Short out the choke coil and notice that the RFI increases. You may detect this RFI by bringing an FM radio close to the dimmer circuit. Since there may be long lengths of mains wire between the triac and the lamp load which will radiate this RFI an L-C RFI suppression network is usually built into these types of circuits. It is greatest when the triac is triggered at 90 o and least when it is triggered at close to zero or 180 o of the mains AC waveform. Each time the triac is turned on the load current changes very quickly – a few micro seconds – from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. The main problem with this circuit is radio frequency interference (RFI). The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit. The ac supply is applied to the load for only a controlled fraction of each cycle. The circuit here controls the average power to a load through the triac by phase control.
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