Diodes: The One-Way Valve of Electronics
Discover the fundamental semiconductor device that controls current flow.
What is a Diode?
A diode is a two-terminal electronic component that acts as a one-way switch for electric current. It allows current to flow in one direction (known as forward bias) but strongly resists current flow in the opposite direction (reverse bias). However like mechanical one way valves diodes are not perfect. When a voltage is applied to a diode in a forward Bias direction, the diode will only let voltage flow if it is above its threshold voltage, and even when enough voltage is applied there is still a voltage drop over the diode (around 0.5-2v). Similarly when reverse biased a diode cannot block everything. Diodes have a breakdown voltage, when the voltage is too high the diode breaks down internally allowing current to flow in a backwards direction. Also potentially damaging the diode.
Exploring the I-V Curve
The Current-Voltage (I-V) curve is key to understanding diode behavior. Use the slider to apply voltage and see how the diode responds.
Resulting Current:
0.00 A
Bias State
Voltage is applied in reverse. No current flows.
Diode Forward Voltage Drop (Vf)
When a diode is forward biased and conducting current, there’s a small voltage drop across its terminals, known as the forward voltage (Vf). This voltage drop is a characteristic of the diode’s material and construction, and it’s an important factor to consider when designing circuits.
- Silicon Diodes: Typically have a forward voltage drop of around 0.6V to 0.7V. These are the most common general-purpose diodes.
- Germanium Diodes: Have a lower forward voltage drop, usually around 0.2V to 0.3V. They are less common now but were historically used in sensitive radio circuits.
- Schottky Diodes: Characterized by a very low forward voltage drop, often in the range of 0.15V to 0.45V. They are used in high-frequency applications and for power efficiency where minimizing voltage loss is critical.
- Light Emitting Diodes (LEDs): The Vf of an LED depends on its color. For example:
- Red LED: 1.8V – 2.2V
- Green LED: 2.0V – 3.5V
- Blue/White LED: 3.0V – 3.6V
Understanding Vf helps engineers select the right diode for an application, especially when dealing with power efficiency or signal integrity.
Application: The Half-Wave Rectifier
One of the most common uses for a diode is converting AC to DC. A rectifier circuit does this. See how a single diode blocks the negative half of an AC sine wave.
AC Input Voltage
Rectified DC Output Voltage
Application: The Full-Bridge Rectifier
While a half-wave rectifier works, it is pretty terrible only rectifying half the wave. This leads to an exceedingly inefficient circuit, so in the real word a full-bridge rectifier (FBR) is used. An FBR uses four diodes to convert both positive and negative halves of the AC input into a pulsating DC output. While the symbol for a full bridge rectifier is 4 diodes in a Diamond configuration, I find it much more useful to look at it as a positive and negative wire with a 2diodes connected to each wire. This way you can visualise the diode facing from negative and towards positive on each wire. We will talk more about FBRs in the more advanced courses coming soon.
Conceptual Full-Bridge Rectifier Diagram
Full bridge rectifier symbol, and conceptual veiw
AC Input Voltage
Full-Wave Rectified DC Output Voltage
More Common Diode Applications
Voltage Regulation
Zener diodes provide a stable reference voltage.
Lighting (LEDs)
Light Emitting Diodes are efficient light sources.
Circuit Protection
Protect against reverse voltage or voltage spikes.
Signal Demodulation
Used in radios to extract audio from radio waves.
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