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Diodes 101 – What is a Diode?

Blog Post created by Daniel_Bogdanoff Employee on May 24, 2018

The diode is a crucial component to master if you want to grow your electronics prowess. So, it’s crucial to have a solid understanding of how diodes behave under varying loads. Today we’re going to look at some diode fundamentals, then take a look at a video covering 4 ½ practical uses for a diode.

 

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What are Diodes?

Diodes are relatively simple but more complicated than many basic passive components you probably already know. Diodes are non-linear components. Resistors, capacitors, and inductors are linear devices, meaning they can be characterized using first order differential equations. As much as I’d like to geek out on the math, I’ll spare you. That’s what Wikipedia is for. So, what are diodes?

 

Diodes are nonlinear devices. They don’t follow Ohm’s law, and for circuit analysis, you can’t replace them with a Thevenin equivalent.

 

Diodes are passive devices, which means they don’t need power to function.

 

Diodes are two-port devices. There’s a positive input, which is known as the anode, and there’s a negative output, which is known the cathode (Figure 1).

 

A diode’s circuit symbol. The anode is on the left, and the cathode is on the right.

Figure 1. A diode’s circuit symbol. The anode is on the left, and the cathode is on the right.

 

Diodes may be simple, but they are extremely useful because of their V-I curve, shown in Figure 2. The X-axis is voltage, and the Y-axis tells you how much current can flow through the diode at that voltage level.

 

A diode’s IV curve.

Figure 2. A diode’s IV curve.

 

Let’s take a closer look at the V-I curve. What does it mean? More importantly, how can you use it to your advantage?

 

Positively Biased Diodes

In Figure 2, there are a few things worth noticing. Let’s start with positively biased diodes. At moderate positive voltages, a diode basically acts as a short. However, there is a small voltage drop, usually called the “forward voltage drop.” It’s also known as the “cut-in voltage” or simply “on voltage.”

 

You can see the forward voltage drop on the V-I curve in Figure 2 around .6/.7 V, where the current spikes. A .6/.7V drop is standard for silicon diodes, but for other diode materials, the forward voltage drop will vary.

 

You can measure the forward voltage for a specific diode using a multimeter with a diode testing capability. You can see that this silicon diode has a forward voltage of roughly .62V (Figure 3).

 

Using a Keysight U1282A multimeter to measure the forward voltage drop of a diode.

Figure 3. Using a Keysight U1282A multimeter to measure the forward voltage drop of a diode.

 

But what you need to remember is that, when exposed to a moderate voltage – say 5V, a diode will pass through 5V minus the forward voltage. So, 4.3V for a standard silicon diode. There are some methods for compensating for this drop, but that’s beyond the scope of this article.

 

Negatively Biased Diodes

Let’s now move to the left side of the Y-axis in Figure 2. When exposed to a negative voltage, there will be a nano-amp reverse current. You can generally approximate it as 0A in most situations. That is, until you get to the other big swing on the VI curve, known as the breakdown voltage.

 

If your diode is exposed to a high level of reverse bias, you blew it. Often literally. Diodes essentially can’t hold up to that level of negative voltage, and the device physically breaks down, allowing negative current flow.

 

Long story short, you can essentially think of a diode as a one-way conductor with a voltage drop. Enough preface, let’s look at a few different ways you can use diodes for your circuits.

 

How to Use a Diode in Your Designs

Now that you know how a diode works, how can you use one in your designs? Check out this video for 4 ½ practical uses for a diode:

 

 

Diodes are Great!

Diodes are extremely useful components, and if you’re working with electronics, you need to build a solid knowledge of how diodes work. The video covered a few ways to use diodes, but that’s just the beginning! How are you using diodes in your designs? Let me know in the comments here or on the Keysight Labs YouTube channel.

 

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