Hardware Reference
In-Depth Information
Voltage
The difference in charge between two points
Current
The rate at which charge is flowing
Resistance
A material's tendency to resist the flow of charge (current)
If you know two of them, you can figure out the third, thanks to Ohm's law,
which says that V=I*R, or the voltage (volts) is equal to the current (amps)
times the resistance (ohms). Those who are mathematically disinclined can
get an assist from the handy tool at the online
Ohm's Law Calculator
.
Keep this in mind when things act funny and you're not sure why: it might be worth a
quick power check.
You can also use the Pi's test points to test its polyfuse. A
polyfuse
is a type of fuse
that can repair itself after it has been blown. The Raspberry Pi has at least one of these,
labeled F3 on the bottom of the board, as shown in
Figure 1-16
.
Figure 1-16.
F3 polyfuse beneath SD card slot
Earlier Pis also had two on the USB ports, which have since been replaced by 0
Ohm resistors. They were known as F1 and F2.
You'll find F3 to the left of the SD card slot if you turn the Pi over and hold the SD slot
toward you.
