What a diode is

A diode is a part that lets current through one way and blocks it the other. That one sentence is most of what you need — and the four circuits it turns up in are the reason there is a diode on nearly every board you will ever own.

Every part you have met so far treats both of its ends the same. A resistor does not care which way you fit it. A capacitor mostly does not either. A diode is the first part on this site that has a front and a back, and turning it around changes everything about what the circuit does.

Press the button with the battery one way round, then the other.

Push it one way, then the other

forwards · flowing

Nothing has moved yet. Press the button and watch both halves at once — the flap in the pipe and the diode in the circuit are the same idea drawn two ways.

Same battery, same lamp, same wire. The only thing that changed is which direction you asked the current to go, and one of those directions is simply not available.

The valve in the drawing is not a metaphor for the maths

It is a metaphor for the behaviour, and it is a good one. Flow arrives, the flap swings open, everything carries on. Flow arrives from the other side and presses the flap against its seat, and the harder it pushes the more firmly the flap is held shut. A diode does exactly that, with no moving parts at all — the next page is about how.

Two things a valve does that a diode also does, and that catch people out:

It costs something to open. The flap has a spring; the diode has a threshold of about 0.7 V. Below that, nothing goes through in either direction. This is the forward voltage, and it is the price of the valve.
It has a pressure it cannot survive. Push hard enough backwards and the flap tears off its hinge. A diode's number is printed in its datasheet, and past it the part stops blocking — permanently.

Where you will actually meet one

Four jobs, four circuits

protect

One part, in series with the supply. Fitted the right way the board runs; fitted backwards nothing happens at all, which is a much better afternoon than the alternative. The cost is about 0.4–0.7 V off your supply, every day, for ever.

Those four tabs are not a taxonomy, they are where the diodes on your desk are right now. There is one behind the barrel jack of anything that takes a plug pack. There are several in the USB charger itself. There is one across the coil on every relay module. And there is one on the board you are reading this on, in series with the battery.

The one sentence to keep

Current enters at the anode and leaves at the cathode, and never the other way. The cathode is the end with the stripe painted on it, and the end the bar is drawn on in the symbol. Everything else in this chapter is detail hung off that.

What the detail is for: the 0.7 V matters when you are running from a battery and cannot spare it, the reverse rating matters when there is a motor or a mains supply anywhere nearby, and which-end-is-which matters the first time you solder one in.

The valve in the drawing is not a metaphor for the maths

Where you will actually meet one

The one sentence to keep

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