To get effective soft start control for an AC load such as a heater, be it a tungsten or nichrome wire type or a quartz rod type heater, it is necessary to actually limit the current into the heater. This can only be attained by using phase angle control which limits the applied voltage to the load to lower than normal level.

The cycle proportional type of control that I've presented in my other posting here is very effective for temperature control but is not so useful for soft start. A complete half cycle or whole cycle with zero voltage switching into a stone cold heater core is still going to conduct full current during that cycle.

As far as I am concerned the reason to use soft start is to control the immediate current surge at startup. This is effective for eliminating the 10 to 15X sized current surges through a cold circuit which could lead to fracture of wires that are under mechanical stress. Rapid thermal expansion could also lead to fracture and that can be controlled by either phase angle control or by cycle proportional type control.

Michael Karas