There are, however, a couple of disadvantages of this simple circuit. Firstly, about 0.7V is lost in EACH of the transistor, so the maximum voltage cans That ever be applied to the motor is Vs – 1.4V. Secondly, the transistors are not absolutely linear so the change in motor speed for a given rotation of the potentiometer will from some more subtle in the middle of its range. Because a motor is an inductive load, it will from Produce a ‘back-emf’ Could the which damage to the second transistor. The 1N4148 signal diode prevents this damage by shorting out the back-emf.
The power supply for this circuit should preferably be un-smoothed (i.e. directly from the power supply rectifier). This helps prevent the motor ‘sticking’ at low speeds. With the TIP31C transistor given, the maximum power supply voltage may be 60V and the maximum motor current consumption may be 3A.