Our circuit of a priority switch is shown in Figure 1 and uses SCRs as basic elements. Assuming that both SCRs are off and S1 is closed before S2, the SCRs are activated first, providing current to Load2.
Figure 1 Priority switch.
With the SCR2 turned on, the voltage in its anode falls to about 2 V, which is not enough to turn on SCR1 if S2 is closed.
Note that this circuit is self-latched, meaning that to turn the triggered SCR on or off, the power must be cut momentarily. The SCRs must be mounted on heat sink if the loads draw more than 500 mA.
