What is a thyristor and how do they work?
It's a variation on a transistor, (well two or more heavy duty transistors in a package).
Generally they are used to switch on/off a large current across it's two main power terminals by applying a small current to a third terminal called a gate.
(Switching OFF is a little more complicated, entailing that the main power source has dropped or has reversed.)
They more or less replace the job that used to be done by electromagnetic relays.
Their most interesting property is the "latching" property. Once turned ON they stay ON until the current through them is turned OFF by other means. In an AC circuit the current reduces to zero twice per cycle so turning them OFF isn't usually a problem
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