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Diodes

Definition:  A Diode is a semi-conductor silicon device which passes current in only one direction. Think of it as a one way valve. It is constructed of a semi-conductor material similar to transistors. It has two terminals, positive and negative.  The direction of current is from positive to negative. See sketch for diode symbol and picture of markings on a typical diode.

Simple circuit demonstrating diode action:

In the sketch on the left, the diode is oriented so that current from the 12 volt battery can flow to the lamp thus permitting the lamp to turn on. In actuality there will be a small voltage of  0.6 to 1.7 volts when conducting.

In the sketch on the right, the diode is oriented so that current from the 12 volt battery cannot flow to the lamp thus preventing the lamp from turning on.

Diode Applications

Notice in the figure below that the diode passes only the positive part on the sine wave, thus creating a relatively poor positive only dc power supply.  The resulting dc value is also shown in the sketch. This process is known as ½ wave rectification.

Notice in the above circuit how the diode combination passes the positive and the negative part of the sine wave, thus creating a relatively positive only dc power supply which is better than the previous example.

During the positive half of the sine wave the current will pass from the source through diode #1 , through the load and then through diode #2. During the negative half of the sine wave the current will pass from the source through diode #3, through the load and then through diode #4.

The resulting dc value is also shown in the sketch. The process is known as full wave rectification. In practice, as part of power supply design, a capacitor is often added across the load to provide a dc voltage with minimum variations or ripple.

There are many types of diodes which have many applications in the model train world such as the following:

Light Emitting Diodes (LEDs) possess all the properties of a conventional semiconductor  diode  described in another section which passes current in one direction with the additional property of emitting light.   Refer to the following link for more information:  Light Emitting Diodes

A Zener diode is a special case of  ordinary semiconductor diode.  Specifically, the Zener displays the following property: The direction of current is from positive to negative and it blocks current in the minus to plus direction (Reversed Bias).  If a positive voltage is applied to the anode terminal of a Zener diode and a negative voltage is applied to cathode terminal, the Zener diode will behave as an ordinary semiconductor diode.  However, when a positive voltage is applied to the negative, the Zener diode will breakdown when it is reversed biased by a voltage greater than the breakdown voltage. As a result a Zener voltage (Vz) will appear across the diode terminals, with its cathode being positive and the anode being negative.   Refer to the following link for more information: Zener Diodes

Infrared Diodes  are different regular LEDs in the following ways:  They do not give off visible light. They have a lower forward voltage and a higher rated current.   Infrared Leds are used in remote control devices.  In the world of model railroading use Infrared hand held wireless throttles and wireless receivers.

Bridge Rectifier Diodes are comprised of a package of four diodes configured as described in a previous paragraph associated with full wave rectification.  They are available in various voltage and current ratings.

Switching Diodes as the name implies are used  in switching and logic networks.  The 1N914 is an example of such a diode.

High Current Diodes  as the name implies are used  in high voltage power supplies, 6 amps to 100 amp range.

Schottky Diode has the following properties: Low Forward Voltage Drop = 0.15 to 0.45 volts when conducting. Typical Voltage Drop for regular semi-conductor diode=0.6 to 1.7 volts when conducting .  High switching action.

Laser Diodes have the following properties: Transmits analog or digital signals for sending through optical fibers via a very tiny beam.

High Voltage Diodes  as the name implies are used  in high voltage power applications,  10kvolts max

The Infrared Laser Diode is similar to the Laser Diode except it is based on the gallium arsenide compound in the 750 to 900 nanometers wavelength in the infrared spectrum or the indium gallium phosphide in the 1200 to 1700 nanometers in the infrared spectrum.

A Surface Mounted Diode (smd diode) is one that is mounted directly to a Printed Circuit Board (PCB)  without the use of wire leads attached to the diode which in turn are fed through holes in the circuit board.

A Current Regulator Diode is similar to a zener diode except that it regulates current over a wide voltage range.

In general, the following vendors are recommended for any of the above electronic components:

Radio Shack local store or at www.radioshack.com

Futurlec at www.futurlec.com

DeMar Electronics at www.demarelectronics.com

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics

 

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