Power source selection systems

A power source selection system, comprising:

• a primary source line configured to be connected to a primary source having a primary voltage;
• a backup source line configured to be connected to a backup source having a backup voltage;
• a voltage divider and limiter connected to the primary source line to receive the primary voltage and to provide a sense signal on a sense line;
• a NAND gate including a first input connected to the sense line to receive the sense signal and a second input connected to a voltage supply line configured to connect to a supply voltage, wherein the NAND gate is configured to provide a gate signal to a gate line based on the sense signal;
• a switchover circuit connected to the backup source line and the gate line to receive the backup voltage and the gate signal, wherein the switchover circuit is configured to output the backup voltage to a switch line in a first state and to block the backup voltage from the switch line in a second state, and wherein the switchover circuit is configured to switch between the first state and the second state based on the gate signal;
• and
• an ORing circuit connected to the primary source line and to the switch line, wherein the ORing circuit is configured to select between the primary voltage and the backup voltage and to output the selected voltage as an output voltage.