Diodes 101: How PN Junction Diodes Work, Biasing, and Applications

In this Engineering Mindset video, Paul explains what a diode is, how it conducts current in only one direction, and the meaning of forward and reverse bias. The explanation covers the PN junction formed by P-type and N-type silicon, the depletion region, and a typical barrier of about 0.7 volts. The video also introduces diode symbols, the concept of voltage drop, and the IV characteristics that define when a diode acts as a conductor or an insulator. Practical uses such as protecting circuits from reverse polarity and converting AC to DC via rectification are discussed, including a bridge rectifier and capacitor smoothing. Multimeter testing and in-circuit voltage measurements are shown to illustrate real-world diode testing and fault detection.

Introduction and overview

Paul from Engineering Mindset introduces diodes, their purpose, and common physical form. He explains that diodes allow current to flow in only one direction and distinguishes anode and cathode, noting other diode types like xenodiodes and LEDs but focusing on the basics of silicon diodes.

What a diode does in a circuit

Diodes control the direction of current. Depending on orientation, a diode can act as a conductor or an insulator. The video uses the water pipe analogy to illustrate forward bias (current flows) and reverse bias (current is blocked).

Semiconductor fundamentals and PN junctions

The video provides a primer on conductors, insulators, and semiconductors. Silicon is doped with donors (N-type) and acceptors (P-type) to create a PN junction. At the interface, a depletion region forms as electrons and holes migrate, creating a barrier that yields a typical forward voltage of about 0.7 volts. Forward bias reduces this barrier and allows current to flow; reverse bias widens it, blocking current.

Diode structure, symbol, and identification

Internally, diodes have a PN junction, and externally the stripe marks the negative side. The symbol includes a line and a triangle or arrow indicating conventional current direction. Each diode carries identification numbers and IV data used to look up specifications.

IV characteristics and voltage drop

The IV curve shows conduction in forward bias and blocking in reverse bias. The barrier limits current until the forward voltage is reached, after which current rises rapidly. In practice, a diode like the 1N4001 may show a forward drop around 0.71 volts.

Applications: protection and rectification

Diodes protect circuits from reverse polarity and are essential for converting AC to DC. A single diode yields half wave rectification; a four diode bridge (full-wave rectifier) improves DC quality, and adding capacitors smooths ripple to closely resemble DC.

Testing diodes with a multimeter

Testing involves placing the multimeter leads on the diode ends, forward-biased reading around 0.5–0.7V, and reverse-biased reading as OL (open circuit). If both orientations show a reading, the diode is likely faulty. In-circuit tests show the forward voltage drop (for example, 0.71V) once connected in a DC circuit.

Wrap-up and next steps

The video invites viewers to explore related content and follow Engineering Mindset on social media for more lessons on electronics and capacitors.