper.’ However, it consisted of grass, so it was not very accurate. In the 20th century, two main types came up: film resistors and wire-wound resistors. Both had a wide range of resistance values. They worked as fire rockets and many other electrical devices of the time. These are not common now. However, they are still manufacturing them for high-power applications that need low resistance.

Today we have many different types of resistors on the market. However, most store more than one value of resistance in a single piece of material through various processes. This also allows prices to vary, and some have an improved range of values.

The history of modern resistors can be split into two sections. The first was when engineers started to replace chemical batteries with electric currents. The second is from around the 1920s to the present day.

The first step in resisting resistors was to replace a chemical battery with an electric current. This allowed engineers to create electronic circuits which we could use in place of a battery. In addition, it allowed for the creation of different circuits. Some could store information and help to control other machines.

The second period for the evolution of resistors was from around the 1920s to the present day. This period saw further development in electronic engineering and resistors in circuits. During this time, resistors’ value, color, and tolerance became more accurate. This meant that their values were easily identifiable. It is useful for Rayming PCB & Assembly when quickly assembling circuits.

Types of resistors

There are many types of resistors today. Some help in power supplies to help regulate or stabilize the voltage. Others provide extra current flow when the user activates a specific circuit.

A pull-up resistor is a specific resistor that increases the current flowing through a circuit. It does this by increasing the resistance to any other flow of current. The value of this resistor also determines how much resistance will increase. Therefore, how much current will be affected.

A pull-down resistor works similarly as it decreases current flow through a circuit. The value of the pull-down resistor determines how much it will decrease, which also is a constant.

There is another type of resistor called a J for the current limit. These resistors have different properties and are helpful in different situations. For example, we can divide other resistors into series, parallel, or power calculation styles.

It is worth noting that the word ‘resistor’ can be helpful in other ways than outlined above, such as ‘resistive’ or ‘resistance.’

Series resistance (also known as parallel resistance) resists one current path. It allows another to flow through it in the opposite direction through its adjacent path. Parallel resistance is similar to series resistance. However, it will not allow one current path but resist two or more.

In power formula-style, the calculations depend on the value of R, which is essential in Ohm’s Law. This law states that resistance (R) times voltage (V) is equal to current (I). Therefore, when using a resistor, we must note the equations for all three different types we may use. However, they will come out with different results.

1. Pull-up Resistors

The most common use of pull-up resistors is in circuits with a push button. When we push the button, a tiny amount of current goes through it. It activates a switch that allows the circuit to work. The resistor stops any other current from flowing through the circuit. Also, it may cause unwanted power concentration problems.

As mentioned earlier, pull-up resistors increase the amount of current flowing through a circuit through increasing resistance. Therefore, it is best to use parallel resistance when using pull-up resistors as this gives the correct calculation for Ohm’s Law. If we only used series resistance, Ohm’s Law would not be correct and could give an unwanted reading on how much power the circuit used.

Concept:

The concept behind pull-up resistors is simple. When we push a button, the resistance that prevents the current flow in one direction will cause it to flow in the opposite direction. The value of the resistor determines how much current we use. With this knowledge, engineers can create circuits that will respond correctly. However, this depends on the amount of current flowing through a piece of material.

The concept behind pull-down resistors is like pull-up resistors. However, it works differently. A push-button works by allowing a small amount of current to pass through it and thus into the circuit. To lead the resistance out of this circuit, we put it in a reverse state where the circuit activates and stops the current flow. If we can alter the output level positively, a switch can make the process easier.

How to calculate the actual values for pull-up resistors

We can calculate the actual value of the resistance of a pull-up resistor by using Ohm’s law to work out the power in a circuit. The input voltage, output voltage, and input current are all needed for this calculation. We also need to know whether we will place the resistor before or after the load device.

R pull-up = (V supply – VH(min)) / Isink

Where:

VH(min) = The minimum drop in voltage.

Isink = The load current of the load device.

The voltage drop must be more significant than VH(min) for the current to flow through the circuit. Therefore, if we place the resistance in series, the output voltage will be less than before as there will now be a voltage drop across the resistor. This means there will not be enough power for the circuit to function unless output voltage increases. However, when we place a pull-up resistor parallel with a load device, it is only effective up to a certain current value.

Why use Pull up resistors

A tiny amount of current passes through a button when pushed. This allows the circuit to work. However, if the user were not to use a pull-up resistor, there is no way for the circuit to work as the user does not want the button pressed. Therefore, pull-up resistors will stop any unwanted power consumption. In addition, it allows current through in one direction and stops it from flowing in a different direction.

When using pull-up resistors, it is best to use parallel resistors as this gives the correct calculation for Ohm’s Law. If we only used series resistance, Ohm’s Law would not be correct and could give an unwanted reading on how much power the circuit uses.

The size of the pull-up resistor will determine how much power it can absorb without damaging the circuit. We measure it in ‘joules’ (J). The resistor then needs to be the correct size for these joules. For example, light switches tend to use 1.5 to 6J resistors. On the other hand, switches on computers use 2.2j, and power sockets use around 10J. The amount of current that we pass through a circuit will also depend on how dark the button is and how bright the light switch is.

The digital circuit has three states; high, low, and off. When we press the button, it enters the low state as a current flows through it. The resistor adds resistance to this process so that the circuit enters the high state. It will activate a switch to turn the light on or off. As soon as we activate this switch, no more current passes through the button. This increases its resistance value. When this happens, another process happens. It allows another current to pass through the button and into a different circuit.

Limitations of Pull up resistors

One limitation of using these resistors is that they cannot be helpful with more than one button. This is because if there are more buttons, the resistor will not push the current through them all. In addition, as the resistor allows one current to flow through it in only one direction, it cannot change for other directions. However, we can use a different circuit, such as a ‘Schmitt trigger’ with a larger effect than just one push button.

When we push buttons quickly, there is not enough time for the circuit to switch from high to low. So, there is no power released from the resistance, and thus no current flows through the circuit. We call this ‘Hold-off.’ It can result in many problems as functions within the circuit may stop working.

Pull-up resistors cannot work with light switches as the current would not flow through them. Therefore, it prevents the light from coming on. We also do not use Pull-up resistors with a switch turned off for long periods. It will not allow the current to flow, so it needs to be turned off for longer than usual, saving money.

2. Pull-down resistors

Pull-down resistors are helpful in many situations where circuits need switching on and off. For example, a push-button can work with the pull-down resistor to allow the circuit to switch on or off. A pull-down resistor works similarly to a push-button. It will enable current to flow through it at a certain pressure level or press certain buttons.

Concept

Pull-down resistors work with a small amount of resistance that allows current to pass through it and activate a switch when desired. When using pull-down resistors, it is best to use series resistance to calculate Ohm’s Law correctly. Pull-down resistors can only be helpful with positive polarity, and we must place them with the powering circuit. If not, the circuit will cut off the current because the current cannot flow through the resistor.

When a switch turns on, it activates several components simultaneously. This will cause a current to flow through each of these components simultaneously. It eventually causes a positive voltage across all of them. The resistor prevents this because it allows only one direction of current flow. So, the voltage across it is purely positive and cannot go negative. Therefore, any negative voltage will result in the resistor switching the circuit off. This is the opposite of what we want.

We cannot use Pull-down resistors with many buttons. The current will not flow through them and thus prevent their function from working. Using a pull-down resistor with a switch that does not switch on for long periods will stop the current from flowing through it. Therefore, it causes it to switch off completely.

How to calculate the actual values for pull-Down resistors

We can calculate the actual value of the resistance of a pull-down resistor by using Ohm’s Law to work out the power in a circuit. We will call the voltage logic Low. Isource will be the device’s current that the pull-down resistor controls. VL(max) will equal the minimum voltage drop across it.

R pull-down = (VL(max) – 0) / Isource

We can change this formula depending on whether we place the resistor before or after the load device.

There will be a large amount of current flowing through the resistor. So, there may be a large voltage drop across it. This means that we should choose this value carefully not to damage the circuit or any components.

We can also combine the pull-up and pull-down resistors to create a single component that performs both tasks simultaneously. We refer to these as bidirectional resistors. This is because they allow current to pass through them in either direction and thus perform both functions depending on how we use them.

Advantages of the pull-down transistor

The advantages of using pull-down transistors are that they can be helpful with one or more circuits. They are small and do not need to be placed parallel with the circuit. The pull-down transistor can also be helpful with larger resistors than its counterparts and can accept voltages from 2.5V up to 5V. Pull-down transistors are also helpful for controlling circuits that need smaller amounts of power. It will control the amount of current flowing through them by how sensitive the pull-down resistor is. Pull-down transistors can also make circuits turn on or off more quickly. It is helpful in situations such as control panels.

Limitations of the pull-down transistor

When using a pull-down transistor with more than one resistor, the voltage across each resistor needs to add the same value as the other not to switch off any of them. Also, if all resistors are of different values, this will mean that the circuit does not switch on or off and will not work properly. Finally, placing a pull-down transistor too close to a heat source can cause problems. Examples include causing high temperatures or even causing the melting of insulation.

Pull up resistor vs. pull down; The Differences

There is a slight difference between a pull-up and pull-down resistors. Pull-up resistors are only used with positive polarities. Pull-down resistors may work in positive and negative polarities.

The difference is that when a current is flowing through a resistor, it will cause a voltage drop across it. Pull-up resistors help to “pull up” the input signal’s voltage to be at the right level for what we need. This allows us to use less power for the circuit to function. However, pull-down resistors are helpful to lower voltages. So, they are not too high and will not cause damage or incorrect operation of some connected devices.

When using pull-down resistors, we need to realize that a large voltage drop across it when the input current is high may damage some devices or even cause the device to go out of limits. Generally, when an input voltage is higher than 5 V, it is best to use a pull-up resistor for circuit needs as these will not cause any damage. The problems arise when using voltages higher than this, which we deal with using series-connected resistors.

However, when we combine pull-down resistors with transistors to control the state of logic circuits, then-current is only passed for a certain amount of time. Therefore, the current will not flow for as long as other resistors. So, we need to consider this when working out how much power to use. The main advantage of their use over pull-up resistors alone is that they do not cause any damage. Also, they do not need any protection from overheating or causing voltages to rise too high.

1. Frequency characteristics

Pull-up power frequency response is ±50% of the line range, and pull-down power frequency response is less than ±25%. However, the capacitance between the input and output can cause the frequency response of a pull-down circuit to be greater than 50%. Therefore, we recommend that the capacitance between the inputs and outputs is as low as possible.

2. High and Low Level

Pull-up resistor output low level is 0.8 V ~ 2 V. The high input level of the pull-down circuit is 1.5 times larger than the low output level. Finally, the high output level is 3 V ~ 15V. However, it cannot exceed 30V. Therefore, the pull-down signal current threshold level is 0.5A (for pull-down resistor with input impedance 50Ω).

3. Driving Demand of the Lower-level Circuit

The driving demand of the lower-level circuit pull-down signal is 0.1mA. The pull-up resistor does not need to be driven by the lower level. When the high level is output, the driving current of the pull-up resistor is lower than 5mA.

4. Driving capability and Power Consumption

The maximum output signal of the pull-up resistor is at least 20mW, and the maximum input signal of a pull-down circuit is 10W. Drive pull-down signal logic requires 50Ω ~ 100Ω resistance to drive, and when using transistor output buffer, it should be lower than 50Ω. The power consumption of a pull-up resistor is 3~5W, the power consumption of a pull-down circuit is less than 1W.

Applications

We combine pull-up and pull-down resistors to create bidirectional circuits, apart from being used as discrete components—these work by having an input current flow through the resistor due to some bias mechanism. Depending on whether this is above or below a certain level will determine whether it outputs the voltage or not.

For example, they are helpful in many logic gates, such as the half adder, where there is a logic 1 to enable current to flow through the transistor. Thus a voltage drop across the resistor turns on the power for the circuit. Another example is in the use of tri-state buffers. One buffer side has a pull-up resistor, while the other has only a pull-down. This means that the current will only flow through to the load when logic 0 is present and will not cause any power loss. This circuit is then used during transmission between devices to prevent data from being lost by keeping power off until it is necessary.

The main disadvantage of using pull-up and pull-down resistors is that they are less efficient than they could be. As a result, it will convert some energy into heat rather than work. There can also be large voltage drops across them. This leads to damage or incorrect function in connected devices if you do not take suitable precautions.

We can build a modified version of this circuit without requiring additional components. It provides an input voltage present at the device to interface with.

What computer components can use this?

This can be used for power supplies and debugging. For example, when you plug your computer into something that uses a pull-down resistor, the switch will turn on or off depending on whether you are using positive ground or negative ground. Therefore, if you use a pull-down resistor, you shouldn’t use positive ground and negative ground simultaneously.

Conclusion

Of all the approaches, pull-up and pull-down resistors have the advantage of requiring very little power to operate. It makes them suitable for low-power designs. They also do not require any additional buffers or other components to work together, as each one can generate a voltage of its own. Where low power is needed, there are many different solutions available to use, such as diodes and transistors. Still, these may not provide the same efficiency level as a resistor. It would lower the overall efficiency of a circuit built using this method.

However, there may not be a choice for some applications as these circuits may prove to be the most suitable. So, it would be best if you considered their overall cost-effectiveness.