The TIP139 is also in emitter follower and supplies current to the motor. The emitter voltage is fed back to the inverting input of the 741. An op-amp with negative feedback will acts to maintain the balance between its inputs. The voltage applied to the non-inverting input via the control will be the same as the inverting input and this ensures a constant voltage across the motor.
The constant voltage means that the motors speed will be maintained, independently of the mechanical load applied to the motor. Should the motor load increase, then the motor armature takes more current, this causes the voltage at the inverting input to fall, and the op-amp will increase its voltage output, countering the effect of increased motor load.
With motor circuits, always measure the current drawn by the motor with a load, as this is the current the series pass transistor has to supply. Dissipation in the transistor will be the product of collector-emitter voltage and emitter current. This must not exceed the limit for the transistor, the BD139 can handle 8 Watts of power maximum, maximum collector current is 1 Amp and maximum collector-emitter voltage is 80 Volts. Alternative power transistors such as TIP31C may also be used.
The 1N4002 rectifier dissipates any back EMF from the motor when the power supply is removed.
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