What is the difference between Zero Fired and Phase Angle Fired SCR Power Controls?

Phase Angle Fired SCR Power Controls

Phase Angle SCR Wave

Phase angle SCR power controls adjust the output voltage/wattage by delaying the turn on of the output voltage during each half cycle.  The end result is very smooth virtually infinite voltage control.
 
1) Can be used with resistive or transformer coupled loads.
2) These units are available with current limiting and soft start options.  This allows the SCR control to be used with heating elements that significantly change resistance  from cold to hot.
3) Other options such as current or power regulation are available.
 

Zero Fired SCR Power Controls

Zero Fired Wave Form

 

Zero fired controls adjust the average wattage over time by cycling full line cycles on and off.  Zero fired controls only switch the line voltage of the heaters when the sine wave from the source passes through zero volts.  The voltage is cycled very rapidly for virtually infinite voltage/wattage control.

 
1) Since the load is only energized at zero volts there is very little electrical noise generated.
2) Zero fired controls are for resistive loads only.  Should not use zero fired with transformers or other inductive loads.
3) Should not be used with loads that have large resistance changes from cold to hot.