Diode Basics
A diode only lets current flow one way — get the orientation wrong and, depending on the circuit, it either does nothing or fails immediately. Here's how to read one correctly.
Reading Polarity
Every diode has a cathode (the negative side) marked with a stripe or band on the body. Current flows from anode to cathode when the diode is forward-biased. In a schematic, the diode symbol's flat line — the same side the physical stripe is on — is the cathode.
Common Diode Types
- Rectifier diodes — general-purpose, used to convert AC to DC or block reverse current
- Zener diodes — designed to conduct in reverse above a specific voltage, used for voltage regulation and protection
- Schottky diodes — low forward-voltage drop and fast switching, common in power supplies
- LEDs — technically diodes too; the longer lead (or the flat edge on the housing) is the anode
Testing a Diode with a Multimeter
Switch your multimeter to diode-test mode (the diode symbol). Connect red to the anode and black to the cathode — a good diode reads roughly 0.3–0.7V forward voltage drop. Reverse the leads and it should read open (OL). A reading of 0V or OL in both directions means the diode has failed.
Common Mistakes
- Installing an LED backwards — it simply won't light, it isn't usually damaged
- Reversing a rectifier diode in a power path — can short the supply or let current flow the wrong way entirely
- Confusing a Zener's reverse breakdown voltage with a forward voltage drop when reading a datasheet