The line runs fine for months, then one morning the PLC is dead. The 24 V supply faulted, the program lost its state, and the machine comes up in a fault condition. Nobody pressed e-stop. The building power dipped for two seconds at 5 AM.
The 24 V rail is more fragile than you think
A PLC runs on 24 V DC from a switching power supply. The supply accepts 100 to 240 V AC and puts out a regulated 24 V. When the building voltage dips below 85 V for a few cycles, the supply drops out. The PLC loses power, the RAM state clears, and on restart it boots into whatever the boot program says.
A controlled shutdown takes time. If the power supply dies instantly, the PLC doesn’t get to save its state. Position values, recipe selections, and interlock latches all reset to default. On restart, the machine might try to start an axis that was mid-motion. That’s why the boot program should start in a safe, idle state, not resume where it left off.
What a UPS buys you
A small UPS on the 24 V rail keeps the PLC alive through a 500 ms dip. That’s enough to ride through most momentary sags. It’s not enough for a multi-second outage. For that, you need the supply to ride through long enough to park the axes and save state, or you need the machine to come back up in a safe condition.
Size the UPS for the PLC, the network switches, and the HMI. Don’t put the motor drives on the UPS. They draw hundreds of watts and will drain the battery in seconds. The goal is to keep the brain alive, not to keep the motors moving.
Redundant supplies for critical lines
On a line that can’t afford a dead PLC, use two 24 V supplies in parallel with diode OR-ing. If one supply fails, the other takes over without interruption. This costs double the supply price but removes a single point of failure. For a packaging line that runs two shifts, it pays for itself in one avoided stoppage.
Don’t just wire two supplies to the same rail without the diodes. Backfeeding one supply through the other can damage both.
Brownout reset vs hard reset
Most PLCs have a configurable behavior on power-up. Set it to come up in program mode, not run mode, until an operator presses start. This prevents the machine from surprising someone after a power blip. It costs a few seconds of restart time, but it’s safer than an axis moving on its own.
Log the power cycle. The PLC can record a timestamp in a retained register. When maintenance asks why the line stopped at 5:12 AM, you have the answer instead of guessing.
Bottom line
The power supply is the most boring and most failure-prone component in the cabinet. Size it with margin, add a small UPS for sag ride-through, and program the boot sequence to come up safe. The PLC itself rarely dies. The power feeds it.
