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Thyristors are switching devices that do not require any control current once they are turned on. A small control pulse of current is all that is required to turn them on and when that pulse is removed the thyristor continues to conduct, assuming a minimum holding current is maintained by such devices as SCRs or triacs. Then there are pulse generating thyristors in which a holding current is meaningless as in the cases of unijunction transistors and Shockley diodes.