〖 mod note: moved from technical forum, so template is missing 〗 〖 mod note: click on thumbprint image to see legible image 〗 I was wondering if anyone could provide some more insight as to how to find the current through Rc. The first picture TRANSFER1 I can find the current Ic through Rc by the following method: Ic = (Vin - Vout)/Rc Vout = -Vin(1/(R*s*C)) Ic = Vin(1+1/(R*s*C)) However, if I add a voltage divider network before the input, the problem becomes much more difficult for me here is my latest attempt: Ic = (Vin - Vout - I1*R1)/Rc Vout = (-Vin+I1*R1)/(Ra*s*C) Ic = Vin + (Vin-I1*R1)/(Ra*s*C) - I1*R1. Now the problem is that this current will be multiplied by CTR from an optocoupler then multiplied by a pullup resistor to get a voltage Vfb on the other end. There's no way I can think of to write this equation as Vfb/Vin. I'm essentially modelling a TL431 and an optocoupler per the following: http://www.onsemi.com/pub_link/Collateral/TND381-D.PDF on page 18 I appreciate any help and if this is not the appropriate place to post this, please let me know where I can. I'll keep trying and if I figure it out I'll update this post. Thanks in advance.