collector and emitter drops to a low value, the transistor switches so that
very little of the Vcc voltage appears at the output (Figure 7.8(b)). We
thus have an electronic switch.
Because the base current needed to drive a bipolar power transistor is
fairly large, a second transistor is often needed to take the small current
and produce a large enough current to the base of the transistor used for
the switching and so enable switching to be obtained with the relatively
small currents supplied, for example, by a microprocessor. Such a pair of
transistors (Figure 7.9) is termed a Darlington pair and they are
available as single-chip devices. Since such a circuit is often used with
inductive loads and large transient voltages can occur when switching
occurs, a protection diode is generally connected in parallel with the
switching transistor to prevent damage to it when it is switched off. As
an indication of what is available, the integrated circuit ULN2001N
contains seven separate Darlington pairs, each pair being provided with
a protection diode.
Open-loop control of d.c. motor speed can be achieved by pulse-width
modulation (PWM). This technique involves the switching on and off of
a d.c. voltage to control its average value (Figure 7.10). The greater the
fraction of a cycle that the d.c. voltage is switched on the closer its
average value is to the input voltage. Figure 7.11 shows how pulse width
modulation can be achieved by means of a basic transistor circuit. The
transistor is switched on and off by means of a signal applied to its base,
e.g. the signal from a microprocessor as a sequence of pulses. By varying
the time for which the transistor is switched on so the average voltage
applied to the motor can be varied and its speed controlled. Because the
motor when rotating acts as a generator, the diode is used to provide a
path for the current which arises when the transistor is off.