How SCR fires with a zero crossing and a speed reference

Discussion in 'General Electronics Chat' started by trherzfeld, Jul 2, 2015.

  1. trherzfeld

    Thread Starter New Member

    Jul 2, 2015
    Hello, I'm trying to come up with an explanation of how exactly this circuit works for a class. It is a circuit that fires an SCR based off a speed reference coming into the op amp of U3A from where D9 is. I'm not exactly sure how the SCR fires based off the zero crossing and the speed reference. I know that in order to adjust how much power is delivered to the load when firing a SCR, you have to change how quickly the gate triggers, so then it triggers at different points along the AC wave. I'm just not sure how the zero crossing and speed reference work together with that op amp and Q3 to the fire the SCR at different points along the wave. Also, I'm not entirely sure how that zero crossing works by itself. Thank you for any potential help you might be able to give.