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VOLUME 6

Live and let diode

Hi guys and gals. Today we will take a look at a component that most people, regardless of their knowledge of electronics, probably interact with on a daily basis: the diode.

Diodes are based on a very simple concept: the PN (positive-negative) junction. While we will try to avoid turning this into an advanced quantum physics class, just keep in mind that standard current flow is from anode (positive) to cathode (negative) and that modern semiconductors are built on silicon that is “doped” with other elements to alter their conductivity. A PN junction is just two parts of silicon that are doped to control the current flow.

There, we now have a diode.

Diodes don’t really have any significant resistance and will simply conduct like a straight wire, or won’t conduct at all like an open circuit. In reality that’s not 100% accurate, remember when we said no resistance is impossible, but for this discussion, let’s act like it is. What they have though is what we call a “forward voltage” - which is a voltage at which they will start conducting.

Most silicon PN junctions, such as most silicon diodes, except LEDs (Light Emitting Diodes), have a Vf (forward voltage) of about 0.6 volts. LEDs are special in that they usually have a higher Vf but they still work on the same principles. Very popular with vintage effect builders, there are also PN junctions made with germanium instead of silicon. Those have a Vf that’s closer to 0.25 volts.

Basically, you can think of a diode like a pressure valve in a water pipe that won't let the water through until it reaches a certain pressure and if you were to measure the pressure on the other side, it would be the same as what came in minus the force required to open the pressure valve. In the case of a diode, it's called a "voltage drop" - the voltage on the other side will always be Vf lower than the voltage applied to it.

PN junctions also have a reverse voltage, Vr, but once that Vr is broken, the diode is usually fried, except for a special type of diodes called zener diodes but that’s a subject for another day.

In guitar effects, diodes have many uses. They can be used in power supplies or envelope followers for rectification, reverse polarity protection and clipping. Clipping is very common and is an electronic operation made on a signal with the intent of creating harmonic content, or distortion, which is hopefully pleasing to the ear. More on that later.

Diodes also have a maximum current limit which cannot be exceeded. Again, remember when we said that we can’t build a circuit with no resistance. If you feed your supply through a diode and then to ground with no significant resistance in its path, the current will reach way beyond the diode’s maximum current limit and fry it in seconds. Every diode needs at least a current limiting resistor which is normally calculated using Ohm’s law and the diode’s current rating which is available on the datasheet.

We will see many uses for diodes in detail later.

Talk soon!