What is the resistance in Raspberry Pi 5 GPIO pin?
The resistance of a Raspberry Pi 5 GPIO (General Purpose Input/Output) pin isn't a single fixed value; it depends on whether the pin is acting as an input or an output, and the state it's in.
Here’s a breakdown of the different scenarios:
1. As a Digital INPUT Pin
When you configure a GPIO pin as an input (e.g., to read a button press), its key characteristic is very high input impedance.
Effective Resistance: Typically >1 MΩ (over 1,000,000 Ohms).
Why? A high-impedance input requires almost no current to read the voltage level. This is ideal for sensing because it doesn't disturb the circuit you are measuring. It's like listening very carefully without talking.
Practical Implication: You must use an external pull-up or pull-down resistor (either physical or internal) to define the pin's state when it's not being actively driven (e.g., when a button is not pressed). The internal pull-up/pull-down resistors are typically in the range of 50kΩ - 65kΩ.
2. As a Digital OUTPUT Pin (LOW or HIGH)
When you set a GPIO pin as an output to drive a component (e.g., turn an LED on or off), the situation changes completely. The resistance you care about is the on-state resistance of the internal transistors that switch the pin.
Effective Resistance: Approximately 50Ω or less.
Why? A low impedance output allows the pin to source (provide current when set to HIGH) or sink (accept current when set to LOW) a meaningful amount of current to drive external devices.
Practical Implication: This low internal resistance, combined with the current-limiting capability of the Pi's SOC, is why GPIO pins have strict current limits.
The Most Important Concept: Current Limits, Not Resistance
For the Raspberry Pi 5 (and all Pis), you should not think of the GPIO pins in terms of their resistance for output purposes. Instead, you must always design your circuits around their absolute maximum current ratings to avoid damaging the board.
Per GPIO Pin: 16 mA (milliamps)
Total Combined from All GPIO Pins: 50 mA
Example: If you try to draw 20 mA from a single pin (exceeding the 16 mA limit), the internal resistance will increase to limit the current (due to the transistor's properties), but this will cause the voltage on the pin to sag (drop below 3.3V) and generate excessive heat inside the BCM2712 chip, potentially damaging it.
Practical Example: Driving an LED
You wouldn't just connect an LED directly between a GPIO pin and GND. The low internal resistance (~50Ω) would allow too much current to flow, destroying both the LED and the GPIO pin.
The correct way is to use a series resistor:
How to calculate the resistor value (Ohm's Law):
GPIO Voltage (
V_gpio): 3.3VLED Forward Voltage (
V_led): ~1.8V (typical for red LED)Desired Current (
I): 8 mA (well under the 16mA limit)Resistor Value (
R):R = (V_gpio - V_led) / I = (3.3V - 1.8V) / 0.008A = 187.5Ω
A standard 220Ω resistor is a common and safe choice for this application.
Summary Table
| GPIO Mode | Effective Resistance | Key Characteristic | Design Consideration |
|---|---|---|---|
| INPUT | Very High (>1 MΩ) | High Impedance | Requires pull-up/down resistor |
| OUTPUT (HIGH) | Low (~50Ω) | Can source current | Stay under 16mA per pin |
| OUTPUT (LOW) | Low (~50Ω) | Can sink current | Stay under 16mA per pin |
Conclusion: While the internal resistance is a real hardware property, you should always design your circuits based on the Raspberry Pi's 3.3V logic level and, most importantly, its strict current limits (16mA per pin, 50mA total). For any device drawing more than a few milliamps (like motors, relays, or large LED arrays), you must use an intermediary component like a transistor, MOSFET, or motor driver board (e.g., H-Bridge) to protect the GPIO pins.
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