Pull-Up Resistor for Logic

R = (Vcc − VOH) / Isink

Quick answer

This calculator solves R_pullup = (V_cc − V_OL_max) / I_sink for Pull-Up Resistor. With Supply Voltage 120 V, Max Low-Side Voltage 56.4 V, Sink Current 0.2 A, the result is 0.318 kΩ. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Pull-Up Resistor (kΩ)
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Formula

R_pullup = (V_cc − V_OL_max) / I_sink

Description

Open-drain and open-collector outputs require an external pull-up resistor to define the high-state voltage. The resistor value must be small enough that the sink current through it (when the output is low) does not cause the low-state voltage to rise above VOL_max. The formula gives the maximum pull-up resistance. Additionally, the pull-up must be large enough that the open-drain device can sink the current without exceeding its IOL rating. I²C bus pull-ups, interrupt lines, reset circuits, and wired-OR buses all require properly sized pull-up resistors.

Variables

  • R — Pull-up resistor value (Ω)
  • V_cc — Supply voltage (V)
  • V_OL — Maximum acceptable low output voltage (V)
  • I_sink — Current the open-drain output can sink (A)

Practical Notes

For I²C at 3.3 V standard mode (100 kHz): R = 1.5-4.7 kΩ. For I²C fast mode (400 kHz): R = 1-2.2 kΩ. Smaller R gives faster rise times but increases power consumption and IOL current. The rise time is determined by the RC time constant: t_rise ≈ 2.2 × R × C_bus. For multiple devices on the bus, the total bus capacitance C_bus is the sum of all device input capacitances plus trace capacitance.

Symbols and units

Symbol Meaning Default unit Accepted units
R Pull-Up Resistor kΩ mΩ, Ω, kΩ, MΩ
Vcc Supply Voltage V pV, nV, µV, mV, V, kV
VOH Max Low-Side Voltage V pV, nV, µV, mV, V, kV
Isink Sink Current mA pA, nA, µA, mA, A, kA

Step by step

  1. 1. What you know: Supply Voltage 120 V, Max Low-Side Voltage 56.4 V, Sink Current 0.2 A
  2. 2. The relation: R_pullup = (V_cc − V_OL_max) / I_sink
  3. 3. Substitute: Vcc = 120 V, VOH = 56.4 V, Isink = 0.2 A
  4. 4. Result: Pull-Up Resistor = 0.318 kΩ

Related quantities from the same inputs

  • P = V × I = 24 W

Nearest standard component values

Series Nearest value Error
E12 0.33 kΩ 3.8%
E24 0.33 kΩ 3.8%

Worked examples

Minimum range

Given: Supply Voltage 3.3 V, Max Low-Side Voltage 1.55 V, Sink Current 1 mA

Result: Pull-Up Resistor = 1.75 kΩ

Low range

Given: Supply Voltage 33 V, Max Low-Side Voltage 15.5 V, Sink Current 10 mA

Result: Pull-Up Resistor = 1.75 kΩ

Typical

Given: Supply Voltage 120 V, Max Low-Side Voltage 56.4 V, Sink Current 0.2 A

Result: Pull-Up Resistor = 0.318 kΩ

High range

Given: Supply Voltage 2.3 kV, Max Low-Side Voltage 1.08 kV, Sink Current 5 A

Result: Pull-Up Resistor = 0.244 kΩ

Maximum range

Given: Supply Voltage 23 kV, Max Low-Side Voltage 10.8 kV, Sink Current 50 A

Result: Pull-Up Resistor = 0.244 kΩ

Solving for each variable

Solve for Pull-Up Resistor

Supply Voltage 3.3 V, Max Low-Side Voltage 1.55 V, Sink Current 1 mA → 1.75 kΩ

Solve for Supply Voltage

Pull-Up Resistor 100 Ω, Max Low-Side Voltage 0.726 V, Sink Current 100 µA → 0.736 V

Solve for Max Low-Side Voltage

Pull-Up Resistor 100 Ω, Supply Voltage 0.33 V, Sink Current 100 µA → 0.32 V

Solve for Sink Current

Pull-Up Resistor 100 Ω, Supply Voltage 0.33 V, Max Low-Side Voltage 0.726 V → -3.96 mA

How Pull-Up Resistor changes with Supply Voltage

Supply Voltage (input) Pull-Up Resistor (result)
60 V 18 Ω
90 V 0.168 kΩ
120 V 0.318 kΩ
0.18 kV 0.618 kΩ
0.24 kV 0.918 kΩ
0.48 kV 2.118 kΩ
0.84 kV 3.918 kΩ
1.2 kV 5.718 kΩ

The same result in other units

Unit Value
mΩ 318000
Ω 318
kΩ 0.318
MΩ 0.000318

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Frequently asked questions

How do you calculate Pull-Up Resistor for Logic?

Use R_pullup = (V_cc − V_OL_max) / I_sink. With Supply Voltage 120 V, Max Low-Side Voltage 56.4 V, Sink Current 0.2 A, Pull-Up Resistor works out to 0.318 kΩ.

What is the formula for Pull-Up Resistor for Logic?

The formula is R_pullup = (V_cc − V_OL_max) / I_sink. Open-drain and open-collector outputs require an external pull-up resistor to define the high-state voltage.

How do you find Supply Voltage when you know the other values?

Rearrange for Supply Voltage: with Pull-Up Resistor 100 Ω, Max Low-Side Voltage 0.726 V, Sink Current 100 µA, Supply Voltage is 0.736 V.

How do you find Max Low-Side Voltage when you know the other values?

Rearrange for Max Low-Side Voltage: with Pull-Up Resistor 100 Ω, Supply Voltage 0.33 V, Sink Current 100 µA, Max Low-Side Voltage is 0.32 V.

What units does Pull-Up Resistor for Logic use?

Pull-Up Resistor is shown in mΩ, Ω, kΩ, MΩ. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

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