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In the circuit diagram,
block A generates pulses
at the same frequency as
the ac power supply.
Block B is a phase shift
circuit which enables
the phase of the pulses
to be shifted with
respect the ac supply.
These pulses are applied
to the gate.
In waveforms A and B,
the gate pulses coincide
with the start of the
positive half cycles of
the mains supply.
They turn the thyristor
on, and current flows
for the complete half
cycle.
There is no current flow
during negative half
cycles.
The current flow is
shown at C.
Waveform D shows the
gate pulses shifted and
occurring half way
through the positive
half cycles of the ac
supply.
The thyristor starts to
conduct at this time.
Current flow is shown at
E.
F and G show the gate
pulses occuring near the
end of the positive half
cycles, and current
flowing for a very short
time.
The circuit acts as a
lamp dimmer.
However, current only
flows during half of the
ac cycle, and full
brightness cannot be
achieved.
The high power thyristor
circuits can be isolated
from the control
circuits by means of
transformers or opto
isolators.
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