thyristor passes negligible current when reverse biased and when forward
biased the current is also negligible until the forward breakdown voltage,
e.g. 300 V, is exceeded. Thus, if such a thyristor is used in a circuit in
series with a resistance of 30 f2 (Figure 7.5), before breakdown we have
a very high resistance in series with the 30 f2 and so virtually all the
300 V is across the thyristor with its high resistance and there is
negligible current. When forward breakdown occurs, the resistance of the
thyristor drops to a low value and now, of the 300 V, only about 2 V
might be dropped across the thyristor. There is now 300 - 2 = 298 V
across the 30 f2 resistor and so the current rises from its negligible value
to 298/30 = 9.9 A. When once switched on the thyristor remains on until
the forward current is reduced to below a level of a few milliamps. The
voltage at which forward breakdown occurs is controlled by a gate input
current, the higher the current the lower the breakdown voltage. Thus, by
controlling the gate current we can determine when the thyristor will
switch from a high to low resistance.
As an illustration of the use of a thyristor, Figure 7.6 shows how it can
be used to control the power supplied to a resistive load by chopping a
d.c. voltage I,'. An alternating current signal is applied to the gate so that
periodically the voltage l,' becomes high enough to switch the thyristor
off and so the voltage I/" off. The supply voltage can be chopped and an
intermittent voltage produced with an average value which is varied and
controlled by the alternating signal to the gate.
Another example of control using a thyristor is that of a.c. for electric
heaters, electric motors or lamp dimmers. Figure 7.7 shows a circuit that
can be used. The alternating current is applied across the load, e.g. the
lamp for a lamp dimming circuit, in series with a thyristor. R~ is a
current-limiting resistor and R2 is a potentiometer which sets the level at
which the thyristor is triggered. The diode in the gate input is to prevent
the negative part of the alternating voltage cycle being applied to the
gate. By moving the potentiometer slider the gate current can be varied
and so the thyristor can be made to trigger at any point between 0 ~ and
90 ~ in the positive half-cycle of the applied alternating voltage. When the
thyristor is triggered near the beginning of the cycle it conducts for the
entire positive half-cycle and the maximum power is delivered to the
load. When triggering is delayed to later in the cycle it conducts for less
time and so the power delivered to the load is reduced. Hence the
position of the potentiometer slider controls the power delivered to the
load; with the light dimming circuit the slider position controls the
power delivered to the lamp and so its brightness.
Another form of electronic switching is provided by the junction
transistor. For the junction transistor in the circuit shown in Figure
7.8(a), when the base current 1B is zero both the base-emitter and the
base-collector junctions are reverse biased. When the base current 1B is
increased to a high enough value the base-collector junction becomes
forward biased. By switching the base current between 0 and such a
value, bipolar transistors can be used as switches. When there is no input
voltage F= then virtually the entire l,'cc voltage appears at the output as
the resistance between the collector and emitter is high. When the input
voltage is made sut~ciently high so that the resistance between the