What is the maximum input voltage that Raspberry Pi can handle? 5.25 V is what is claimed to be the maximum. Beyond that, damage can result to your Raspberry Pi. Unless you have a Raspberry Pi 3, then it handles this in an interesting way… there is a TVS diode that will short if there is an overvoltage, and that will cause a high current that will trip the polyfuse, thereby preventing any damage from the overvoltage. Please note that your polyfuse’s max voltage should be above the clamping voltage of the TVS diode, else your polyfuse will get damaged before the TVS diode trips.
20200322/DuckDuckGo raspberry pi 5.2 maximum voltage
20200322/https://lb.raspberrypi.org/forums/viewtopic.php?p=1604897
So, what is a TVS diode? Ah, yes, the missing link that I was merely hinting at in my summary of protective circuits. Yes, indeed this is the missing link… if you want a semiconductor diode voltage regulator to provide a barrier against higher voltages in your circuit, but you want something that can specifically tolerate higher voltage transient spikes, a TVS diode will do that. A TVS diode is placed in reverse polarity in a circuit so it is normally non-conducting.
20200322/https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode
Also related is a “crowbar device.” Unlike a TVS diode that dissipates much more excess energy as heat from itself, a crowbar device creates a short circuit path for the excess current to flow through.
Please note: If you are using a TVS diode in an application where reverse polarity is assumed to “never happen,” then you can use a unidirectional TVS diode. Otherwise, if you are using a Schottky diode for polarity protection, you should use a bidirectional TVS diode which is placed in front of your Schottky diode but behind your polyfuse. This way, the Schottky diode is protected from transient voltages, and the polyfuse is also in the proper location to trip the circuit if the TVS diode must act.
N.B.: Check the max voltage and max current ratings of your polyfuse before using in relation with a TVS diode. If either one of these are exceeded, then your polyfuse may be damaged.
20200331/DuckDuckGo exceed max voltage of polyfuse
20200331/https://electronics.stackexchange.com/questions/370847/specifications-to-consider-when-selecting-a-polyfuse
UPDATE 2020-04-07: I looked more carefully at the details of the polyfuse and TVS diode combination in the Raspberry Pi 3 B+, and it turns out that these are not actually matched such that the TVS diode tripping will also trip the polyfuse. The polyfuse simply takes too long to trip at the rated current, and the TVS diode is not rated to last this long. Nope, the graphs show a 10 ms spike as the maximum duration of a transient voltage spike. That’s nothing compared to the 5 second time to trip the polyfuse.
So, the end all be-all of this conclusion is this… if you wire up a constant voltage source to the Raspberry Pi 3, you will still have the same risk of hardware damage as you would on the earlier Raspberry Pi’s. The TVS diode there will only protect against transient overvoltages, not persistent ones. A persistent overvoltage would first damage the TVS diode such that it provides no more protection ability, and since this is parallel-wired, the overvoltage would simply continue to flow through the main electronics power supply path and cause the same damage as if there were no TVS diode.