Which way round a diode goes

A diode fitted backwards is not a subtle fault. Either nothing works, or nothing is protected and you find out later. Both are avoidable by knowing which end is which before the iron comes out.

The four views are the same fact four times: the marked end is the cathode, and current leaves there. Anode in, cathode out. Manufacturers do not vary this — there is no brand whose stripe means the other end.

The words, once

• Anode — where current goes in. The flat side of the triangle in the symbol. The long leg of an LED. Unmarked on the part itself.
• Cathode — where current comes out. The bar in the symbol, the painted stripe on the body, the flat edge and short leg of an LED, and the thick line on a board's silkscreen.

Two ways to keep it straight when you are tired. The bar is a wall, and current piles up against it if you try to come back through. And the letter K, which is how a cathode is marked on a schematic and on many boards, is a wall with two sticks leaning on it.

Proving it on the bench

You do not have to trust the stripe. A multimeter's diode range settles it in ten seconds, and the same test tells you whether the part is alive.

The diode range is not the ohms range. It pushes about a milliamp through the part and displays the voltage that took — so a working silicon diode reads around 0.5–0.7 V one way and OL the other, and a Schottky reads 0.2–0.4 V. Red and green LEDs usually read 1.6–2.0 V and glow faintly while you do it. Blue and white ones often read OL in both directions, because their forward voltage is higher than the meter can supply; that is a meter limit, not a dead LED.

Fitting it

• Line the stripe up with the printed bar on the silkscreen. Through-hole parts go in either way round mechanically, so the board cannot stop you.
• Check before you solder both legs. One leg is thirty seconds to undo. Two is a solder sucker and a lot of patience.
• Surface-mount parts have the same stripe, usually a fine line at one end of a black or brown body. It is small. A phone camera zoomed in is a legitimate tool.
• An LED with trimmed legs still has the flat on its rim. If it has been cut flush too, put it on the meter.

When backwards costs you nothing, and when it costs the board

An LED fitted backwards does not light and is not harmed — reverse voltage on an indicator LED in a 3.3 V or 5 V circuit is well inside what it survives, so turn it round and carry on.

A signal diode in a logic circuit fitted backwards means the circuit does not work, and nothing is damaged.

A protection diode in series with a supply, fitted backwards, means the board never powers up at all — which is annoying, and harmless, and much better than the failure it was fitted to prevent.

The one that costs money is the flyback diode across a coil. Fitted backwards it is directly across the supply, conducting, which shorts out the rail and destroys either the diode or whatever was driving it, usually in under a second. That is the one to check twice.