Isolation of trigger sources in AC voltage controller

In a single-phase AC voltage controller, the two anti-parallel SCRs do not share a common cathode, so their gate circuits must be electrically isolated from each other (and from the power circuit).

        +---T1 ->|---+
  A o---+            +---o B --[Load]
        +---|<- T2---+
   K(T1) = B,   K(T2) = A
   G1 referred to B, G2 referred to A

 Gate pulse ->[Pulse Tr 1]-> G1,K1
 Gate pulse ->[Pulse Tr 2]-> G2,K2

Reasons:

• A gate signal is always applied between the gate and its own cathode. The cathode of T1 is at point B and the cathode of T2 is at point A.
• Points A and B differ by the voltage across the controller, which can be the full peak supply voltage (325 V for a 230 V supply) when both SCRs are off.
• If both firing circuits shared one common reference, points A and B would be connected through the trigger circuits. This would short the SCRs and the supply through the gate circuits, damaging them and causing false firing.

Method: use a separate pulse transformer (or opto-coupler) for each SCR, so that each gate–cathode pair floats at its own potential while the control circuit stays at ground potential.