zero crossing dimmers
"Dimmers can't be zero-crossing, how could they work?."
This is bit more complicated. In fact I have not seen such a "dimmer" yet. However the statement was brought in by me by transferring the solution from a heater control device.
Traditional dimmers: it is a phase controlled (say it is a triac) device, where - by altering the firing point of the triac within the 0-180 degrees angle we will achieve the 100-0% control of the brightness.
The zero crossing device: It can not use the "phase" controll because it has to turn ON and turn OFF at "zero". Rather then controll the phase, we need to control the number of AC cycles the device is ON and OFF.
For a crude example, if we could turn the load ON at "zero" for just one cycle then keep it OFF for one cycle, then turn it ON again for one cycle e.t.c. .. we will have a 50% power point.
If we keep it ON for two cycles and OFF for one cycle e.t.c. - we will be at around 66%
If we keep it ON for 3 cycles and OFF for one cycle e.t.c. - we will be at around 75%
If we keep it ON for 1 cycle and OFF for 3 cycles - we will be at around 25%
This will work ofcourse but the zero crossing devices do have their own "time delay" - on an average it takes about 8[ms] to ensure the turn ON/OFF at "zero". One mains cycle lasts 20[ms] for the 50Hz frequency or 16[ms] for 60 Hz (2 zero crossings in each Mains cycle).
If we are trying to do the light dimming application we will have to do it pretty fast to avoid flickering, thus it has to be done on a cycle by cycle basis - therefore number of possible power control points in one "control cycle time" is limited to the toal number of (half) Mains cycles the device is OFF without introducing the "flicker".
For a heating application however, this solution is much easier because the time inertia of a heater is larger than the inertia of a lighting bulb.
With heater then - we can easy establish a control cycle of (say) 3 seconds that gives us 3 x 50 x 2 = 300 possible "zero" crossing points for 50Hz and 3 x 60 x 2 = 320 possible "zero" crossing points for mains frequency of 60Hz ( number of zero crossings = one control cycle time lenght in seconds times the mains frequency in Hz times the number of zero crossing in one mains cycle).
Having 300 control points to turn ON or OFF in one control cycle of 3 seconds gives a pretty good resolution for the power control of 100%/300= 0.3%
There will always be ONE mains cycle lost for the turn ON/OFF operation but the benefit of zero crossing is to obvious to pass by.
Also, the larger is the inertia of the load (heater,furnace etc) - the control cycle time can be elongated giving even more zero crossing points per control cycle and even finer resolution of the power control.
This is little above the original subject of reducing the mains power for the XMAS lights but I felt compelled to respond to the query of "zero crossing" dimmers.
Regards,
xanadunow