Diodes Explained — How Diodes Work and Types
A diode is a one-way valve for electrical current. It allows current to flow in one direction (forward) while blocking it in the other (reverse). This simple property makes diodes essential for rectification, protection, signal processing, and light emission.
Open Diode SimulatorDiode Rectifier Flow
How a Diode Works
A standard silicon diode (like the 1N4007) has an anode (positive terminal, marked with a triangle on schematic symbols) and a cathode (negative terminal, marked with a bar or line). Current flows easily from anode to cathode (forward bias) but is blocked from cathode to anode (reverse bias).
- Forward voltage drop (V_F): Silicon diodes drop approximately 0.6–0.7V when conducting. Germanium diodes drop 0.2–0.3V. This voltage must be accounted for in circuit calculations.
- Reverse breakdown: If reverse voltage exceeds the diode's rated Peak Inverse Voltage (PIV), the diode breaks down and may be destroyed. The 1N4007 has a PIV of 1000V.
Diode Types
| Type | V_F | Key Feature | Common Use |
|---|---|---|---|
| Silicon (1N4007) | 0.6–0.7V | General purpose, 1A, 1000V PIV | Rectifiers, protection |
| Schottky (1N5819) | 0.2–0.4V | Low forward voltage, fast switching | Power supplies, logic, RF |
| Zener | Varies | Stable reverse breakdown voltage | Voltage reference, regulation |
| LED | 1.8–3.5V | Emits light when conducting | Indicators, displays, lighting |
| Schottky barrier | 0.15–0.3V | Very fast, low drop | High-frequency rectification |
Rectifier Circuits
Rectifiers convert AC to pulsating DC by blocking the negative half-cycles. A half-wave rectifier uses one diode and passes only the positive half-cycles. A full-wave bridge rectifier uses four diodes and converts both half-cycles to positive DC — doubling the output frequency and reducing ripple.
Simulate Half-Wave Rectifier Simulate Full-Wave RectifierZener Diode Voltage Reference
A Zener diode is designed to operate in reverse breakdown at a specific voltage (the Zener voltage, typically 2.4V to 200V). Connect it in reverse with a series resistor from the supply: the Zener maintains a stable voltage across itself regardless of current variations. Used for simple voltage regulation and reference.
LED Current Limiting
LEDs must have their current limited by a series resistor — without it, they draw too much current and burn out. The required resistor value depends on supply voltage, LED forward voltage, and desired current.
Example: Red LED, 5V supply, 20mA → R = (5 - 2) / 0.02 = 150Ω. Use 150Ω or 180Ω.
Common Beginner Mistakes
Connecting LED Without Resistor
An LED without a current-limiting series resistor will draw maximum current and burn out within seconds. Always calculate R = (Vsupply - VLED) / ILED before connecting an LED to any power source.
Reversed Diode Polarity
A diode connected backwards blocks current entirely — the circuit simply won't work. Check the cathode marking (stripe, bar, or K label) and ensure it connects toward the more negative side of the circuit.
Ignoring Reverse Voltage Rating
Every diode has a Peak Inverse Voltage (PIV) rating. In a mains rectifier, the reverse voltage spikes can exceed 400V. Using a 1N4148 (100V PIV) instead of a 1N4007 (1000V PIV) in a mains circuit will destroy it.
No Flyback Diode on Inductive Loads
Switching off a relay, motor, or solenoid coil generates a large reverse voltage spike. Without a freewheeling diode in reverse across the load, this spike destroys the switching transistor or MOSFET.