Download Diodes and Rectifiers

Diodes and Rectifiers
The difference between a diode and a rectifier is more a matter of use than
anything that shows in a schematic symbol. The same symbol is used for both. In general
a rectifier is used to change AC to DC in a power supply and is made for higher currents
than signal diodes.
Signal Diodes
Signal diodes are designed to work at low
currents (100 mA to 200 mA, maximum) and lower
voltages (under 100 Volts). They are typically silicon or
germanium, and used for low power applications like
logic, signal switching, waveform generation and such.
Fast diodes
Whether we are talking about signal diodes or rectifiers we have some devices
that are faster than others. In reality it takes a while to pump enough current into a diode
to get it to turn on. Once it is saturated and conducting it typically takes even longer for
the charges to flow out of a diode after we have stopped pumping. The charges inside the
diode drain off slowly. What we have just described is a capacitance. There is a built-in
capacitance to the junction in all diodes. This is inherent in the physical design of the
diode itself. Lower capacitance means faster speed of the diode. Some data sheets rate the
parts by capacitance. Some rate them by Turn-On and Turn-Off time in nano-seconds or
even micro-seconds. Some do not rate the speed at all. As a general rule, if it doesn't rate
the speed it is a general-purpose (not fast) rectifier (1N400x and 1N540x class device).
Schottky diodes
Schottky diodes differ from other diodes in their physical construction. They only
have about half the forward voltage drop we find in Silicon diodes. They are also a little
faster than normal silicon diodes. Their structure has a drawback. Where it is not hard to
find even a signal diode with a reverse voltage of 100 volts most basic Schottky diodes
have a limit of 20 to 40 Volts.
Constant Current Diodes
Constant Current Diodes are two terminal devices that are functionally designed
to work in reverse bias mode and keep the current in a circuit set at a certain level. Within
limitations this statement is true. More than just a reverse biased diode they will limit the
current flowing in a circuit. They will allow a lower current to flow in a circuit up to a
certain limit then limit current flow above that limit.
Varactors
Diodes have a certain capacitance when reverse biased. It takes a certain amount
of time to turn a diode on and more time to turn it off. This is effectively a capacitance.
We can construct a diode to enhance this capacitance and we can control it by the reverse
biased voltage we apply. By controlling the level of reverse bias we control the
capacitance of a diode used as a tuned circuit.