24VDC
480VAC --- fuse ---resistor ---*---------*-----*------*---- regulator
                               |         |     |      |
                           transient    zener  |      |
                          suppressor          |      |
                               |              |    reverse
                               |              |    biased
                               |              |     diode
                               |             SCR      |
                               |              |       |
gnd ---------------------------*--------------*-------*