In this guide
The 24 V DC supply is easy to overlook and often the cause of baffling faults. When it sags, controllers reset, I/O drops out and nothing in the program is wrong.
Sizing it
Add the maximum current of every load on the rail, using the datasheet maximum rather than a typical figure:
- PLC CPU and I/O modules
- Relay and contactor coils
- Sensors and encoders
- HMI, switches and other network gear
Then add headroom of 20 to 30 per cent and pick the next standard supply size. Common ratings are 2.5, 5, 10, 20 and 40 A. Our power supply calculator does the arithmetic.
Why headroom matters
- Panels grow. Loads get added over the years
- Some loads have inrush currents higher than their running current
- Supplies derate when the panel is hot, so a supply working near its limit on a cool day may struggle in summer
- Components age and give less than their label rating
Protect the branches
One short circuit on a field device should not take the controller down with it. Use separate protected outputs, or electronic circuit protection per branch, so a fault trips its own branch and leaves the PLC powered.
Warning signs of a failing supply
- Random CPU resets or modules dropping out under load
- Output voltage that sags when load increases
- A supply that runs very hot, hums, or has a bulging case
- Output ripple visible on an oscilloscope
- A panel that restarts after load shedding returns
Electrolytic capacitors age faster when hot. A common rule of thumb is that their life roughly halves for every 10 °C rise, so cool panels last longer.
How to check one
With the machine running, measure the output voltage at the load end of the rail, not just at the supply. It should stay within the tolerance the PLC allows. Compare the measured current with the supply's rating.
Redundancy
Where a stopped 24 V rail is unacceptable, two supplies can be joined through a redundancy module so one can fail without interrupting the other.
What to send us
- The supply's full order number and a photo of the label
- The total load, if known, and the input voltage
- The panel's ambient temperature