Transistor motor controller problem

[In2Dusk]

I am using TIP122 NPN transistor to control geared DC motor using this scheme:

I ordered transistors and they arrived today so I tested them in this circuit:

But the motor was rotating at just half speed as when connected straight to battery (without the transistor).
What could be the problem? Please help me...

 

[IanP]

The Emitter voltage is roughly 2*0.6V =>1.2V lower than the Base voltage (TIP122 is Darlington, that is, has two transistors inside) ..
In a bridge like yours you should consider PNP transistors as the "upper" switches ..

:wink:
IanP