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Part 1 explored the role of pull-up and pull-down resistors to ensure that the binary (digital) circuit points are at an unambiguous 1 or 0 level. This part explores basic issues of sourcing, sinking current, and resistor sizing.

Sourcing and sinking current

Q: What does “sinking” mean?
A: In sinking, the “top” side of the load (resistor or other component) is connected to the power rail, and the transistor as switch interrupts the current flow between the other side of the load and ground, Figure 1 (left). One side of the transistor is grounded, and it “sinks” current from the power rail and loads to that ground. This circuit topology is generally easier to implement since the driver transistor is grounded. It is often used between circuits on a circuit board, for example.

Q: Does sinking have a complementary arrangement?
A: Yes, called sourcing, Figure 1 (right). In this case, the transistor provides current to the load’s ungrounded (top) side, and the other side is connected directly to the ground.

Q: Why does source versus sink matter?
A: In many real-world applications, the external loads must be grounded for safety or performance reasons. One good example is the automobile, where the many electrical loads such as motors (there are at least 30 to 40 in a modern car!), SCR and MOSFET switches, ignition-system components, and more must be connected to the common ground of the chassis. Further, many loads require that their low side is at the ground, without an intervening switch element for load-loop stability.

Specifying the pull-up/pull-down resistor

Q: Is a particular resistor needed for the pull-up or pull-on function?
A: No, a basic, plain resistor is appropriate in almost every case as long as it has the correct power rating and resistance.

Q: How do you determine the resistance value?
A: This is a classic tradeoff situation with relatively wide boundaries. The resistor value is generally not critical, so the resistors are often sized at round values such as 10 kΩ, 50 kΩ k, or 100 kΩ. (Node that tradeoffs are inherent in engineering: consider the simple calculation for the “right” value of the current-sensing resistor in series with a load.) Power dissipation and pin voltage must be balanced to calculate the resistance value.

Q: How do you size the resistance value of the pull-up or pull-down resistor?
A: Consider a basic push-button connected to an input pin, which will be pulled low when the switch is pressed. The value of the resistor determines the amount of current going from the supply rail through the button and then to the ground. If the resistance value is low, a larger current will flow through the pull-up resistor. This, in turn, will result in higher dissipation at the resistor when the switch is closed. This is called a “strong” pull-up, which should be generalized and minimized if low power consumption is required.

Q: So why not make the resistor larger to minimize power dissipation?
A: When the button is not pressed, the input pin is pulled high via the pull-up resistor, which, therefore, 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 may be insufficient to keep at its high logic level, called a weak pull-up.

Q: So what do you do?
A: The pull-up resistor’s actual value depends on the impedance of the input pin, which is closely related to the pin’s leakage current. As a first-pass guide, you should use a resistor with a value at least ten times smaller than the value of the input pin impedance.

For example, in 5-V bipolar-logic families, the typical pull-up resistor value is between one and five kΩ, while for switch and resistive-sensor applications, it is between one and ten kΩ. For CMOS devices with much smaller input-leakage current, much higher resistance values, from around 10 kΩ up to 1 MΩ, are used.

Q: What about for pull-down resistors?
A: The pull-down resistor should always have a larger resistance than the logic circuit’s impedance. If it does not, it will pull the voltage down too much, and the input voltage at the pin will remain at a constant logical low value regardless of whether the switch is on or off.

Q: Why not just make the resistance as large as can be tolerated?
A: The pairing of the resistor value combined with the pin and wire capacitance at the switching node forms an RC circuit. The circuit’s time constant determines how fast that node can be switched. A larger RC value may inhibit the circuit from switching at the needed rate. This is a non-issue for slowly changing loads such as relays; this can be a serious one for RF circuits and even LEDs used for driving data links.

Q: How does the dissipation factor fit into the calculation?
A: These resistors generally do not carry much current, and their power dissipation is low. In most circuits driving other circuits and ICs, resistors of ¼ watt or smaller are adequate. Of course, once you have determined the required resistance value and current, the resistor’s power rating is easily calculated.

Conclusion

The topic of pull-up and pull-down resistors applies to many digital circuits. It is not complicated, yet it illustrates how even simple topics still have issues that must be acknowledged and tradeoffs to be made, even if the assessment has only modest criticality.

Related EE World content

What is an open drain?
FAQ: How to wire a switch to a microcontroller with pullup resistors
150-V fast high-side-protected N-channel MOSFET driver provides 100% duty cycle
Choosing the right encoder – open-collector, push-pull, or differential output?
When to use NPN and PNP transistors and FETs
Why I still like electromechanical relays – and you should, too (maybe)

External references

Sparkfun, “Pull-up Resistors”
Circuit Basics, “Pull-up and Pull-down Resistors”
Wikipedia, “Pull-up resistor”
EE Power, “Pull-up and Pull-down Resistors”
Utmel Electronic, “What are the Differences Between Pull up and Pull down Resistors?”
Electronics Tutorials, ”Pull-up Resistors”
Robu.in, “What are Pull-up and Pull-down resistors?”

The post FAQ on pull-up/pull-down resistors: part 2 appeared first on Power Electronic Tips.

In the crisp, clean, theoretical binary world, signals exist in only two unambiguous states, generally called 1 and 0 (one and zero). However, when engineering students, hobbyists, and digital-only professionals get into the hands-on real world of circuits and systems, they find that binary circuits have three states: 1, 0, and undefined (or indeterminant).

That last state is undesirable and can lead to circuit and system malfunctions, erratic operation, and even hard faults, so it must be avoided in nearly all designs. Fortunately, that is fairly easy to accomplish. Note that technically, “binary” is a specific case and subset of “digital,” but the two terms are often used interchangeably.

This FAQ will explore the role of pull-up and pull-down resistors to ensure that the binary (digital) circuit points are indisputably at an unambiguous 1 or 0 level and that the intermittent state cannot occur. These resistors are often used when interfacing a switch or other components with microcontroller input/output (I/O) pins or other digital gates. Many microcontrollers include them, but others do not, allowing interface flexibility.

Q: What is the reality of a digital-circuit logic level?
A: A digital logic circuit actually has three logic states: high, low, and “floating” (or high impedance); the latter is indeterminate. When probed with a meter, it may appear as a high or low or alternate between them randomly, but that is misleading, and it is not a valid logic-level reading.

Q: Is this a problem for circuit inputs as well as outputs?
A: Yes, you can have a floating input or output. They may have different “appearances” and impacts despite having the same underlying cause.

Q: Are there basic principles to keep in mind when looking at logic levels in real circuits?
A: Yes, there are two: 1) electrical current needs a path to flow, and 2) voltage only has meaning when defined between two known points. Of course, voltage and current coexist and interact, and you need to understand the actions of one to understand the other.

Q: Do concerns about a floating signal in a circuit relate to both input and output points?
A: Yes. Input is either the base of a bipolar transistor or the gate of a CMOS transistor (Figure 1), where the output driving base or gate is from another IC, or it could be a discrete mechanical switch. The concerns are very similar in both input and output cases.

Pull-up versus pull-down resistors

Q: What is a pull-up resistor? What is a pull-down resistor?
A: A pull-up resistor is a resistor used to ensure that a circuit point is “pulled” to a high logic level in the absence of an input signal; a pull-down resistor ensures that the circuit point goes too low (almost always “ground” or common). These resistors are used to correctly bias the inputs of digital gates to stop them from floating randomly. They are also used on the outputs of digital gates.

Q: Can we see this “in action?”
A: Consider a simple on/off pushbutton switch connected to the input of a logic gate (remember, gates in the real world are built of bipolar or CMOS transistors; they are not some abstract concept). Electrically, the switch looks like a short or open circuit — simple enough.

If there is no pull-up resistor (Figure 2), the logic-gate input is floating and not at 1 or 0 when the switch is open; when the switch is closed, the input is hard-connected to ground, logic 0.

Consider the same input but with a pull-up resistor (Figure 3).

When the switch is open, the voltage at the gate input is pulled up to the level of the input voltage (designated as Vin, VCC, VDD, or the supply rail, depending on the circuit design and its designer) and is at logic 1. When the switch is closed, the input voltage at the gate goes directly to the ground.

In short, you must use a pull-up resistor when you have a low default impedance state and wish to pull the signal to ‘high.’

Q: This all makes sense for a “hard” contact-closure on/off switch, but how does it apply to an input being driven by the output of another digital-logic gate?
A: If you draw the schematic and model the circuit, it’s a very similar situation. When the driving output is low, there is a low impedance to ground, almost but not quite a short circuit; when the output is high, there is a high-impedance path and virtually no current flow, appearing almost as an open circuit.

Q: What about pull-down resistors on inputs?
A: It’s the same principle except as its complement (Figure 4). When the switch is open, the voltage of the gate input is pulled down to the ground level. When the switch is closed, the input voltage at the gate goes to Vin. The voltage levels would virtually float between the two voltages without the resistor. The pull-down resistor holds the logic signal near zero volts (0 V) as it pulls the input voltage down to the ground to prevent an undefined state at the input.

Q: Can you briefly compare and contrast pull-up and pull-down resistor arrangements?
A: Figure 5 is a brief overview of their relative attributes.

Q: What about pull-up and pull-down on the output side of a gate or even a basic transistor?
A: It’s a similar problem. If the output collector or drain is connected to “nothing,” it will float and be unable to attain a true logic of 0 or 1, as there is no current path. For this reason, most logic gates have a built-in pull-up or pull-down output resistor.

Q: Why do I sometimes see gates with “open collector” or “open drain” outputs?
A: This is needed when you want the transistor’s output to drive an external, “real-world” load. The open-collector output refers to an output connected to the collector of the transistor (here, an NPN device) (Figure 6). The NPN transistor allows the sinking of current to ground (more correctly called circuit common).

You can make the load function as the pull-up resistor by using an open collector or drain. The load can be a discrete resistor, relay, motor, lamp, LED, or other component.

The next section discusses sourcing and sinking current, as well as resistor sizing.

Related EE World content

What is an open drain?
FAQ: How to wire a switch to a microcontroller with pullup resistors
150-V fast high-side-protected N-channel MOSFET driver provides 100% duty cycle
Choosing the right encoder – open-collector, push-pull or differential output?
When to use NPN and PNP transistors and FETs
Why I still like electromechanical relays – and you should, too (maybe)

External references

Sparkfun, “Pull-up Resistors”
Circuit Basics, “Pull-up and Pull-down Resistors”
Wikipedia, “Pull-up resistor”
EE Power, “Pull-up and Pull-down Resistors”
Utmel Electronic, “What are the Differences Between Pull up and Pull down Resistors?”
Electronics Tutorials, ”Pull-up Resistors”
Robu.in, “What are Pull-up and Pull-down resistors?”

The post FAQ on pull-up/pull-down resistors: part 1 appeared first on Power Electronic Tips.