(ii) Electronic change over of transformer taps. Fig. 21.12 shows the circuit of electronic change over of power transformer input taps. Two triacs TR1 and TR2 are used for the purpose. When triac TR1 is turned on and TR2 is turned off, the line input is connected across the full transformer primary AC. However, if it is de- sired to change the tapping so that input appears across part AB of the primary, then TR2 is turned on and TR1 is turned off. The gate control signals are so controlled that both triacs are never switched on together. This
avoids a dangerous short circuit on the section BC of the primary.