Power Supplies for Industrial Automation

Power supplies for industrial automation feed the 24 V control rail that PLCs, sensors, relays, HMIs and actuators share. The supply is rarely the most expensive part in the cabinet, but it is the one everything else depends on, and it is the part most often chosen on rated wattage alone.

That shortcut is where most field failures start. A supply picked to match the sum of the nameplate currents will be running at its limit on the day it is commissioned, inside an enclosure that is warmer than the room, with no margin for the moment several outputs switch together. The sections below work through the decisions in the order they actually bite.

What the 24 V control rail has to hold

IEC 61131-2, the equipment standard for programmable controllers, sets the rated operating range for a 24 V DC supply at −15% to +20%. In absolute terms the controller has to keep working between 20.4 V and 28.8 V, and the standard additionally permits an AC component with a peak value of 5% of the rated voltage.

Read that as a budget rather than a guarantee. The regulated output of the supply is only the starting voltage; by the time it has crossed a terminal block, a length of rail wiring and a fused distribution strip, the device at the far end sees less. Long sensor runs are usually where the margin disappears, because the drop is proportional to current and cable length and nobody measures it at commissioning.

Two habits keep the rail inside the window. Use a supply with an adjustable output and set it near the top of the range rather than at a nominal 24.0 V, and treat the devices most sensitive to brownout — the controller itself, and anything holding retentive memory — as the point where the voltage has to be verified.

Sizing: continuous current is only half the answer

Add up the continuous current on the rail first, then account separately for what happens at switch-on. Several classes of load draw far more current at the moment they energise than they do in steady state:

• Devices with a capacitive input stage — most DC/DC converters, drives and electronic modules — charge their input capacitance through an almost unlimited peak that the supply sees as a momentary short.
• Contactor and relay coils pull in at several times their hold current.
• Incandescent and halogen signalling lamps, where still in use, have a cold filament resistance a fraction of their hot value.
• Motor starters and anything driving a mechanical brake.

A supply with a foldback or hiccup response to overload will fail to start a system whose combined inrush exceeds the limit, even though that same system sits comfortably within the rating a second later. A supply with a peak-power rating for a defined interval, or one sized with genuine headroom, will not.

As a working rule, specify 25% to 30% above the continuous figure and verify that the supply can deliver the summed inrush. If the plant is likely to grow — and control cabinets almost always do — the headroom is also what absorbs the next two sensors without a new supply and a new panel drawing.

Derating: specify for the cabinet, not for the catalogue

Rated power is quoted at a stated ambient temperature. Above that point every switching supply derates, typically along a linear curve down to some fraction of full output at the top of its range, and the derating curve — not the headline wattage — is the number that matters in an enclosure.

The ambient that counts is the air immediately around the supply, not the temperature of the plant room. A sealed cabinet in a warm factory, with a drive and a transformer dissipating into the same volume, runs well above room temperature, and the top of the cabinet where supplies are usually mounted is the hottest part of it.

Mounting affects the figure too. Convection-cooled supplies are characterised in a specific orientation, and the derating published for vertical DIN rail mounting does not hold when the unit is laid flat or stacked against a neighbour. Where the manufacturer specifies clearance on each side, that clearance is part of the specification.

DIN rail or enclosed chassis?

Both form factors do the same electrical job, and the choice is usually about the panel rather than the circuit.

DIN rail supplies clip onto the same rail as the controllers and terminal blocks, so they are wired like any other cabinet component and swapped in minutes. The AC/DC DIN rail category covers the range from small single-phase units through to three-phase supplies for larger rails — the MDR and HDR families at the compact end, SDR and the XDR and XDR-E families where the rail carries a real load. Rail width is the constraint that bites in a crowded panel, and it is worth checking against the drawing before ordering.

Enclosed chassis supplies in the AC/DC enclosed category — LRS, RS and RD in the G3 family, HRP, MSP, NSP, UHP — mount to the panel backplate on studs. They generally cost less per watt and go higher in power than a rail-mounted unit of the same rating, which is why machine builders use them where the supply is fitted once and not expected to be touched. The trade is that replacing one is a wiring job rather than a clip-out.

A practical tie-breaker: if the cabinet is maintained by the end customer's own electricians, the rail-mounted unit usually wins on the day it fails.

Redundancy, buffering and controlled shutdown

On a line where an unplanned stop costs more than the entire control panel, a single supply is the obvious weak point. Three different problems get solved with three different devices, and they are often confused with one another.

• Redundancy means two supplies feeding one rail through a decoupling module, so the failure of either leaves the rail up. The module matters: paralleling two outputs directly lets a failed unit drag the rail down through its own output stage. Redundancy modules such as the DRDN units in the DIN rail peripheral family exist for exactly this.
• Buffering covers the millisecond-scale dropouts that a contactor or a mains dip produces. A buffer module such as DBUF charges a capacitor bank from the rail and holds the voltage up through the gap, which is enough to stop a controller resetting.
• Backup covers a real outage, and needs stored energy and a controlled shutdown — a DC UPS module such as DUPS with a battery, so the controller can close its files before the rail collapses.

Deciding between them starts with the question of what a power loss actually costs. If the answer is a reset, buffering is enough. If the answer is a corrupted recipe file or a half-finished batch, it is not.

Getting a second voltage from the rail

Plenty of cabinets need 12 V or 48 V alongside the main 24 V rail — for a radio, a sensor with a non-standard supply, or a subsystem bought in from another vendor. A second AC/DC supply is one answer, but it means another mains connection, another fuse way and another point of failure.

A DIN rail DC/DC converter takes the existing 24 V rail and produces the second voltage from it. The DC/DC DIN rail category holds the DDR family and the higher-power DDRH units. Because they sit downstream of the main supply, they inherit its isolation from the mains and are usually simpler to justify in the safety case.

Remember to count the converter's own load when sizing the AC/DC supply upstream: its input current is the output power divided by efficiency, at the lowest rail voltage rather than the nominal one.

Standards a control cabinet is usually held to

Which standards apply is decided by the end equipment and the market it ships to, not by the supply in isolation. A few recur often enough to be worth checking at selection time rather than at certification.

Safety for industrial control equipment sits under IEC 61010-2-201, which took over the product safety requirements previously in IEC 61131-2. Immunity and emissions for the installed equipment are usually demonstrated against the generic industrial EMC standards. Where the 24 V rail is treated as SELV or PELV, the isolation and earthing arrangement of the supply is part of that claim and cannot be assumed from the fact that the output is low voltage.

North American panels bring their own question, because a supply accepted under one scheme is not automatically accepted under another. If the machine ships to the United States or Canada, confirm the marks on the specific model against the requirement early — it is a cheap check before an order and an expensive one after a panel is built.