Learn Automation with MINATA #02: Electrical Power — Why the Right Voltage Can Still Overload
A power supply is rated 24 VDC, and the solenoid valve is also rated 24 VDC. Put them together and it runs — but not necessarily reliably.
The right voltage only answers the question "do the devices use the same potential level." To know whether the power supply can feed the whole system, we also have to calculate current and power. This is exactly where many control panels run fine when each mechanism is tested on its own, yet sag in voltage or restart when several loads switch on together.
1. What does electrical power tell us?
In a direct-current (DC) circuit, the basic calculation is:
P = V × I
Where:
- P is power, in watts (W).
- V is voltage, in volts (V).
- I is current, in amperes (A).
For example, a 24 VDC solenoid coil drawing 0.25 A has a power of about:
24 × 0.25 = 6 W
According to NIST, one watt equals one joule per second; in an electrical circuit it can be pictured as the power produced by 1 ampere passing through a potential difference of 1 volt.
For a purely resistive load, we also have:
These two formulas explain why a joint whose contact resistance has risen can heat up very quickly: the heat rises with the square of the current. A loose lug sometimes does not stop the machine right away, but it is turning electrical energy into heat exactly where heat should not be generated.
2. "A 24 V supply" does not mean unlimited power
The 24 VDC label states the nominal output voltage. The power supply also has a current limit, for example 2.5 A or 5 A.
| Power supply | Rated current | Approximate output power |
|---|
| 24 VDC – 1 A | 1 A | 24 W |
| 24 VDC – 2.5 A | 2.5 A | 60 W |
| 24 VDC – 5 A | 5 A | 120 W |
If the total load requires 3 A but the supply only delivers 2.5 A continuously, the system may:
- sag in voltage;
- trip its overcurrent protection;
- cut out and recover in a cycle;
- make the PLC or HMI restart;
- make sensors report intermittently;
- make solenoid coils pull in indecisively.
That is why a machine can be "exactly 24 V" and still fault.
3. Budgeting power for the control panel
Consider a small machine with the following 24 VDC loads:
| Load group | Quantity | Power per load | Diversity factor | Calculated power |
|---|
| PLC and I/O modules | 1 set | 8 W | 100% | 8 W |
| Sensors | 6 | 1.2 W | 100% | 7.2 W |
| Solenoid valves | 4 | 6 W | 50% | 12 W |
| HMI | 1 | 12 W | 100% | 12 W |
| Expected total load | | | | 39.2 W |
| Plus 25% margin | | | | 49 W |
If the machine logic guarantees that at most two of the four valves are on at the same time, we can use a 50% diversity factor. But that condition must truly be locked in by the design and the program, not merely assumed.
Arithmetically, a 60 W supply can meet 49 W. In practice, the engineer must also read the manufacturer's characteristic curves and consider panel temperature, ventilation, inrush current and expansion plans. When the environment is hot or the load has large current peaks, choosing a 72 W or 120 W class can be more sensible — not because "bigger is always better," but because the operating conditions have been fully considered.
4. Continuous load and peak load are not the same
A device may consume little while running steadily but need a high current for a short time at power-on. That phenomenon is called inrush current or peak current.
Loads to watch:
- contactor coils and solenoid valves;
- DC motors;
- high-power LEDs;
- DC–DC converters;
- HMIs, IPCs and devices with input capacitors;
- many devices starting together after the main power is switched on.
If you only add up the nominal powers, the supply can still hit its limit for a few milliseconds to a few seconds. So you need to read the supply's short-term power boost capability, overcurrent limit and startup characteristics.
5. Derating as temperature rises
Derating is the required reduction of the allowable load when the operating conditions are harsher than the rated conditions. For a power supply, the common factors are:
- high temperature inside the panel;
- mounting too tightly, poor convection;
- high altitude reducing heat-dissipation capability;
- input voltage near its limit;
- load held high for a long time.
Do not take the exact power number printed on the label and run it at 100% year-round without checking the technical documentation. The derating curve of the exact supply model is the real basis.
6. Don't forget losses along the wiring
The supply may hold 24 V at its terminals, but a load at the end of a long cable receives less because of voltage drop. The larger the current, the smaller the wire and the longer the run, the more significant the loss.
A pragmatic check sequence:
- Measure the voltage right at the supply output.
- Measure the voltage at the load while it is operating.
- Compare the two values.
- Check terminals, lugs, terminal blocks and wire cross-section.
- Use a thermal camera if there are signs of abnormal heating.
Measuring at no load usually fails to reveal a high-resistance joint.
7. Supply power does not replace protective devices
A high-power 24 VDC supply does not mean you can drop the fuses or branch breakers. Protection must coordinate with:
- the current-carrying capacity of the wire;
- the operating and inrush current of the load;
- the interrupting capacity under a short circuit;
- the current-limiting characteristic of the supply;
- the need for zoning so that one faulty branch does not bring down the entire control.
PLCs, sensors and valves can be split into separate protected branches. For a critical machine, the control supply and the actuator supply can also be separated to ease diagnosis and reduce cross-influence.
8. The MINATA checklist before finalizing a power supply
| Question | Evidence needed |
|---|
| Are the voltage and polarity correct? | Wiring diagram, device labels |
| What is the total continuous load? | Power budget table |
| Which loads have inrush current? | Datasheets and power-up sequence |
| Which loads can switch on simultaneously? | Machine sequence and PLC logic |
| Is there margin for expansion? | A margin with a reason |
| Does panel temperature cause derating? | Manufacturer curves |
| Is the end-of-line voltage drop acceptable? | Calculation and real measurement |
| Are load branches suitably protected? | Protection diagram and wire sizes |
Conclusion
When choosing a supply, do not stop at "they are all 24 VDC." Make a load list, calculate the power, and consider the diversity factor, inrush current, temperature and voltage drop. A small spreadsheet at the design stage can avoid many hours of troubleshooting after the machine is built.
Next article: Direct current and alternating current — Don't choose a supply by the volt number alone.
References
- NIST, Watt: https://www.nist.gov/glossary-term/34606
- NIST, SI Units – Electric Current: https://www.nist.gov/pml/owm/si-units-electric-current
- OSHA, Electrical Glossary: https://www.osha.gov/electrical/glossary
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