Diodes
Diode
A one-way valve for current. It conducts from anode to cathode once the forward voltage passes about 0.6 V (silicon), and blocks the reverse direction. An LED is a diode that turns that energy into light.
Think of it as: A one-way flap valve in a pipe.
Pins
- A (anode)
- : Current goes in here. On an LED it is the longer leg.
- K (cathode)
- : Current comes out here. Marked by the band on the body, or the flat edge of an LED.
The triangle in the symbol points the way current flows; the bar is the cathode, and so is the band printed on the real part.
What to look for inside
- The left side is P-type silicon (holes ○), the right side is N-type (electrons ●). The dark band in the middle is the depletion region.
- The voltage slowly swings from 4 V backwards to 0.75 V forwards and back.
- Backwards (reverse bias) the dark band widens and nothing gets across. That is the blocking direction.
- Forwards, past about 0.6 V, the band collapses and carriers flood across: current flows. In an LED each meeting of an electron and a hole gives off light.
The numbers that matter
| Spec | Typical |
|---|---|
Forward voltage V_F The voltage it "uses up" when conducting. | 0.3 V Schottky · 0.7 V silicon · 2 to 3 V LED |
Max forward current I_F Above it the junction overheats. An LED needs a resistor to stay under it. | 20 mA LED · 200 mA 1N4148 · 1 A 1N4007 |
Reverse voltage V_R How much backwards voltage it can block before it breaks down. | 30 V to 1000 V |
Zener voltage V_Z For a zener diode, the reverse voltage it holds steady. | 2.4 V to 47 V |
Where you’ll use it
- Reverse-polarity protection: blocks current if the battery goes in the wrong way.
- Flyback diode across a relay or motor coil to soak up the voltage spike when it switches off (see the inductor lesson).
- Turning AC into DC in a power supply (rectifier).
- Indicator lights (LEDs), and TVS diodes that clamp static-electricity spikes.
Common mistakes
- An LED with no series resistor. It draws far too much current and burns out in moments.
- Fitting it backwards. The band (cathode) must face the lower voltage.
- Forgetting the forward drop: a diode in series with a 3.3 V supply leaves only about 2.6 V.
Lessons that use it
- Lesson 3.1: Diodes & LEDs: Forward Voltage and the I-V Curve
- Lesson 0.1: Your First Circuit: Voltage, Current & Resistance
- Lesson 2.4: Inductors & the Flyback Diode Explained
- Lesson 5.2: Input Protection: Reverse Polarity, TVS & Fuses
Go deeper
Shockley equation: the current grows exponentially with voltage, which is why the curve has a sharp knee.
Choosing the LED resistor: e.g. (5 V − 2 V) ÷ 10 mA = 300 Ω, use 330 Ω.