Diodes & rectifiers

In one line

A diode lets current flow one way and blocks the other — a check valve for electricity.

<figure style="text-align:center;margin:1.5rem 0"> <svg viewBox="0 0 280 100" role="img" aria-label="Diode symbol: a triangle pointing into a bar" style="width:100%;max-width:320px;height:auto;color:var(--c-iot)"> <line x1="14" y1="50" x2="108" y2="50" stroke="currentColor" stroke-width="2"/> <polygon points="108,32 108,68 146,50" fill="none" stroke="currentColor" stroke-width="2.5"/> <line x1="146" y1="30" x2="146" y2="70" stroke="currentColor" stroke-width="3"/> <line x1="146" y1="50" x2="266" y2="50" stroke="currentColor" stroke-width="2"/> <text x="92" y="28" font-size="10" fill="var(--text-3)" text-anchor="middle">anode (+)</text> <text x="170" y="28" font-size="10" fill="var(--text-3)" text-anchor="middle">cathode (−)</text> <text x="140" y="92" font-size="11" fill="currentColor" text-anchor="middle">current flows only this way</text> </svg> </figure>

How it works

A diode conducts when the anode is more positive than the cathode (~0.7 V drop) and blocks when reversed.

• Rectification — turn AC into DC. A bridge of four diodes plus a capacitor gives steady DC.
• Protection — block a reversed battery; a flyback diode absorbs a coil's spike.

Where it shows up in digital health

• The power supply of every mains device starts with a rectifier.
• Reverse-polarity protection saves a board from a backwards cell.
• A special diode — the LED — is the light in a pulse oximeter.

Watch for

Diodes have a max reverse voltage and forward current; exceed either and they fail — often shorted. Sizing matters.