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the voltage across the C leads the mains voltage by some angle dependent on the value of the pot. This AC voltage is applied to a bi directional diode to the gate of the triac, which then fires (goes short circuit) until the mains goes through a zero volts in its cycle. the incoming mains then rises again and the process is repeated. Altering the pot changes the time at which the triac fires and so varies the shape of the AC current pulse going through the load. These switches produce a lot of interference hence the inclusion of the 50 micro H choke and the .iMF capacitor as a filter.