Zero crossing switch

  
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. Rl 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, the transistor will conduct, shunting any thyristor gate current to ground
Zero crossing switch - schematic

When the line potential is less than about 2 V, it is possible to trigger the thyristor. The diode Dl is to remove any negative potential that might cause reverse breakdown.




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