Diodes kinds

A diode is like a one way valve for electrical current. Current can flow in one direction, and will be blocked in the other direction up to the diodes rated breakdown voltage.

Diodes of all types have two connections, an anode (A) and a cathode (K). With the exception of zener diodes, "conventional" current flow can only pass from the anode to the cathode and will always suffer a small drop in EMF. (usually about 0.6V) This effect is called "forward voltage drop", "forward" because it occurs when the diode is forward biased.

Before going on, a bit of terminology. A diode is often said to be "forward biased" or "reverse biased", which can be quite confusing if you're not in the know. When the voltage on the anode is greater than the voltage on the cathode by the diodes forward voltage drop, current will flow from the anode to the cathode and the diode is said to be forward biased. If the difference between A and K is less than the forward voltage drop the diode will not conduct, voltages less than the forward voltage drop are therefore lost.

Likewise, when the voltage on the cathode is greater than the voltage on the anode the diode is said to be reverse biased and will not conduct. (with the exception of zener diodes)

Diode Types:
The four most common diode types are pictured below:

1. Power Diode: Used whenever a diode junction is required to withstand high electrical current. The diode pictured here is rated for 6A and is quite bulky. Smaller 1A diodes are more commonly used. Almost all power diodes have a forward voltage drop of ~0.6V.

2. Signal Diode: The diode in the above image is a modern version of the original "cats-whisker" diode used in AM detection. Signal diodes have very small voltage drops, (often considerably less than 0.1V) in order to be used with low voltage signals. AM detection would be impossible with a power diode because the input signal is unlikely to rise above 0.6V.

3. Zener Diodes: Works just like a power diode with a similar power rating for current flowing from the anode to the cathode, and blocks current from cathode to anode up until the zener voltage. In other words, zener diodes block reverse current only up to a preset voltage which then becomes the voltage drop for current flowing from the cathode to the anode.

4. Light Emitting Diode: Words like a normal diode but glows with an intensity proportional to the forward current applied. The semiconductor which does the glowing in LEDs cannot withstand large current flow so the forward current must be limited to about 25mA. (depending on the LED)

Power Diodes:
Diodes designed to withstand heavy loads are called power diodes. The most commonly used power diode is the 1N400x. (Where "x" is any number, eg. 1N4004, 1N4001... etc) The 1N400x has a maximum rating of 1A and a voltage rating which varies with "x" but is almost always several hundred volts. 1N400x diodes all have a forward voltage drop of ~0.6V, as do most power diodes.

Power diodes are usually used in power supplies (for AC rectification or reverse power supply protection) but can be used in any situation where the "one way valve" operation of any diode is required with large current capacity.

AC Rectification:
Because diodes only allow current to flow in one direction they can be used to rectify AC current, providing a supply which does not reverse in polarity.

There's two ways to rectify an AC waveform; full-wave and half-wave rectification. Half-wave rectification involves discarding the negative (reverse current) part of the AC waveform, leaving a positive waveform with gaps (0V) where the negative half was. This is illustrated below.

Full-wave rectification is slightly more involved, taking the negative half of an AC waveform and inverting it to fill the gap that would result if it was simply discarded. Full-wave rectification is considerably more efficient than half-wave rectification as it utilises the entire input waveform. This is illustrated below:

The circuitry behind either method of AC rectification is quite simple. For half-wave rectification a diode is simply placed in series with the path of the AC voltage, blocking the reverse flow. (see image below) Full wave rectification is achieved with an arrangement of four diodes which directs both the negative and positive sides of the input wave to one direction. This arrangement of diodes can be found in a single package called a "bridge diode".

The full-wave rectifier (or bridge diode) is quite simple. No matter what side of the AC input the current is flowing from, the diodes force it to flow from positive to negative on the output. (this is conventional flow, or course) Trace the path current must take from each input and you will soon see how it works.

Another use for the bridge diode configuration is to protect against power supply reversal. If a DC power supply is connected either way across the bridge diode's input the correct polarity will be obtained on the output.

Signal Diodes:
Signal diodes have a different physical construction from other diode types in order to achieve very low forward voltage drops. Commonly used is the modern equivalent of a "cats whisker" diode, where the diode junction is created by a thin wire resting on a semiconductor. This sort of diode is used to "detect" (rectify) AM radio signals without eliminating the signal like a power diode would.

Signal diodes work just like power diodes, but without the larger forward voltage drop. Use a signal diode to pass signals which are close to or less than 0.6V.

Zener Diodes:
Zener diodes are an exception to the diode rule. They pass forward current like a normal diode with a standard ~0.6V forward voltage drop, but will only block reverse current up to the "zener voltage" which is specific to individual parts. When the zener is exceeded while reverse biasing the diode, the diode junction will conduct and the excess voltage will appear on the anode. (ie. the zener voltage is also the "reverse voltage drop")

This unusual behaviour can be used to clamp a voltage level in a circuit. If the zener diode is connected with it's anode to ground and the cathode to the positive supply, voltages in excess of the zener voltage of the diode will be shorted to ground and not appear on the positive rail. This can also be applied to logic inputs to protect sensitive integrated circuits from voltage spikes or incorrect connections. (see image below)


For more robust spike protection, place a small capacitor across the zener to absorb the transient voltages which can occur as the zener diode conducts. The larger the capacitor the more effective the circuit will be, but it will also have more effect on the rise time of voltages applied. (see capacitors page for more information) If the circuit is required to pass high frequency signals, (like serial data) then capacitors should be smaller so they don't interfere with the signal timing.

Light Emitting Diodes:
LEDs are diodes which glow a certain color when forward biased. The intensity with which they glow is roughly proportional to the current allowed to pass through the diode junction. LEDs typically have larger forward voltage drops than other diodes and require current limiting as the diode junction can only withstand small amounts of current. (~30mA) When a LED passes excessive current it will glow off color and heat up. If a red LED is glowing orange, it's about to open circuit and become useless plastic.

As this type of diode is more often used for emitting light than as a diode, much emphasis is placed on the visual appeal of most LEDs. You can get just about any color you want, in a variety of sizes and profiles, or even with more than one diode in the same package. Below is a picture of a tri-color LED in a standard 5mm package. The colors are red, green and blue and can be pulse width modulated to obtain just about any color.
Because LEDs can only withstand small amounts of forward current, they should always be used with a series resistor. The correct resistor for the voltage being used can be calculated with ohm's law. See the ohm's law section of the resistors page for more details.