What Are Diodes and How Do They Work?
Diodes are electrical components with two terminals that primarily conduct current only one way. They basically make a wire or any other conductor a one-way street. Just like all of these other components, diodes come in various types. We’ll talk about some of them later, but first: the basics.
If you apply voltage on the “correct” or “apropriate” side of the diode, it is given so called forward bias. If the voltage is above a certain threshold, current will start to flow through the diode. This threshold is called the knee voltage (often 0.7 V for standard diodes) and is important because this will also be the minimum voltage drop across the diode. Larger current leads to bigger voltage drop due to increase in temperature.
Keep in mind that these components arent ideal. For instance, if the diode is given forward bias with voltage below the knee voltage, some current will still pass through.
If you switch the polarity on the voltage applied to the diode, you get reverse bias. This is where the diode stops current from flowing through it. However, you have the same non-ideal issue here: a very small amount of current will pass through the diode, aka. reverse leakage current, which is something to keep in mind.
If the voltage in reverse bias gets sufficiently large, the diode goes into breakdown. In this case the pressure gets too large for the diode to handle and current starts to flow in the “wrong” direction. This usually damages the diode permanently. However, some diodes are made to operate in this breakdown area for various reasons, and these diodes can handle this breakdown state.
Types of Diodes and Applications
Let’s take a quick look at SOME of the types of diodes available (there are many).
p-n Junction Diode
This is the most common type of diode which usually is just called a diode. Normal diodes have many areas of application, for instance in rectifiers or as flyback diodes as we’ve talked about in a couple of posts in the past.
With a low knee voltage (typically 0.3 V) and very fast switching action this diode is well suited for a wide variety of applications (for instance switched-mode power supplies). However, they are a bit more expensive, have a higher reverse leakage current, are a tad larger physically and normally have a lower reverse voltage rating. As always: read the datasheet!
Avalanche and Zener Diodes
These are diodes made for operating in the breakdown area without getting damaged. They are very similar and often mistaken for one another with subtle differences that we’re not going to go into here. Avalanche diodes and zener diodes are often used in surge protectors and other applications where something has to be protected from high peaks in voltage. When the voltage gets high enough in reverse bias, the diode will go into breakdown and pass current through, relieving the application from a too high voltage.
You guessed it, thse are LEDs. LEDs are actually p-n junction diodes that emits light when current passes through them in forward bias. Thus they can be used as normal diodes as well as light sources. As you might have noticed, we like to make things with LEDs.
These diodes actually converts (photons) light into current and is the “main component” in solar cells.
Of the types we’re NOT going to talk about here are tunnel diodes, varicap diodes, transient-voltage-suppression diodes, Gunn diodes, laser diodes, PIN diodes, BARITT diodes and the list goes on. We feel we have mentioned the most important ones, and there’s not a lack of other online resources if you want to dig a bit deeper.
We’ve now gone through the basics of diodes, how they work, what they can be used for as well as shown some examples of different types of diodes.
If you’ve read the posts about capacitors, inductors, transistors, relays and now diodes you’re hopefully well on your way to get a good grip of the most common components used in electronics.
What we haven’t talked about yet (besides the Electronics 101 post) is basic electronic theory. This is an essential subject when it comes to designing electronics. It doesn’t matter how much you know about the components as long as you’re unaware of the basic electronic principles. This is something we’ll address in one or several future posts.