Transistor basic connection for switching

[Aman bharti]

http://www.circuitspedia.com/transistor-as-switch-working-how-transistor-works/

For more detail click here

A small Positive current  required at the Base terminal for the turning on the transistor.  By sending varying levels of current to the base, the amount of current flowing through the collector to emitter may be regulated.

When we apply a small Positive supply at base , then Current between emitter and collector will passed and we say that transistor is turn on.  A very small amount of current may be used to control a large amount of current, this property is known as amplifier . .

A Diode with Parallel of relay coil in reverse connection is necessary for protection the transsistor. This must connected on both PNP and NPN

 

[audioguru2]

Why do you show text that talks about an NPN transistor but you show a schematic with a PNP transistor?

The Circuitspedia PNP circuit is missing the important diode parallel with the relay coil. The diode is shown on their NPN circuit.