Diodes are arguably the simplest active electronic component you can find.
They, like other components, can fail during a power surge or a short circuit. In those situations, being able to identify a diode is useful—they come in various shapes, sizes, and types so it’s very important to distinguish them from one another.
Types of Diodes
All diodes, under normal operating conditions, allow current to flow in only one direction (positive to negative).
Some diodes, such as Zener diodes, will also flow in one direction; but can clamp voltage movement when reversed.
Schottky diodes are capable of low, forward voltage drops that make them useful for power applications.
Diodes can be in axial through-hole form, SMD, or even in special packages such as full-wave rectifiers—but they always have the component indentation, “D” when on a PCB.
Note: This guide will not include optic-related diodes such as IR diodes, photodiodes, or laser diodes, as they are in a separate group.
Signal diodes are a basic type of diode that allows current to flow in one direction. They are commonly made using silicon and have forward voltage drops of 0.6V.
This kind of diode comes in many different packages; the main types are made as either through-hole or surface mounts using glass or plastic surface mounts.
Through-hole Glass Diodes
Through-hole signal diodes with a glass housing are the most common diode type. They include a band that identifies the cathode (negative lead).
The glass structure comes with the diode type printed on it.
Example: A common diode is the 1N4148.
These diodes are common, but not worth salvaging. They are also fragile because of their glass structure and require careful handling.
Through-hole glass diodes are easily mistaken for Zener diodes, which are also available in a glass housing; but they can be distinguished by the reverse voltage printed on the Zener casing.
SMD Signal Diodes
SMD signal diodes are hard to tell apart from other SMD diodes if they aren’t in a glass housing. The easiest way to identify them is by reading the printed code on their packaging.
Smaller SMD diodes will have two indented letters that make up a code, many don’t have indentations at all which makes identifying them nearly impossible.
Zener diodes are diodes that behave like normal diodes when used in forward-biased mode (voltage drop of 0.6V for example) but behave differently when reverse-biased.
When in reverse biased mode, the zener will clamp the voltage to a factory determined level such that the voltage cannot go beyond a certain value. This is very handy in situations where sensitive circuits (such as CMOS microcontrollers), connect to outside devices that are prone to sudden voltage spikes.
Through-hole Glass Zener Diodes
Through-hole zener diodes often come in glass housings, just like signal diodes. While signal diodes have their part number printed on their housing, zener diodes have the reverse-breakdown voltage displayed.
This makes them easy to identify, as you will see part numbers such as “9V” and “4V7” as opposed to 1N4817 on them.
Note: Just like other glass diodes, zener glass diodes are fragile and easily shattered if handled with pliers.
SMD Zener Diodes
SMD zener diodes are available in a range of packages; one of the more popular includes three pins.
The use of three pin packages ensures that the diode is inserted correctly (a three pin SOT-23 part can only be inserted one way).
zener diodes are also available in glass housing with plastic pin parts. The glass structure identifies the cathode with a thick blue band and the plastic varieties have a thick line beside the cathode.
Schottky diodes, unlike other diodes, are constructed using a piece of metal and a single type of semiconductor.
The specific metal, N Jun N junction has a very low forward voltage drop (typically 0.2V ~ 0.5V) which makes them useful for power applications.
These types of diodes are found in rectifiers and ESD protection circuits. They are available in both through-hole and SMD packages.
Through-hole Schottky Diodes
Most through-hole Schottky diodes are housed in a black plastic cylinder with a single silver ring that indicates the cathode.
These diodes can range in size, but are almost always much larger than signal diodes (one of the common ones being the 1N5817).
Through-hole Schottky diodes have their part number printed on the outside of the cylinder body and are easy to spot on a PCB.
SMD Schottky Diodes
SMD Schottky diodes, like other diodes, come in many different packages, but unlike Zener diodes, some Schottky packages have more than one.
Two pin packages often have a thick bar next to the cathode and a part identifier; three pin packages only have a part number.
The BAT54 range of Schottky diodes are the well known SMD diodes, as they contain two diodes in different configurations for ESD protection.
For example, if a circuit needs to connect to an external device that may experience ESD, then the BAT54 is used to protect both digital and analog connections that are linked to a microcontroller or IC.
Full wave rectifiers are diode packages that combine four diodes in a full-wave bridge rectifier arrangement.
These packages are often more convenient than other diodes as they combine all components in one package and are available in both through-hole and SMD.
Through-hole Rectifier Diodes
Through-hole rectifier diodes range in size and their size typically corresponds with their power capability.
Small rectifiers are not much larger than TO-92 transistors and are cylindrical in shape. Large rectifiers have big, thin, rectangular houses with thick legs.
Sometimes, rectifiers have metal backs that allow them to be directly mounted to heat sinks.
They are mainly AC/DC conversion in power supplies. They aren’t the most valuable component to salvage otherwise.
Through-hole rectifiers often have printed part numbers and a pin indication such as positive (+), negative (-), and AC input (~).
SMD Rectifiers are easy to spot. They are square in shape and have markings next to each pin that indicates their function (positive +, negative -, and AC input ~).
These parts, like other SMD parts, are not worth salvaging, as they are difficult to use in projects, require SMD soldering, and have limited applications.
SMD rectifiers are also inexpensive and easier to purchase as new parts rather than using older components.