Choosing a 24 V DIN rail power supply for a control panel

Choosing a 24 V DIN rail power supply for a control panel

A 24 V DIN rail power supply can look comfortably sized on paper and still be too close to its limit once the panel is working as a system. The usual reason is simple. The selection has been made from a headline wattage figure rather than from the way the 24 V loads actually behave together.

A PLC, HMI, communications device, relay coil and solenoid valve do not all present the same demand. Some run continuously. Some operate only during certain sequences. Some draw more current when they start or pick up. The usable output of the power supply can also change with the conditions inside the finished enclosure.

Start with current, timing and operating conditions rather than catalogue wattage alone.

Pairui PIS120-24 24 V DC DIN rail power supply.

Start with the loads that can operate together

Build a 24 V load schedule around the maximum credible simultaneous demand.

Record the current required by the PLC and I/O, HMI, sensors, communications equipment, interposing relays, contactors, solenoid valves and other 24 V devices supplied from the same source. Use the relevant maximum figures from the manufacturers' data rather than a typical consumption figure where the device can legitimately demand more.

Then establish which loads can really be on at the same time.

If a machine has six valves but the control sequence guarantees that only three can be energised together, the simultaneous load can reflect that. The same assumption has to remain true during manual operation, recovery modes and other credible states. If another mode can energise more devices, include it.

At 24 V DC, 5 A equals 120 W. Working in amps makes it easier to compare the load schedule directly with the rated current of the supply.

Worked example: an illustrative 24 V panel load

This example is illustrative. The device currents are assumptions for the calculation, not measured product data.

The simultaneous running load is 2.97 A, equal to 71.28 W at 24 V.

Now add a credible transient. In this example, the HMI start-up demand is assumed to rise from 0.60 A to 1.00 A while two contactors pick up together at 0.55 A each.

2.97 - 0.60 - 0.18 + 1.00 + (2 × 0.55) = 4.29 A

So a panel that normally runs at 2.97 A can present a 4.29 A transient.

This is different from AC input inrush at the power supply. AC input inrush concerns what the supply itself draws from the incoming AC source when energised. The 4.29 A figure here is a 24 V load-side start-up demand.

Figure 1. Illustrative running and transient 24 V demand against a 5 A nominal rating. This is calculation data, not product test data.

Allow for the additions you actually expect

Future capacity is often handled with an arbitrary spare percentage. A clearer approach is to identify the likely additions and put them into the same current calculation.

Suppose this panel is expected to gain another 0.50 A of 24 V load. The expanded running demand becomes 3.47 A. The expanded transient becomes 4.79 A, equal to 114.96 W at 24 V.

A nominal 5 A supply is now only 0.21 A above that illustrative transient.

That does not automatically make a 5 A supply unsuitable. It does mean the exact model's transient capability, derating information and operating conditions now matter to the selection.

Power supply derating can change the answer

The rated current printed on a supply is not necessarily the current available under every permitted operating condition.

Temperature is a common example. The temperature that matters is the environment around the supply inside the enclosure, not simply the room temperature outside the panel. Input conditions and other limits defined by the manufacturer can also affect usable output.

An operating temperature range should therefore not be read as a promise of full rated output across that whole range. Check the exact model's published derating information against the panel conditions.

To show why this matters, consider the 5 A nominal rating only as arithmetic. If a set of operating conditions left 80% of that nominal value available, usable current would be 4.00 A. If only 60% were available, it would be 3.00 A.

Those percentages are not Pairui or GIC derating figures and are not tied to any temperature. They simply demonstrate the effect on the earlier calculation.

At 4.00 A, the expanded 3.47 A running load would fit, but the 4.79 A transient would not. At 3.00 A, even the expanded running load would be above the illustrative available current.

Figure 2. Illustrative effect of 100%, 80% and 60% of a 5 A nominal rating compared with the expanded running and transient calculations. These percentages are arithmetic examples, not a product derating curve.

Comparing Pairui PIS and MIS 24 V supplies

Pairui PIS and MIS give panel builders different choices within the 24 V DC supply category. The useful comparison starts with current and power, then moves into the exact model data.

For PIS120-24, the verified product data gives a 90 to 264 V AC input and a 24 V DC, 5 A, 120 W output. Pairui also publishes peak-load capability for this model. The permitted duration and duty cycle need to be checked in the exact datasheet before that capability is used in a design.

MIS120-24 has the same 24 V DC, 5 A and 120 W headline output. MIS240-24 increases nominal output to 10 A and 240 W.

Those figures narrow the choice, but do not complete it. Check the exact model datasheet for input conditions, status output requirements and other model-specific functions.

What to check after the load calculation

Once the running and transient currents are known, check the candidate supply against the actual application.

The incoming supply is one part of that check. PIS120-24 is specified for 90 to 264 V AC input. GIC 24AS244D6D is a 230 V AC nominal input model. The choice depends on the panel supply and the manufacturer's published limits.

Also confirm the required 24 V output, any permitted adjustment and the current and power limits that apply at the chosen setting.

Then check the model-specific derating information for the enclosure conditions. If usable current falls under those conditions, compare that reduced capacity with both the continuous demand and the credible transient.

Status and monitoring requirements can matter too. If the control design needs a particular indication or status output, confirm that function on the exact model rather than assuming it from the family name.

Current sharing and redundancy are separate functions. Do not assume a supply supports either purpose because another model in the range does. Physical fit and any required spacing should also be taken from the current model documentation used for the build.

A practical selection sequence

1. List the 24 V loads and their relevant maximum currents.
2. Establish the maximum credible combination that can operate together.
3. Add credible start-up or pick-up demand.
4. Add identified future loads in amps rather than applying an arbitrary reserve percentage.
5. Check the exact power supply's usable output under the panel's operating conditions.
6. Confirm the incoming supply and any required status functions.
7. Compare the final requirement with the rated current and power of the candidate model.

This gives the panel builder a reasoned basis for choosing between a 5 A and 10 A supply, or deciding that another rating is required.

Frequently asked questions

Is a 120 W supply always enough for a panel with a 100 W load?

No. At 24 V, 100 W is about 4.17 A, but the selection still needs to account for which loads operate together, credible start-up demand, identified future additions and the usable output under the panel's actual operating conditions.

Should I add every 24 V device nameplate current together?

Not automatically. Use the maximum credible simultaneous demand. Only reduce the total where the machine or control design genuinely prevents those loads from operating together in all relevant operating modes.

How much spare capacity should I leave on a 24 V DIN rail power supply?

There is no universal percentage that suits every panel. Add the current for identified future loads, recalculate the running and transient demand, then check the exact model's usable output under the expected operating conditions.

Does the operating temperature range mean full output is available across that range?

No. An operating range does not by itself prove that full rated current is available at every temperature. Check the exact model's published derating information against the conditions around the supply inside the enclosure.

Can I connect two same-voltage power supplies together?

Do not assume so. Current sharing and redundancy are different functions and depend on the exact model. Use the manufacturer's published information for the specific supply and application.

Need a second check on the selection?

Send Charter Controls the 24 V load list, incoming supply details and expected panel operating conditions. The technical team can help you compare the requirement with the appropriate Pairui PIS, Pairui MIS or other suitable supply. Call 01424 850660.

Charter Controls is the trading name of Unipower (UK) Ltd, St Leonards-on-Sea, East Sussex.

For electrical work, always follow the manufacturer's current published instructions and use a competent person where required.