Zero crossing and EMI

As switch-on for each heating element occurs when the MOC3041 chip detects rising zero crossing of the main power supply, and switch-off of the power triac naturally occurs when the voltage falls to zero crossing at the end of a half cycle of that supply voltage, sudden voltage ramps are avoided. Sudden voltage ramps occur during every half cycle of the main power supply when triacs are phase controlled, as in small light dimmers, and this is a major source of electromagnetic interference, EMI. It is therefore totally irresponsible to use such "dimmer" circuits to control large power loads unless the controller is certified as being fully and effectively screened to eliminate EMI. Switching on and off at zero crossing, and then only twice every two seconds, greatly reduces the possibility of EMI. However, as no zero crossing detector circuit is perfect, and we want to suppress even tiny amounts of EMI that may occur, we've included an RF choke in our controller circuit to be on the safe side. In this manner, with EMI suppression and with constant loading on the power supply, we've ensured that this controller is as "clean" as it's possible to get with a home-made device.

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