From page 240 of the GE SCR Manual (1979) is the following:




         E                    1
Eo =  ---------   sqrt(pi-a+ --- sin(2a))
      sqrt(2*pi)              2



        where

        Eo is the RMS output voltage
        E is peak AC voltage
        a is turn-on phase delay in radians (trigger triac a radians past
zero-cross)

        This is for full-wave control (triac or inverse parallel SCRs)
into a resistive load.
        Glad I didn't have to derive it!

Harold

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