Diode Fundamentals: How It Regulates Voltage and Controls Current

If you’re still building your foundation in basic electricity, start with this beginner-friendly overview: 🔹 “Electricity 101: The Complete Beginner’s Guide to How Power Really Works”
After reading it, the concepts in this article will make a lot more sense.

Watch now: what is a diode and how does it control current?

This short video walks you through how a diode turns electric current into a “one-way street.”
You’ll see where it shows up in power supplies, LED indicators, and voltage protection, and you’ll also get a quick look at how the PN junction makes current choose only one direction.

What is a diode? One-sentence explanation

If someone asks you, “So… what is a diode, exactly?”, here’s the one-line answer:

A diode is an electronic component that lets current flow almost entirely in one direction, and is used to prevent reverse current, rectify AC to DC, emit light, or stabilize voltage.

In everyday electronics — from phone chargers and laptop power bricks to USB adapters, LED bulbs, and industrial control panels — you’ll find diodes everywhere.

In this article we’ll keep the math light and answer the question “what is a diode” step by step. We’ll focus on three things:

• what’s going on inside a diode,
• the most common types you’ll actually see,
• and one simple LED experiment that makes its “one-way” behavior very visible.

If you want a more formal physics treatment and full device classifications, you can also check the Wikipedia entry for “Diode” or this introductory guide on diode applications from All About Circuits.

Basic principle of a diode

The PN junction: turning current into a one-way street

The core of a typical semiconductor diode is the PN junction, made from two differently doped regions:

If you’ve ever googled “what is a diode” while fixing a project, this PN junction is the real answer to what the diode is doing inside your circuit.

• P-type semiconductor – has lots of “holes,” which you can picture as missing electrons.
• N-type semiconductor – has plenty of free electrons.

When you join P-type and N-type together, they form a depletion region in the middle.
You can think of this as a small “no-man’s-land” barrier:

• Some electrons and holes recombine near the junction.
• That leaves behind fixed charges and creates an internal electric field.
• This field forms a potential barrier that makes it hard for charge carriers to cross when there is no external voltage.

That built-in barrier is the reason a diode can prefer one direction for current flow.

Forward bias vs reverse bias: when does it conduct?

To make a diode do anything useful, we apply voltage across its two terminals. We call this biasing.

• Forward bias (conducting direction)
  • Connect the P side to the positive terminal of the source, and the N side to the negative terminal.
  • This pushes holes and electrons toward the junction and lowers the potential barrier.
  • Once the applied voltage reaches a certain level (the forward voltage), current starts to flow significantly.
  • We say the diode is “forward-biased” and conducting.
• Reverse bias (blocking direction)
  • Connect the P side to the negative terminal and the N side to the positive terminal.
  • The internal barrier gets even higher, the depletion region widens, and under normal conditions almost no current flows.
  • In this state, the diode behaves like a switch that is turned off.

So in a circuit, you can think of a diode as a one-way gate or check valve for current.
It helps you:

• keep current from flowing backward into places it shouldn’t go,
• make AC behave more like DC,
• and protect sensitive parts from wiring mistakes or voltage spikes.

Common types of diodes and where you’ll see them

All of these diodes use some form of a junction, but they’re optimized for different jobs.
Here are five of the most common ones you’ll see in textbooks, repair work, and real-world panels.

1. Rectifier diode

Main job:
Convert AC (alternating current) into DC (direct current). Rectifier diodes are the basic building blocks of power supplies.

Where you’ll see them:

• Inside AC-to-DC power adapters for routers, monitors, and small appliances
• Bridge rectifiers right after the secondary winding of a transformer
• DC power modules inside control panels and industrial machines

Without rectifier diodes, you don’t get a clean DC rail — so in that sense, almost every electronic device starts with them.

2. LED (light-emitting diode)

Main job:
Under forward bias, when current flows through the junction, energy is released as light — so the diode glows.

Where you’ll see them:

• Little indicator lights (power status, on/off, machine states)
• LED bulbs, strips, and floodlights
• Backlights in displays, car headlights, traffic signals

The tiny indicator on your power strip or charger?
Almost certainly an LED doing its job quietly in the background.

3. Zener diode

Main job:
Work under reverse bias. When the reverse voltage reaches a specific value (the Zener voltage), the diode intentionally starts conducting, helping to:

• Clamp or stabilize voltage, and
• Protect downstream circuits from over-voltage.

Where you’ll see them:

• Simple reference or clamp circuits (for example, a 5 V or 12 V reference)
• Over-voltage protection circuits that shunt extra voltage away from sensitive ICs

You can think of a Zener diode as a safety valve that only kicks in when the reverse voltage is too high.

4. Schottky diode

Main job (practical focus):

• Lower forward voltage drop (less voltage lost across the diode)
• Very fast switching, so it can handle high-frequency signals

Where you’ll see them:

• High-efficiency DC-DC converters
• High-frequency rectifiers (such as in switching power supplies)
• Protection circuits where you really care about low loss and high speed

From a field-tech point of view, the key points are:
less heat, better efficiency, and fast response compared to a regular silicon diode.

5. Photodiode

Main job:
Convert light intensity into current — it’s a type of light sensor.

Where you’ll see them:

• Light sensors for auto-on lights or automatic screen brightness
• The receiver side of infrared remote controls
• Fiber-optic communication systems
• Simple light-measurement or lab circuits

If you think of an LED as “electricity → light”, a photodiode is “light → electricity.”

Simple experiment: use an LED to see a diode’s one-way behavior

This little experiment is safe, quick, and easy to do on a desk.
No mains power required — just a battery.

⚠️ Safety note:
For this experiment, use only batteries.
Do not connect it directly to household mains (120 V / 240 V AC).

📌 What you’ll need

• 1 × LED (light-emitting diode)
• A battery around 3 V total (for example, two AA cells in series)
• 1 × resistor around 330 Ω (anything from 220–1 kΩ works; brightness will change)
• 1 simple switch (or you can plug and unplug a wire by hand)
• A few jumper wires and a breadboard (optional, but convenient)

📌 Steps

1. Forward connection (make the LED light up)
   • Connect the LED’s long leg (anode, +) to the battery positive.
   • Connect the short leg (cathode, –) to one end of the resistor, then the other end of the resistor back to the battery negative.
   • Put a switch anywhere in series to turn the circuit on and off.
2. Close the switch
   • When the switch is closed, the LED should light up.
   • This is the LED under forward bias — the diode is conducting.
3. Reverse the LED
   • Now flip the LED around: connect the long leg to battery negative, and the short leg through the resistor to battery positive.
   • Close the switch again. This time, the LED should stay off or barely glow.

🔎 How to interpret what you saw

• Forward connection:
  • The LED lights up.
  • That means under forward bias, the diode lets current pass.
• Reverse connection:
  • The LED stays dark.
  • Under reverse bias, the diode blocks current (at this low voltage).

This simple demo is the real-world picture behind the textbook sentence:

“A diode has one-way (unidirectional) conduction.”

Later, when you see the diode symbol — an arrow with a line — in rectifier circuits, protection circuits, or LED drivers, you can immediately recall this simple LED experiment.

FAQ

Summary: why understanding diodes matters

By now, you not only know the short answer to “what is a diode”, you’ve also seen that:

• What a diode is:
  a device that makes current behave like it’s on a “one-way street.”
• How it does that:
  by using a PN junction and the difference between forward and reverse bias.
• Where it shows up:
  in phone chargers, LED lights, voltage regulators, and all kinds of industrial control and power systems.
• How to see it in action:
  with a simple LED + resistor + battery experiment that makes the one-way behavior visible.

Whether you’re:

• brushing up on basic electronics after years away from school,
• thinking about going deeper into electronics, repair, or electrical work,
• or just trying to read datasheets and schematics without getting lost,

getting comfortable with this one small component — the diode — will make later topics like rectifiers, regulators, switching power supplies, and sensor circuits much easier to learn.

Recommended next reads

• “Basic Circuits: Power Source, Wires, and Loads”
  Build an intuitive picture of what a circuit actually looks like — source, conductors, and loads. Once that’s clear, it’s much easier to see where diodes fit and what role they play.
• “Current vs Voltage: A DIY-Friendly Guide”
  Uses everyday examples to explain the difference between voltage and current, and why the same diode behaves very differently at different voltages or in different directions.
• “How to Design a Simple Zener-Based Regulator” (coming soon)
  A hands-on walkthrough of using a Zener diode to clamp voltage and protect sensitive parts in a basic regulator circuit.
• “How LED Lighting Has Changed the Way We Use Electricity” (coming soon)
  From tiny indicator lights to high-power fixtures, this article looks at how LED technology reshaped lighting and energy usage.

💡 If you still have questions about diodes, drop them in the comments.
And if you’d like more circuit experiments and career-change content in your inbox, you’re welcome to subscribe to the newsletter. 🚀