Diodes

A diode is a component which only allows current flow in one direction. This property is commonly used in many different ways, from rectifying and regulating voltage, to combining and otherwise dealing with analog and digital signals, to more esoteric applications such as in oscillators and RF circuits of various kinds.

Diodes have two poles, called the anode (+) and cathode (-). As a convenient mnemonic, you can think of the arrow in the symbol as showing the (conventional) direction the current can flow, and the line as “stopping” any current. On many diodes, the line in the symbol corresponds to a color marking at one end of the diode’s capsule.

Important parameters for diodes depend on the application, but include:

Forward voltage drop (V_F)
The voltage needed for the diode to begin conducting. 0.6V is a common value.

Voltage (maximum reverse voltage, V_R(max) – peak inverse voltage/maximum reverse voltage)
How high a voltage the diode can block. If exceeded, the diode will be destroyed! Smaller signal diodes can normally withstand less than 100V, while rectifier diodes often will often withstand 1000V or more. A rectifier diode that can withstand 1000V can replace one with a lower resistance, but never the other way around!

Current (I_F(max) – maximum forward current)
Maximum current that can pass through the diode. When current passes through the diode, it gets warm. Diodes that can withstand higher currents do this by warming up less and/or withstanding higher temperatures. For large diodes, mounting on a heatsink may be required for them to be capable of the rated current. Just as with voltage, it is fine or good to use a diod with higher current handling than strictly necessary. Lower heat generation is always a good thing, both for the diode itself, the PCB and nearby components.

There are many different types of diodes, with different areas of use – read more in the categories.

Silicon diode. The most common type of diode. Available both for lower voltage and current – ​​often called signal diodes – and more powerful types, which are often called rectifier diodes or just rectifiers. Silicon diodes are used, among other things, for protection against incorrect polarity, protection against current spikes and for rectifying alternating current.

Diode bridge. Consist of four internal diodes, commonly used to rectify alternating voltage. Diode bridges are available in many different packages, from small IC-style chips to bigger devices for chassis mounting with a bolt. Often called rectifier bridges or just rectifiers.

Schottky diode. Similar to silicon diodes in many respects, and share many common uses. However, schottky diodes are characterized by a lower voltage drop, typically 0.45V. For this reason, they are found in particularly efficient circuits such as switched power supplies, often a single diode – less often in diode bridges in linear power supplies. They generally withstand lower voltages than silicon diodes.

Zener diode. Zener diodes have a certain voltage at which they start conducting current “backwards”, known as the Zener voltage. They are normally used in this way, which means they are wired "backwards". In the other direction, they function much like ordinary signal diodes. Used to regulate voltages, for protection, or as voltage references.

TVS diode. Transient Voltage Suppression diodes are specially designed to protect against overvoltages and spikes from e.g. static electricity. Similar to zener diodes, but are optimized to work in other ways. Available in bidirectional and unidirectional versions, and in combined packages with a combination of silicon diodes and transient protection, for one or more channels.

Avalanche diode. Similar to zener diodes and are also connected in the opposite direction to reach their breakdown voltage and start conducting. They are based on a different technology than zeners, and the voltage is more constant with varying current compared to zener diodes.

Germanium diode. Similar to silicon and schottky diodes but with an even lower forward voltage, down to 0.2V. The germanium diode was the first diode made of semiconductor material, and is mainly found in older electronics, but also in modern effects pedals.

Varicap diode. Short for variable capacitance diode, this diode changes capacitance according to the voltage it sees. Commonly used in oscillators in RF applications, and functions as a voltage-controlled capacitor rather than as a diode. The symbol is a combination of capacitor and diode. Other common names are varactor or tuning diode.

PIN diode. A special type of diode which has a lightly doped middle region between the P and N regions of the diode (hence the I in the name). Used in RF applications . Often has very low and stable capacitance, and can function as transient protection for sensitive electronics for high frequency signals.

Light emitting diode. Diodes that light up when they conduct current – ​​LEDs. Have higher voltage drops than regular silicon diodes, but in principle function just like them.

Photodiode. A special kind of PIN diode used to detect light.