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A thyristor is a semiconductor device which acts as a
switch. However, when switched on it can only pass current
in one direction. It is in fact a switchable diode
sometimes known as a silicon controlled rectifier. To
switch alternating current two devices are connected in
inverse parallel.
Each device is turned on at the appropriate time by a
trigger pulse applied to the gate and the device will
remain on until the instantaneous load current through it
drops to zero.
The trigger pulses are generated by a driver - which times
the pulses to ensure the thyristor unit output is a
function of the input demand signal (e.g. from a
controller) in the selected Firing Mode.
Eurotherm thyristor units incorporate the thyristor
devices, together with the driver and the heatsink which
dissipates the heat generated by the thyristor devices.
Many units include internal high speed fuses to protect the
thyristors from high overload currents. The benefits of
thyristor units are numerous. They offer a reliable long
term alternative to electromechanical devices reducing the
necessity for ongoing maintenance.
Thyristor units are particularly cost effective for fast
systems, for complex loads involving transformers and/or
heaters whose resistance changes with temperature or time
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