H-bridge using 2 relays

Dear All,
Greetings, kindly could anyone tell me how this circuit operate! (CHEck Attatchment) actually it works fine on the simulator. i need to know what the 2n2222 do to enable the relay?. is it pulls the connected pin to the ground although it is connected to the 7805 o/p?.

and what will happen if the transistor replaced by PNP ?

BMP image replaced with GIF format which is smaller and can be viewed in the browser [alexan_e]

Click to expand...

thanks

Frequently the control circuit lacks the power to energize a relay coil directly. It cannot source (or sink) much current at the output.

A transistor needs only a tiny current to turn it on. And it can sink or source sufficient current to energize a relay coil.

We tend to use NPN type because its method of operation is a little more intuitive. Also NPN's were easier to manufacture and had better performance specs in the earlier years of solid state technology.

Generally you substitute PNP's by flipping a circuit head over heels, and reversing diode directions, while keeping positive supply at top. Then you drop in PNP's where there are NPN's.

PNP type operation will be more certain if the bias sees low impedance to the positive supply. You turn it on by extracting current through the bias terminal. Putting the load in the emitter leg of a PNP may interfere with it being sufficiently biased. (In PNP nomenclature emitter means emitter of current holes.)

yes here the 2n2222 transistors are used to pull the relay terminal to ground as u guessed. The transistors are used to provide sufficient current for driving the relay.it need about 250ma which ofcourse varies with different relays. this kind of circuits are mainly used with microcontrollers for driving motors.

if you are using pnp relays must be in collector itself but its emitter willbe connected to positive and at base a resistor to ground will be used to turn the transistor on