One disadvantage of the simple Darlington pair motor speed control discussed in the previous article is that about 0.7V is lost in each transistor, so the maximum voltage that can ever be applied to the motor is Vs - 1.4V. The circuit shown in figure 1 partially solves this problem by reducing the voltage loss to 0.7V. It is still a Darlington pair configuration, but this time the second transistor is of a type known as 'PNP'. The first transistor is an 'NPN' type of transistor.
To increase the range of the potentiometer you may want to place a low value resistor (try 470Ω) between the lower end of the potentiometer and 0V. This should start the motor turning slowly as soon a the potentiometer shaft is turned.
Building the Circuit
A stripboard layout for this circuit is given in figure 2. The layout includes space for an optional 6-way screw terminal block to make connecting up the circuit easier.
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 TIP32C transistor given, the maximum power supply voltage may be 60V and the maximum motor current consumption may be 3A.
Pin-out diagrams of the two transistors used in this circuit are given in figure 3. The TIP32C transistor will get hot in use because the high current supplying the motor flows through it. You should therefore fit a heat sink to it to dissipate the heat - a suggested heat sink is given in the Shopping List, or you could make one out of Meccano! Space is provided in the stripboard layout for a heat sink, but the positions and sizes of the holes are not shown for they will depend on the nature of the heat sink you choose.
The list below includes all the parts required to build the Darlington pair circuit on stripboard as shown in figure 2. The 470 Ohm resistor is optional.
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