Understanding Diodes

A diode is a two-terminal electronic component that allows electric current to flow easily in one direction while blocking it in the opposite direction. This one-way behavior makes diodes essential building blocks in electronics, appearing in everything from simple LED indicators to complex power supplies.

Objectives

• Understand how PN junctions create one-way current flow
• Calculate LED current-limiting resistor values
• Identify different diode types and their specific applications
• Read diode markings and correctly identify polarity (anode vs cathode)
• Design basic rectifier, protection, and voltage regulation circuits
• Recognize diode schematic symbols and read circuit diagrams
• Apply diodes in real-world projects from LED indicators to power supplies

Key Takeaways

• Diodes conduct in one direction only - current flows from anode to cathode
• The cathode is marked with a band, flat edge, or shorter lead (on LEDs)
• Always use current-limiting resistors with LEDs: R = (Vs - Vf) / I
• Forward voltage drop varies by type: silicon (0.7V), Schottky (0.3V), LED (1.8-3.5V)
• Zener diodes regulate voltage by operating in controlled reverse breakdown
• Flyback diodes protect against inductive voltage spikes when switching relays/motors
• Bridge rectifiers convert AC to DC using four diodes
• Schottky diodes have lower voltage drop and faster switching than silicon

Applications

• Half-Wave Rectifier: Single diode passes only positive half of AC waveform to create pulsing DC
• Full-Wave Bridge Rectifier: Four diodes convert both AC half-cycles to DC for efficient power supplies
• Flyback Protection: Diode across relay/motor coil absorbs voltage spike when power is removed
• Reverse Polarity Protection: Series diode blocks current if power is connected backwards
• LED Indicators: Visual feedback with current-limiting resistor (R = (Vs - Vf) / I)
• Zener Voltage Regulation: Shunt regulator maintains constant output voltage despite input variations
• Signal Clamping: Limit voltage swings to protect sensitive circuit inputs
• OR-ing Diodes: Combine multiple power sources with automatic failover

Practice Problems

Problem 1: Calculate the current-limiting resistor for a blue LED (Vf = 3.2V) running from a 12V supply at 20mA.

Problem 2: Three red LEDs (Vf = 2V each) are connected in series with a 9V battery. What resistor is needed for 15mA?

Problem 3: A 5.1V Zener diode is used with a 1kΩ series resistor from a 12V supply. What current flows through the Zener (no load)?

Problem 4: A bridge rectifier uses four 1N4007 diodes (Vf = 0.7V). If AC input is 12V RMS, what is the approximate peak DC output?

Problem 5: What flyback diode should you use for a 24V relay coil that draws 100mA?