Diodes

Diodes

The diode is an electronic component, which allows current to flow only in a single direction. The rectifying action of diode is put to good use in radio receivers and power supplies.


Signal Diode and Rectifier Diode



The signal diodes can handle only small currents. But the rectifier diodes can handle much current and they are specially used to rectify alternating currents.



This is a rectify bridge circuit which is used to full wave rectification.

There are pre-made rectifier bridges available. Following are some of them.





Zener Diode



Refer to the characteristic curve of a typical rectifier (diode) in the figure below. The forward characteristic of the curve we have previously described above in the diode section. It is the reverse characteristics we will discuss here.



Notice that as the reverse voltage is increased the leakage current remains essentially constant until the breakdown voltage is reached where the current increases dramatically. This breakdown voltage is the zener voltage for zener diodes. While for the conventional rectifier or diode, it is imperative to operate below this voltage; the zener diode is intended to operate at that voltage, and so finds its greatest application as a voltage regulator.





The basic parameters of a zener diode are:
(a) Obviously, the zener voltage must be specified. The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available.

(b) A tolerance of the specified voltage must be stated. While the most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available. A test current (Iz) must be specified with the voltage and tolerance.

(c) The power handling capability must be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1, 5, 10, and 50 Watts.


Light emitting diode







The rectifier diode and the zener diode are made by creating a p-n junction between two types of semiconductor (n-type & p-type). This sort of junction acts as a one way valve for current flowing through it.

By using specially selected p-type and n-type semiconductor it is possible to make the p-n junction emit colored light to produce a light emitting diode (LED). The light is emitted when current flows through the LED in the forward-biased direction. The LED is used as a low current indicator lamp in many types of consumer and industrial equipments, such as hi-fi system and machinery control panels. The LED is also used in seven-segment displays to show numbers and letters. By selecting the right type of impurity in the semiconductor compound (usually gallium or arsenide, phospide), the color of the light emitted can be made red, yellow, green or blue. The LED does not emit light when it is reverse biased.

LEDs are used in following ways also in practically.


Bar graph Dot matrix Starburst 7-segment


Photo Diode



In the photodiode a small current flowing in the reverse-bias direction varies with the amount of light, which reaches the junction. The photodiode is used in camera light meters to measure light intensity, and also for rapid counting of the rotation of the wheel in control equipment. The photodiode is able to respond much faster to light change than the light dependent resistor (LDR).