Diodes & rectifiers

In one line

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

How it works

A diode conducts when the anode is more positive than the cathode (forward bias), dropping about 0.7 V across itself, and blocks when reversed. From that one behaviour come its jobs:

• Rectification — turn alternating current (AC) into one-directional (DC). Four diodes in a bridge flip the negative half of the wave up, and a capacitor smooths the result into steady DC.
• Protection — a diode in series blocks a reversed battery; a flyback diode across a coil absorbs the voltage spike when current is cut.
• Steering — pick the higher of two power sources (e.g. switch from mains to battery on power loss).

Where it shows up in digital health

• The power supply of every mains-powered device starts with a rectifier turning wall AC into the DC the electronics need.
• Reverse-polarity protection on any battery-powered device — drop a AA cell in backwards and a diode saves the board.
• Flyback diodes protect the switches that drive infusion-pump motors and valves.
• A special diode — the LED — is the light source in a pulse oximeter.

Watch for

Diodes have a maximum reverse voltage and forward current; exceed either and they fail — often shorted, which can let the very current they were meant to block flow freely. Sizing matters.

Key takeaways

• A diode is a one-way valve for current — conducts forward (~0.7 V drop), blocks reverse.
• Jobs: rectification (AC→DC), protection (reverse-battery, flyback), and steering (mains↔battery).
• It's the front of every mains power supply and the guard on every battery board.
• Respect the reverse-voltage and forward-current limits — exceeded diodes often fail shorted.