Imperf3kt wrote: ↑
Sun Jan 20, 2019 11:34 pm
Will5455 wrote: ↑
Sun Jan 20, 2019 9:56 pm
but the polyfuse protects the pi right?
It isn't indestructible. It won't last forever. One trip could be enough to make the polyfuse more of a problem than a safety device.
Some users have reported the polyfuse never returns to its original resistance and as a result, causes more voltage drop after recovery.
It also may not be quick enough, some Pi engineers claim that while the polyfuse will trip in the event of an overvolt, if it doesn't trip fast enough, the 5v regulator will be damaged.
Adding an extra fuse helps mitigate that issue and is far easier / cheaper to replace.
Desoldering the polyfuse and soldering a new one in place, might also damage the new polyfuse (it works on heat after all) or may damage other components on the Pi.
Finally, replacing the polyfuse voids your warranty.
Protection circuits can be effective even if sacrificial. Usually not the polyfuse, but the TVS diode that prevents overvoltage affecting the "guts" of the Pi, converting the fault to overcurrent, which switches the polyfuse to high resistance. In the process it can fail short circuit. Recovery involves removing or replacing the diode. In the former case, a repeat overvoltage will be fatal. Ask Heater.
While the polyfuse is not fast enough to save the diode, the diode is fast enough to save the Pi.
An external fuse serves a different purpose, it protects the wiring from the PSU from burning under fault (short circuit) conditions.
I only remember one example of polyfuse incomplete recovery reported here, and that was when the Pi was refrigerated. I don't know whether that is relevant, but it might well be.
Do you have any evidence that soldering can damage the polyfuse or other components? (It needs some incompetance to do that). After all, the Pi is made by soldering!