Introduction
Throughout the courses, we have primarily dealt with components that are nonpolar (polarity agnostic), meaning that their orientation does not affect their behavior. However, the component we will be exploring today is vastly different in that its characteristics are highly dependent on its polarity. This component is none other than the diode.
What is a diode?
Diode is a semiconductor component that allows current to flow in one direction, but prevents it from flowing the other way. How? Diodes consist of doped silicon. One part is doped to be positive (P) and the other to be negative (N). When P and N doped silicons contact, they form a PN junction. This junction exhibits a strange phenomenon: current can flow in direction from P to N but not in reverse. This has to do with energy levels in atoms and we won’t go into details.
Different kinds of diodes
Diodes are available in several types. The most common is the classic diode, which has no special properties. Schottky diodes are similar to classic diodes, but they have a lower forward voltage, making them suitable for faster switching. These diodes are commonly used in SMPS power supplies. Zener diodes are regular diodes, but they have a very steep reverse breakdown voltage when they are polarized in the reverse direction, making them a good voltage reference. The exact voltage can even be selected during manufacturing, making them even more useful. Lastly, we have light-emitting diodes, commonly known as LEDs. These diodes function like regular diodes, but emit light when current flows through them in the forward direction. Although there are a few more types of diodes, we will limit our discussion to these in this course.
Markings
As mentioned before, polarity of diodes is important. P and N parts have special names. Positive end is called anode, and the negative is called cathode. If you have trouble remembering this, try the acronym PANK (misspelled punk, Positive Anode Negative Cathode). Diodes come in different shapes and sizes. In the days of THT (through hole technology), cathode was marked with a straight line (resembling the line at the end of the arrow on the electrical symbol). This usually still holds true with SMD components. Other option for marking diodes polarity, very common with LEDs, is making the cathode by making its lead shorter. The following photo shows three different diode packages, all are oriented such that anode is at the top and cathode at the bottom.
Forward voltage
Diodes conduct current only in forward direction, but current Vs. voltage characteristic follows an exponential curve. This means they are very lousy conductors at low voltages and very good conductors at high voltages. To simplify calculations, engineers came up with the following simplification: let’s say that voltage drop on a diode is constant and ignore its IU curve altogether.
Using this, we can calculate current through a simple resistor – diode circuit as such:
Forward voltages are different for different diodes. Standard diodes have 0.7 V, Schottky diodes can go down to 0.3 V, and LEDs have 1.6 V for classic red ones or more. Blue white and UV LEDs can easily exceed 3 V.
Reverse breakdown voltage
Diodes conduct current only in forward direction, but that is a simplification. They really conduct a bit of voltage in reverse direction. Total conducted current is quite small and more or less constant, regardless of applied voltage, but beyond reverse breakdown voltage, current increases exponentially. Zener diodes exhibit especially steep characteristic, making them suitable for being used as voltage references.
Written by Luka Pogačnik Edited by Andraž Pirc
This teaching material was created by Red Pitaya & Zavod 404 in the scope of the Smart4All innovation project.