Diodes

A diode is a one-way valve for current. Apply voltage in the forward direction and it conducts; reverse it and it blocks. That simple behaviour is useful in a surprising range of applications — rectification, voltage clamping, reverse-polarity protection, regulation, and light emission. Every power supply has at least one. Most circuits have several.

How They Work

A diode is a P-N junction — a piece of semiconductor doped to be P-type (holes, positive charge carriers) on one side and N-type (electrons) on the other. When forward-biased (positive at anode, negative at cathode), the depletion region narrows and current flows. Reverse-biased, the depletion region widens and current is blocked — until the reverse breakdown voltage is reached, at which point it conducts again (destructively, for most diodes).

A diode conducts conventional current from its anode to its cathode when forward-biased; the vertical bar marks the cathode, matching the stripe printed on many physical diodes.

The forward voltage drop (Vf) is the voltage across the diode when it's conducting — it's not optional and must be accounted for in calculations. For silicon diodes it's about 0.6–0.7 V. For other types it varies considerably.

Types and Characteristics

Type	Vf (forward)	Max reverse voltage	Speed	Primary use
Silicon rectifier (1N4001–7)	0.6–0.7 V	50–1000 V (varies by suffix)	Slow	AC-to-DC rectification in power supplies
Signal diode (1N4148)	0.6–0.7 V	75 V	Fast (4ns)	Signal steering, demodulation, clipping, ESD
Schottky	0.15–0.45 V	20–200 V	Very fast	Switching supplies, high-frequency rectification, OR-ing
Zener	0.6–0.7 V (forward)	Operates in reverse at Vz	Moderate	Voltage reference, clamping, simple regulation
LED	1.8–3.5 V (colour-dependent)	5–7 V	Fast	Indicator lights, backlighting, optocouplers
Transient voltage suppressor (TVS)	—	Clamps at breakdown voltage	Very fast (ps)	ESD protection, surge clamping
Photodiode	—	Reverse-biased in operation	Fast	Light sensing, fibre-optic receivers

LED Forward Voltage by Colour

Always use a current-limiting resistor with an LED — they have no internal resistance and will self-destruct without one:

Colour	Typical Vf	Wavelength
Infrared	1.2–1.8 V	850–950 nm
Red	1.8–2.2 V	620–750 nm
Yellow	2.0–2.2 V	570–590 nm
Green	2.0–3.5 V	495–570 nm
Blue	2.9–3.5 V	450–495 nm
White	3.0–3.5 V	Broadband (phosphor-converted blue)
UV	3.0–4.0 V	315–400 nm

LED resistor formula:
R = (V_supply - V_LED) / I_desired

Example: 5V supply, red LED (Vf = 2V), target 20mA
R = (5V - 2V) / 0.02A = 150Ω → use 150Ω or 180Ω

Rectification

Converting AC to DC — the most common diode application in power supplies.

Half-Wave Rectifier

A half-wave rectifier uses a single diode to block the negative half of the AC cycle. The filter capacitor smooths the resulting pulses into a relatively stable DC voltage.

V_DC ≈ V_peak − 0.7V   (one diode drop)
V_peak = V_RMS × √2

Example: 12V RMS transformer
V_peak = 12 × 1.414 = 16.97V
V_DC ≈ 16.97 − 0.7 = 16.3V (before load, with cap)

Full-Wave Bridge Rectifier

The full-wave bridge rectifier uses four diodes. During the positive half-cycle, D1 and D4 conduct. During the negative half-cycle, D2 and D3 conduct, ensuring the output always receives positive voltage.

V_DC ≈ V_peak − 1.4V   (two diode drops)

Using 12V RMS:
V_DC ≈ 16.97 − 1.4 = 15.6V (with capacitor filtering)

The bridge rectifier uses all four half-cycles — more efficient than half-wave and produces less ripple. The standard approach for almost all linear power supplies.

Common Part Numbers — Rectifiers

Part	Vrrm (max reverse)	If (avg forward current)	Notes
1N4001	50 V	1 A	Low voltage mains rectification
1N4004	400 V	1 A	Most common for mains (120/240V) supplies
1N4007	1000 V	1 A	High voltage; drop-in replacement when unsure
1N5819	40 V	1 A	Schottky — low Vf (0.3V), fast. Common in DC-DC converters
MBR1045	45 V	10 A	Power Schottky for high-current supplies

Zener Diodes

Zener diodes are designed to conduct in reverse breakdown at a specific voltage (Vz). That breakdown voltage is stable and predictable, making them useful as voltage references:

A Zener shunt regulator. The series resistor limits the current, while the Zener diode operates in reverse breakdown to clamp the output to its rated Zener voltage.

Resistor must limit current: I_R = (Vin − Vz) / R_series
Power in Zener: P_Z = Vz × I_Z

Common Zener voltages	Typical use
2.4 V, 2.7 V	Low-voltage references, ADC protection
3.3 V	3.3V rail reference, logic level clamping
5.1 V	5V reference, USB bus clamping
12 V	12V supply regulation, clamp circuits
15 V, 18 V	Op-amp supply clamping

Protection Circuits

Flyback Diode (Freewheeling Diode)

When a transistor switches off an inductive load (relay coil, motor, solenoid), the collapsing magnetic field generates a voltage spike — potentially many times the supply voltage. A diode across the load clamps the spike:

The flyback diode provides a safe path for the inductive load's current to circulate when the transistor turns off, preventing a destructive high-voltage spike at the transistor collector.

Use a fast diode (1N4148 for small loads, 1N4007 or Schottky for larger). Without it, the voltage spike can destroy the transistor or microcontroller driving it.

Reverse Polarity Protection

A series diode is foolproof but wastes power. A P-MOSFET oriented with its source facing the input acts as an ideal diode, turning on fully when voltage is correct and blocking when reversed.

References

Horowitz & Hill — The Art of Electronics, 3rd ed. Chapter 2
Scherz & Monk — Practical Electronics for Inventors, 4th ed.
Electronics-Tutorials — Diodes ↗
Vishay Diode Portfolio (datasheets) ↗