A diode is a semiconductor. Unlike a resistor, they are polarized with an anode and cathode that is represented on their symbol. A semiconductor is a material that conducts current, but only partly. The conductivity of a semiconductor is somewhere between that of an insulator, which has almost no conductivity, and a conductor, which has almost full conductivity. Most semiconductors are crystals made of certain materials, most commonly silicon.
Simple Definition: The most common function of a diode is to allow an electric current to pass in one direction (called the diode’s forward direction), while blocking it in the opposite direction (the reverse direction).
On a schematic, a standard diode looks like D1. Sometimes the schematic will display the actual diode number below the image. 1N4148 is a popular silicium diode. Some diodes are so popular they get there own symbol like the Schottky and Zener diodes.
The Schottky barrier diode(D4 Above) or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action.
The Zener diode (D5 Above) is unique because when the voltage across the terminals is reversed and the potential reaches the Zener voltage (or “knee”), the junction will break down and current will flow in the reverse direction – a desired characteristic.
A diode is composed of two parts:
- A part poor in electrons (P junction, like “Positive conduction”)
- A part rich in electrons (N junction like “Negative conduction”)
Practically speaking, there is a part where the electrons feel good (the P-junction), and a part where it is difficult for them to go through (N junction, there are already too many electrons in there)
At first, electrons do not manage to go through the part rich in electrons and accumulate in the P-junction. When there are enough electrons accumulated, they have enough “power” to cross the N-junction: current is flowing through the diode!
This phenomenon causes what is called the forward voltage (also called voltage drop): below a certain voltage threshold, the current cannot go through the diode. The voltage drop is around 0.7V for a classic silicium diode for instance: below 0.7V, the current will not pass.
Below 0.7V, current does not go through the diode. Above it: it passes. The voltage drop also diminishes the voltage out the diode. If you make 9V goes through a silicium diode, you will have a 8.3V voltage after the diode!
Finally, another very important thing with diodes is that the current can only go through them in one direction! There is often a really small current that can go the other way, named current leakage. If the current applied is too big, the diode breaks passed what is called the breakdown voltage.
Depending on the type of diode, the forward voltage / voltage drop changes. Below are how diodes are depicted.
Here are the famous ones! They are used in vintage effects, but also in modern ones for their special characteristics. They are easy to recognize with their glass capsule style. Their voltage drop is low, around 0.35V, but it can change depending on the model.
Diodes are mainly used to distort the signal, but can also be used to correct temperature bias of vintage germanium transistors. D9E diode shown.
Small Silicium Diodes:
They are smaller than their germanium diodes. However, their forward voltage is higher at 0.7V. They are the most used in overdrives and distortion pedals to generate saturation. The Electro-Harmonix big muff has (4) small silicium diodes. N14148 diode shown.
Silicium Power Diodes:
These bulky diodes are a bit bigger than small silicium diodes. Their breakdown voltage is also way higher: they can handle bigger voltages in their wrong polarity. Same voltage drop as the small silicium diode, around 0.7V. N4001 diode shown.
Voltage drop is determined by the color: Red 1.7V, Orange 2.0V, Yellow 2.1V, Green 2.2V, Blue 3.2V.
These diodes, named after M. Schottky, a German physicist, have the lowest forward voltage among diodes, around 0.15V.
Therefore, they are very useful to prevent polarity inversion, because the voltage drop is very limited compared to the 0.7V drop of silicium power diodes.
Instead of leaving the P-N junction “empty”, there is nano-metric layer of organic molecules that is added instead. This changes the physical properties of diodes. For instance, current can go both ways.
These diodes are still very hard to find in classic retail. They are more a laboratory experiment type of component created by effect manufacturer Dr Scientist.
You can also combine diodes. Two diodes in series will clip the signal less for instance:
It is also possible to make what is called “asymmetrical clipping”. Using two diodes on one side and one on the other. You can also mix the types of diodes…. Here is a new world of experimentation for you!
Diodes can also be used to protect your circuit against polarity inversions… Lets imagine for a minute that a customer of yours invert the battery in your made-with-great-care-and-patience effect. If there is no protection diode, there are good chances that a component of your circuit gets damages (especially polarized capacitors…). Or worse, that they heat too much, catch fire or even explode!
Always add a polarity protection diode. If the polarity of the power supply is inverted, it will not let current pass.