Undersized power supplies are one of the most common causes of intermittent panel failures. A system that works perfectly on the test bench can behave erratically in the field because the power supply cannot handle the real-world load profile. This article provides a practical, step-by-step method for correctly sizing a 24V DC DIN rail power supply.
Step 1: List Every Connected Load
List every device drawing power from the 24V DC bus: PLC CPU and I/O modules, HMI panel, sensors and transmitters, solenoid valves, relays, signal conditioners, communication modules, and indication lamps. Record the continuous current draw from each datasheet.
Step 2: Calculate Total Continuous Load
Sum all continuous current draws. A typical mid-size panel with a compact PLC, 8 to 12 sensors, an HMI, and a few solenoid valves might total 2.5 to 4 amps.
Step 3: Account for Inrush Current
Solenoid valves can draw 3 to 5 times their holding current for the first 20 to 50 milliseconds. If multiple solenoids energise simultaneously during a machine reset, the combined inrush can cause a voltage dip that resets the PLC.
Step 4: Add Headroom
Size the power supply at 70 to 80 percent of its rated capacity for continuous operation. For a 5A supply, your continuous load should be 3.5 to 4A maximum. This accommodates inrush events, future expansion, and ensures longer service life.
Step 5: Consider Derating
Most supplies derate linearly above 40 to 50 degrees Celsius. If your panel reaches 55 degrees and the supply derates to 80 percent, a 5A supply effectively becomes 4A.
Practical Example
Packaging machine panel: PLC CPU 0.5A + 4 I/O modules 0.6A + HMI 0.8A + 6 sensors 0.18A + 4 solenoids 1.2A + comms 0.3A + LEDs 0.1A = 3.68A total. At 75 percent loading: 3.68 / 0.75 = 4.9A. Correct choice: 5A (120W) supply.
VXT-DRI: Right-Sized Options
The VXT-DRI series covers 24V at 1A (25W), 2.5A (60W), 5A (120W), 10A (240W), and 20A (480W). Each model features wide input range, full protection, and compact DIN rail mounting.
