THYRISTORS
Thyristors are high speed switching devices which can be used for
switching currents in DC circuits or for controlling power dissipation in AC
circuits. Thyristors control power in AC circuits by switching the current
on during part of the cycle only. Very little power is dissipated in the
thyristor itself. They are much more efficient than rheostats for controlling
power in AC circuits and are rapidly taking over in applications such as
controlling lights in theatres. Fig 1 shows the symbol used to represent a
thyristor.
+V
Anode
Load
Gate
Trig
Pulse
Cathode
0V
Fig. 1
Fig. 2
Thyristors have three leads - the anode, the cathode and the gate. The
symbol used is similar to that for a diode with the addition of a gate. They
are sometimes referred to as silicon controlled rectifiers (SCRs). This
term should become clear as you read through the notes.
SWITCHING PROPERTIES OF A THYRISTOR
A ordinary silicon diode will conduct when forward biased i.e. when the
anode is more positive than cathode. A voltage of about 0.7V is dropped
across a forward biased diode.
A forward biased thyristor will only start to conduct when a small current
is established in its gate-cathode junction. Conduction will then continue
in the anode-cathode circuit after the gate current has been switched off.
A trigger pulse can be used to latch a forward biased thyristor (Fig 2). A
voltage of less than 1V is dropped across the anode-cathode of a conducting
thyristor.
The switching action could be compared to that for a transistor. The main
difference is that the base-emitter current must be maintained to hold a
transistor switch in saturation. There is no linear region between the ON
and OFF regions in a thyristor.
1
Thyristor Latch
Thyristors are often used for switching large currents in DC circuits. They
are better than mechanical switches for switching large currents since they
contain no moving parts or contacts which can become damaged due to
‘arcing’. We have already seen how a small gate current can be used to
switch a much larger current in the anode-cathode circuit. We shall now
consider how the anode-cathode current can be switched off.
Once the thyristor has been switched on by applying a trigger pulse at the
gate further pulses at the gate will have no effect. Anode-cathode current
could be switched off using a mechanical switch in the supply line. This
method would defeat the reason for using the thyristor as a switching
device. The mechanical switch used for this purpose would have to be able
to handle the large current taken by the load.
The method most commonly used to switch off a thyristor is to apply reverse
voltage across the anode-cathode for a short period of time. Fig 3 shows how
capacitor commutation can be used for switching off a thyristor.
+12V
LOAD
R
C
A B
S
0V
Fig. 3
When the thyristor is triggered, the anode will be near 0V and the capacitor
charges up through resistor R. Plate A will be near 0V and plate B at +12V.
The thyristor can be switched off by momentarily closing switch S.
When switch S is momentarily closed, it pulls down plate B from +12V to
0V. Voltage at plate A falls by the same amount i.e. from near 0V to near
-12V. This action applies reverse voltage across the anode-cathode and
switches off the thyristor. The capacitor charges up through the load and
plate A eventually reaches +12V.
2