Diodes

Diodes

Put simply, a diode is a device that behaves like a one-way valve, allowing current to flow in one direction, but not the other.

Diodes are used most recognizably in rectifiers, in order to convert AC to DC.


Characteristics

Vfbr - forward breakdown voltage. This is how much forward-bias voltage the diode can handle.

Vbr - reverse breakdown voltage. If you put too much voltage across the diode the wrong way (called reverse bias), it will pop. This is the maximum reverse voltage rating.

Vbo - Forward breakover voltage. This is basically how much voltage you have to put across the diode before it will begin to conduct. For germanium, it's about 300mV or 0.3V. In silicon diodes, this is typically around 600mV or 0.6V.

Vrbo - Reverse breakover voltage can be virtually any value. However, forward breakover will usually stay within the 0.3V or 0.6V, depending on whether it is germanium or silicon respectively.

Note the separate usage of 'breakover' and 'breakdown'. Breakdown in this context generally means that it's not supposed to happen, e. g. a failure. However, exceeding breakover voltage is normal since this has to happen for the diode to function correctly.


Diode capacitance & Varactors

Diodes show an intriguing characteristic when reverse-biased: their junction capacitance will vary with the amount of voltage applied.

There are diodes made specifically for the purpose of utilizing this variable-capacitance characteristic, and they are know as Varactor diodes.

When reverse biased a varactor diode's junction capacitance will decrease when the voltage across it decreases. Typical capacitance range is around 0.50pF to 110pF.

Recifier:

A rectifier takes AC and converts it to DC. Semiconductor rectifiers use diode arrangements (there are also tube rectifiers, but that is for another page) in order to do this.

There are two types of rectifiers - full wave and half-wave.

[insert diagrams of full and half wave rectifiers]
Half-wave rectifiers basically work by blocking the undesired half of the AC wave.

Full-wave rectifiers are the most widely used for their efficiency, and the cost for one or three extra diodes isn't enough to worry about for most manufacturers. One if the transformer is center-tapped and three if it is not.

Zener:

Another useful characteristic of diodes is that when their breakdown point is reached, the voltage across them remains relatively constant.

Zener diodes are specifically made for this purpose. In the case of Zeners, it is perfectly safe to exceed the reverse breakdown voltage and that is their purpose.

Zener diodes are typically used in power supplies where an accurate and stable output voltage is required.

In use, Zeners are typically biased with a reverse current of around 3mA to get it off what is called the 'Zener knee'. It is useful to note that the higher the bias current, the less noisy the Zener is.