Crystal Diode is a two terminal active electronic device that is made of a p-n junction in itself. It can convert AC signal to DC. In ancient days a special type vacuum tube is used as diode. The semiconductor diode is known as crystal diode also.
Various kind of crystal diodes are used in electronic circuitry. Their symbols are different according to type. Most common symbols are shown below:
- Diode acts like short circuit in forward bias and current flows through it easily.
- It acts like open circuit in reverse bias and current can’t flow through it easily.
- Forward voltage drop is 0.7V for Si and 0.3V for Ge.
- Special type diode like zener diode has precision reverse breakdown voltage.
- Some diode like LED emits light in forward bias.
Ideal Diode and Real Diode:
The diode which has zero resistance in forward bias and infinite resistance in reverse bias is called Ideal Diode. But in reality we cannot find this type of diode. In actual practice a diode has very small amount of forward resistance and large amount of reverse resistance. This type of diode is called Real Diode.
Diode has two connecting terminals, one is anode and another is cathode. Anode is connected with p-type material and cathode is connected with n-type material which will be cleared from figure in right side.
Forward and Reverse Bias:
When we connect positive terminal of a battery to anode and negative terminal to cathode the diode gets forward bias and current flows through it, and otherwise it gets reverse bias and current can’t flow through it.
Connecting with Circuit:
It is necessary to connect a diode with circuit in proper direction. A silver colored band is marked in one side of diodes’ body. This side indicates the cathode terminal and another is anode. We should connect a diode on a diode’s symbol on PCB and it will be anode to anode and cathode to cathode connection.
It is true that sometimes we can find some faulty diodes among new diodes which are bought from the market just now. A faulty diode can’t perform proper circuit operation. So we should find out faulty diodes and reject them to make a circuit.
Good Condition: Place the selector knob on diode symbol of multi-meter range. Connect red probe of multi-meter with anode of diode and black probe with cathode. A value will show in meter’s display and it may be 0.3 to 0.7. Here the diode is forward biased now by the internal voltage of meter’s battery and the value which showed on display is the barrier voltage of the diode. Now reverse the previous connection and observe reading. Now a value will show in meter’s display like .0L. That means diode is reverse biased and no current flows through the diode. If any diode shows this type of reading we can say the diode is good to use.
Short Circuit Fault: If any diode shows 0.00 value in multi-meter’s display for both forward and reverse connections then we can say the diode is short circuited and it should not be used.
Open Circuit Fault: If any diode shows .0L value in multi-meter’s display for both forward and reverse connections then we can say the diode is open circuited and it should not be used.
Leaky Fault: If any diode shows the ohmic resistance near to zero in forward bias and some amount of resistance in reverse direction then we can say the diode is leaky faulted and it also should not be used. We can test leaky fault by an analog ohm meter easily.
In Rectifier Circuits: Diode is frequently used to convert AC to DC in various rectifiers and SMPS circuits. For example, a bridge rectifier circuit is shown below:
A Rectifier Circuit
In voltage stabilization Circuits: Zener diode is used to create a DC voltage stabilizer circuit. A voltage stabilizer circuit is shown in the following example:
A Rectifier Circuit with Voltage Stabilizer
In clipping circuit: Diode is frequently used in clipping circuits. A clipping circuit is shown below:
A Clipping Circuit