# What is a Diode? - Definition & Types

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• 0:05 Definition of a Diode
• 0:53 Following the Electric Current
• 2:35 How Diodes Work
• 3:35 Some Different Types of Diodes
• 4:41 Lesson Summary

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Lesson Transcript
Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

In this lesson, we explore the fundamental operation of a diode. We also look are some the various types of diodes, such as the zener diode, the Schottky diode, the light emitting diode, and the laser diode.

## Definition of a Diode?

Imagine you're driving through a busy city looking for your friend's home on Light Bulb Avenue. You see her location on the map but the actual streets have signs that only allow traffic to flow in one direction, the 'one way' street sign.

A diode is the 'one way' sign for electrical circuits. The current is allowed to move through the diode in one direction only. Each diode has a positive end, the anode, and a negative end, the cathode. Current flows from the anode to the cathode, but not the other way around.

Instead of traffic flow, we have electric current flow, in other words, energy flowing from a source, like a battery. In a closed circuit this energy flows through wires and returns back to the source.

And just like the arrow in the street sign, the symbol for the diode has an arrow to show the allowed direction of current.

## Following the Electric Current

Very much like using a street map, electrical engineers use a schematic, which are documents showing how wires and components are connected. Most of us don't think of schematics when we turn on a flashlight or a light switch in a house. Just for fun, let's engineer a flashlight.

A flashlight has one or more cells (groups of cells are commonly called a battery), a switch, some wire, and a light bulb. Pretty much the way it's been for a century. Let's draw a schematic for this flashlight.

Our first circuit has a battery, some wire (the green lines) and a light bulb. The current (the orange arrows) leaves the battery at the '+' end and flows through the circuit. This is a closed circuit because there is a continuous path from the battery through the wire, through the light bulb, and back to the battery. Current flows and the light bulb lights. Great!

What if we reverse the battery in the circuit? The current flows out of the positive as before. We still have a closed circuit. The light bulb still lights. So where does the diode come in? First, we connect the diode to the battery and the light bulb with the diode pointing in the same direction as the current flow out of the battery. The diode acts like a switch which can be on or off. Current flows through the diode and the circuit is closed. The light bulb lights. No surprises.

As before, in our next circuit we reverse the battery. This time we have a diode in the current path. The light bulb does not light! The current wants to leave the positive end of the battery, but cannot pass through the wrong 'one way' of the diode. This is an open circuit. No electric current passes through the circuit and the light bulb does not light. This can save us from ruining our circuits by accidentally putting in the battery wrong.

## How Diodes Work

Let's look at how a diode works in a practical circuit. This diode circuit is called a rectifier circuit and is a step in converting AC (alternating current) to DC (direct current). Those plug-in wall adapters (sometimes called ac-adapters) for our cell phones and other electronics have rectifier circuits in them.

Alternating currents actually go from positive to negative directions. Direct currents, though, need to be in one single direction. To convert AC to DC, you have to take away that negative direction, which is just what diodes are for. In a half-wave rectifier, when the voltage is positive, the diode lets electric current flow. When it's negative, it does not. Take a look at the diagram and you'll see what we're talking about.

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