How does an SCR power control circuit control a resistive load?

Answer 1

Two SCR's are connected in an inverse-parallel arrangement to one leg of a single phase circuit. This arrangement allows one SCR to control one half of the sine wave, and the other SCR controls the other. A special circuit is used, sometimes called a "trigger circuit". This circuit sends a pulsed signal to each gate of the two SCR's. The SCR's are so fast to open close the circuit, that they can be "fired" open, then "shut" off in increments far smaller than 1/60th of a second, the typical AC line frquency. So, if you were to tell the trigger circuit to fire each SCR for only 1/240th of a second, (1/2 of 1/2 of a complete cycle) 60 times a second, then each SCR would only be "on" or letting electric current through for 1/2 of each +/- cycle of the sine wave. Therefore output power would be 50% of maximum, but complete in the sense that the 60Hz cycle would remain intact. To put it another way you would still have an alternating current 60 times per second, but the upper and lower portion of the sine wave would be cut off early, and at the same points respectively. This is called Phase-Angle Control, and is the most common form of solid state power control. Resistive loads are the easiest to control with solid state techniques, though power factor is increased with the use of solid state switching. There are ways around that. The advantages of this type of power control is infinitley variable power output, with no mechanical contactors to wear out. SCR's can switch 100's of amps, hundreds of times per second, and as long as you keep them cool, theoretically they will never wear out. If you would like to know more, there is a site: http://www.payneb5.com , for Payne Engineering. They have a couple of animations and schematics depicting what i just explained, but with pictures.