Electronics for Building Things · Lesson 11 · Components in Depth
The one-way valve — and the three jobs it quietly does in your projects.
A diode does one simple thing: it lets current flow one way and blocks the other. That simplicity hides three genuinely useful tricks — including one that can save your board from a backwards battery.
A diode conducts when current flows from its anode (the flat side of the triangle symbol) to its cathode (the bar). Push the other way and it blocks (SparkFun: Diodes).
Two facts to carry: a conducting diode "uses up" a small fixed voltage — about 0.7 V for an ordinary silicon diode, or ~0.3 V for a Schottky (a fast, low-loss type). And the band marks the cathode — that's how you orient one.
From Lesson 6: when you switch off a coil or motor, it
kicks back a high-voltage spike. A flyback diode placed across the coil
(band toward +) gives that spike a harmless loop to die in, protecting your transistor.
This is the diode you'll add most often.
Plug a power supply in backwards and you can instantly destroy a project. A single diode in series
with the incoming + blocks current entirely if the supply is reversed — cheap insurance,
at the cost of that ~0.7 V drop (use a Schottky to lose less).
Because diodes pass only one direction, they convert alternating current (AC) into one-directional (DC) — the first step inside every wall adapter. You rarely build this yourself, but now the symbol on a power-supply schematic makes sense.
See also: Glossary · Component field guide · Rules of thumb