How to test the F3 polyfuse
- Remove all the things plugged into your Raspberry Pi, including SD card.
- Locate the TP2 test point on the top of the board.
- Turn your board over and find the TP2 test point on the bottom of the board. One lead of your mult-meter will always be on the TP2 point on the bottom of the board for all tests.
- Plug your power supply into the micro usb port and power your board.
- Place one lead of your mult-tester on the TP2 point on the bottom of the board and one lead on the side of the F3 fuse closest to the edge of the board. Note the voltage. This is the voltage coming into your RPi from your power supply.
- Keeping one lead on TP2, move the other lead to the side of F3 closest to the SD card slot. This is the voltage coming out of the F3 fuse.
If the voltage is different by more than about 0.03v - 0.10v you have a problem with the F3 fuse.
When polyfuses "blow" their resistence increases dramatically, there by limiting the voltage that can pass through them. If your power problem suddenly appeared after your board was known to be working fine, it is probable the fuse is just "blown" and will return to normal. It can take 24 hours for the resistence to go back down to normal so leave it unpowered and check it again in 24 hours. If your power problem has been since the first time you plugged in your board, the fuse was probably bad when it arrived and should be returned to place you purchased it.