Electricity

Free online electricity lessons for elementary school,  middle school and high school.

 

Electrical components

Diodes

1) Diodes and LED

Diodes are two-terminal components.
There are two kinds of diodes:
- The simple diodes.
- Light emitting diodes (LEDs) that operate as simple diodes but emit light when electric current passes.

Diode standard symbol	LED standard symbol

Comment:
Red or green LEDs are very commonly used in electrical devices (televisions, computers etc.) and serve as controls that indicate operating condition or standby.

2) The fragility of diodes

Diodes are very vulnerable two-terminal components that are easily damaged by excessive electrical current. So they are never used alone and often protected by two-terminal components called resistors whose roles is to limit the electric current.
Standard symbol of a resistor:  

American standard symbol:  

3) Diodes and electric current

Diodes and LEDs are polarized so they don't behave the same way if electric current is reversed.
Example:
We realize ​​two circuits that differ only in the sense of connection of a LED


We can observe the LED lights in circuit n°1: it is traversed by an electric current.
We can observe that the LED remains off in the circuit n°2: no current flows.

In circuit 1 we say that there's a bypass LED.
In circuit 2 way there's a blocking LED.

We can distinguish a bypass diode and a blocking diode on a diagram by comparing the current direction and the diode symbol: when they are pointing in the same direction there's a bypass diode.

A bypass LED
A blocking LED

Comment:
A bypass diode can be compared to an open switch whereas a blocking diode can be compared to a closed switch.

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