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The thyristor is also
known as the silicon
controlled rectifier (S.C.R.).
It has the same
charactaristics as the
diode, current flowing
from cathode to anode,
when the anode is
positive with respect to
the cathode.
However, it will only do
this when the gate is
also positive with
respect to the cathode.
In the circuit, with the
switch open as shown, no
current flows.
When the switch is
closed, the diode begins
to conduct and current
flows from cathode to
anode.
There is a problem.
If the switch is now
opened, current
continues to flow.
Conduction can be
stopped by removing the
the cathode/anode
voltage.
Another method of
stopping current flow is
to reverse the polarity
of the cathode/anode
voltage.
If the thyristor is used
with an ac supply then
it will conduct on the
positive half cycles and
automatically switch off
during the negative half
cycles.
The resistor in series
with the gate connection
limits the gate current
to a safe value.
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