Zero crossing switch


Posted on Jan 11, 2013

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
Click here to download the full size of the above Circuit.

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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