Electronic circuit theory - rectifier diode

Now we get to look at our first semiconductor, which is a device that behaves differently depending upon certain conditions. In this case we are going to look at the most basic semiconductor device called a diode, and specifically a rectifier diode.

Understanding Diodes – The One-Way Valve of Electronics

circuit symbol of a diode and physical image of a diode

One of the most fundamental components in any electronic circuit is the diode. In the video above , we explore exactly how diodes work, the physics behind the P-N junction, and the different types of diodes you will encounter in your projects.

What is a Diode?

At its simplest, a diode acts as a "one-way valve" for electricity. It allows current to flow easily in one direction while blocking it in the other. This direction is determined by the component's polarity: current flows from the Anode (+) to the Cathode (-).

The Physics: The P-N Junction

To understand why a diode behaves this way, we look inside the semiconductor material. A diode is created by joining a P-type region (positive holes) and an N-type region (negative electrons).

Depletion Region: Where these two materials meet, a "depletion region" forms, creating a built-in potential barrier that stops current from flowing freely.

Forward Bias: When we apply enough voltage to overcome this barrier (typically 0.6V–1.0V for silicon diodes), the "valve" opens, and current flows.

Common Types of Diodes

Different applications require different diodes. In the video, we compare several common types:

Rectifier Diode (e.g., 1N4003): These are the workhorses of power electronics, designed to handle high voltages (up to 200V reverse voltage) and moderate currents (1A). They typically have a forward voltage drop of around 1.0V.
Bridge Rectifier (e.g., RS203): Instead of using four individual diodes to convert AC to DC, a bridge rectifier packages them into a single component with four pins. The RS203 model shown can handle 2A of current with a 1.1V drop.
Schottky Diode (e.g., 1N5818): When efficiency is critical, Schottky diodes are preferred because they have a much lower forward voltage drop (between 0.3V and 0.5V). This makes them ideal for battery-powered circuits where preserving voltage is key.
Signal Diode (e.g., 1N4148): Designed for logic and low-power applications, these handle less current (300mA) but offer very fast switching speeds.
Zener Diode (e.g., BZX79-C3V3): Unlike standard diodes, Zener diodes are designed to conduct in reverse once a specific voltage is reached. The example used is a 3.3V Zener, often used for voltage regulation.
Light Emitting Diode (LED): The most visible type of diode! LEDs emit light when current flows and typically have a higher forward voltage drop (1.2V to 3.6V) depending on the color.

Practical Application: Rectification

The video concludes with a demonstration of a Half-Wave Rectifier. By placing a diode in series with an AC signal, we block the negative half of the waveform, effectively converting alternating current (AC) into pulsating direct current (DC).

Zener Diode

Usually the current is completely blocked in the reverse direction and we need to keep the reverse voltage below the breakdown voltage. However the zener diode is a specialised diode designed to allow current to flow not only in the standard forward direction but also in reverse when a specific voltage threshold, known as the Zener voltage, is reached. Unlike a standard diode, which blocks reverse current and can be destroyed if forced to conduct, a Zener diode is engineered to safely operate in this "breakdown" region. When the voltage across it exceeds its rated zener voltage, it acts like a pressure relief valve, conducting current to maintain that steady voltage level across its terminals. This unique property makes it a useful component for voltage regulation, where it is used to stabilize power supplies and protect sensitive circuits from voltage surges.