Published:2009-7-5 22:34:00 Author:May | From:SeekIC
When switching loads with the aid of a thyristor, a large amount of RFI can be generated unless some form of zero crossing switch is used. The circuit shows a simple single transistor zero crossing switch. R1 and R2 act as a potential divider. The potential at their junction is about 10% of the ac voltage. This voltage level is fed, via R3, to the transistor's base. If the voltage at this point is above 0.2, the transistor will conduct, shunting any thyris-tor gate current to ground. When the line po-tential is less than about 2 V, it is possible to trigger the thyristor. The diode D1 is to re-move any negative potential that might cause reverse breakdown.
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