الدايود DIODES

مصطلحات مفتاحية


A diode is a very simple device and it has a lot of applications. We will cover some of its uses and explain exactly how it works in very simple terms.   
If you don't understand any of the points in this discussion, you can contact Colin Mitchell.

A diode is a device that passes current in only one direction.  It is a bit like a water-valve that prevents water back-flowing into the mains from your property. Or a valve in a pump that prevents the water flowing back down a well. 

There are many types of diodes to handle small currents, large currents, high frequencies and high voltages. And there are diodes made from different materials, but they can all be described in a simple way. And that's what we will do. 

A diode has two leads. These are called ANODE and CATHODE.

The cathode end is identified in a circuit diagram and on the body of the device. 
It may be identified with a line, chamfer or dimple or a symbol. There must be something on the diode that identifies this lead and you have to look for it.

A diode does NOT have a positive or negative end. You see this mistake in so many discussions. A diode will have a positive voltage on the anode and a slightly lower (positive) voltage on the cathode. It will not have a positive on the anode and negative on the cathode.

Incorrect marking with "+" and "-"

In the following diagram only the CATHODE is identified with the letter k (for kathode). The other lead is the ANODE.

Correct marking with "k"

Here is a pictorial way to understand how a diode works:



9v comes out of the battery and when it passes through the diode, 07v is LOST (dropped across the diode), resulting in 8.3v available to operate a motor etc.

The most common type of diode is made from SILICON. It can also be made from GERMANIUM. You need to look in the datasheet to find the composition of the diode you are using.

As mentioned above, a diode does not start to TURN ON until a small voltage is present on its ANODE lead. 
For a Germanium diode, this voltage is approx 0.3v. 
For a Schottkey diode, this voltage is 0.3v
For a Silicon diode, this voltage is 0.7v. As the current increases, this voltage can rise to about 1.1v (at full current-flow for the diode). 



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