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I have a 4-wire PWM FAN. The PWM pin it seems to be pulled-up to 12V with a 47k ohm resistor. Now, with this pin floating, it has almost 12V on it. And this pin must be connected to the Raspi PWM output. How is it possible that this voltage does not destroy the Raspi pins if they work at 3.3V? Because I have already made the connection and it works perfectly. When I connect those two, on the PWM pin I read 0 or 3.3V, depending on which state the PWM is in, even if before it had almost 12v on it. Does the Raspi has some zenner protection on it's pins, or what happens ? Should I leave it connected directly ? Or should I make a protection like this ?

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  • Zenner is a poor protection strategy and the impact this would have on the fan circuit is unpredictable. The circuit you have listed would NOT limit voltage. A simple diode clamp would be more effective protection but what the fan would do is unpredictable.
    – Milliways
    Dec 7, 2023 at 11:31

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The Pi has NO protection.

With a 47k resistor (which will limit the current flowing through the parasitic substrate diode) it probably will be low enough not to cause permanent damage.

This is your call but I would use a level converter or transistor.

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