Electronics
How does a diode work?
A diode lets current through one way only: in at its anode and out at its cathode, the end marked with a stripe. Pointed the right way it conducts, keeping a small forward drop for itself: about 0.7 V for a silicon diode and 0.2–0.3 V for a Schottky. Pointed the wrong way, it blocks.
A one-way valve
A diode behaves like the valve on a bike tyre: the pump pushes air in, but it won't come back out, and it takes a little push before it opens.
Unlike a valve, a diode doesn't snap open. The harder you push, the more current flows, so its drop creeps up a little as the current grows. Below about 0.5 V a silicon diode passes only a fraction of a milliamp; by 0.7 V it conducts well.
Live experiment
Flip the diode
Two AA cells (3 V), a diode, 47 Ω and Pip, a red LED, in one loop. Turn the diode round, put the cells in backwards, and switch between silicon and Schottky. The bar underneath shows where the 3 V goes.
“Diodes” is lesson 1 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 28 before it first.
The forward drop
Once a diode conducts, it takes a roughly fixed share of the supply, and the load gets the rest:
Run a small motor from a 6 V pack through a diode. A Schottky leaves it 6 − 0.3 = 5.7 V; a silicon diode leaves 6 − 0.7 = 5.3 V. On two AA cells, 3 V, the silicon diode's 0.7 V is almost a quarter of the supply. In the experiment, the silicon diode leaves Pip about 8.8 mA, and a Schottky about 17 mA.
Battery-proof gadgets
Put a diode in series with a gadget. With the battery the right way round, the diode conducts and the gadget runs. Put the battery in backwards and the diode blocks, so nothing gets damaged. That's why some battery gadgets hide a diode by the battery box, and why protection diodes are usually Schottky: the drop is the price.
The same one-way trick lets a gadget run from USB or its battery, stops a solar panel draining its battery at night, and, with four diodes in a bridge, turns AC into one-way current in a charger.
Inside the diode
An ordinary silicon diode is a PN junction: silicon with spare electrons (N-type) joined to silicon that's short of them (P-type). A Schottky diode joins metal to silicon instead, which is why its drop is smaller. Where they meet, a thin depletion zone forms with no free charge. A forward voltage squeezes it thin and current flows; a reverse voltage widens it. Every LED is a diode too, one whose junction gives off light.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Diodes” is lesson 1 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 28 before it first.
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Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: the cathode. The stripe matches the bar in the symbol, the cathode. Current flows in at the anode and out at the cathode, the way the symbol's arrow points.
- Why not “nothing: a diode works either way round”? A diode only conducts one way, so which end is which matters. The stripe is there to show the cathode.
- Why not “the anode”? The anode is the unmarked end, where current flows in. The stripe matches the bar in the symbol, which is the cathode.
Show the answer
Answer: 0.7 V. It's a silicon PN junction, so it drops about 0.7 V once it conducts, like any silicon diode.
- Why not “0.3 V”? About 0.3 V is typical of germanium and Schottky diodes. A silicon junction needs about 0.7 V before it conducts properly.
- Why not “5 V”? 5 V is a typical supply, not a junction drop. A conducting silicon junction takes only about 0.7 V, whatever the supply.
Show the answer
Answer: It fills at each peak and feeds the load in the dips. The capacitor tops up near each peak, then feeds the load until the next one, so the dips fill in. It can't lift the output above the peak, and with no diodes it just gets pushed back and forth.
- Why not “It turns AC into DC on its own”? Without diodes, AC just charges the capacitor one way and then the other. The diodes do the rectifying, and the capacitor only smooths.
- Why not “It lifts the voltage above the peak”? A capacitor can only charge up to the peak the diodes let through. It holds the voltage near the peak, but it can't raise it above it.
“Diodes” is lesson 1 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 28 before it first.
Questions people ask
What is the voltage drop across a diode?
About 0.7 V for a silicon diode and 0.2–0.3 V for a Schottky, while it's conducting. It isn't fixed: it creeps up with current and falls with temperature. An LED is a diode too, with a bigger drop, about 2 V for a red one.
Which way does current flow through a diode?
From the anode to the cathode. The cathode is the end with the stripe. For the diode to conduct, the anode must be more positive than the cathode by about its forward drop.
What happens if a diode is put in backwards?
It blocks, and only a tiny leakage current flows. Push hard enough the wrong way and an ordinary diode breaks down; a Zener diode is built to break down at a set voltage, which is how it regulates.
Why use a Schottky diode?
It keeps a thinner slice of the supply, 0.2–0.3 V instead of about 0.7 V, so less is wasted. That matters on low supplies: on two AA cells, a silicon diode would waste almost a quarter of the 3 V.
Related topics
- How does a Zener diode work?A diode built to conduct backwards at a set voltage, then hold it: the simplest regulator.
- Transistor as a switch, explainedA trickle into the base switches a big current: cutoff, saturation and the base resistor.
- How does an inductor work?A coil that fights changes in its current, the high-voltage spike it makes, and how to tame it.