The short answer
Panel Power Supplies
A panel power supply converts the control voltage, usually 120 V AC, to 24 V DC for the controller, the I/O, the instruments, the relays, and the network devices. It is sized for the continuous load with margin and for the inrush of the devices it starts, distributed through fused branches so one fault does not drop everything, monitored so a failure is an alarm rather than a mystery, and made redundant with a diode module where the panel cannot be allowed to go dark. Because a switch-mode supply limits current rather than delivering a fault current, the fuses downstream are chosen to clear on what it can deliver.
Key points
- Size for the continuous load plus 25 to 30 percent, and check the inrush of the largest devices against the supply peak rating.
- One supply feeds everything in most panels. That is a single point of failure; decide deliberately whether to accept it.
- Distribute through fused branches: controller, I/O, instruments, network, relays. A shorted float does not take the PLC down.
- A switch-mode supply current-limits on a short. Downstream protection must clear on that limited current.
- Monitor the DC OK contact and the output voltage. A supply dies slowly before it dies completely.
What the supply feeds
Almost everything in a modern control panel runs on 24 V DC: the controller and its I/O modules, the transmitters on loop power, the relays that interface to the starters, the network switches and radios, the HMI panel, and the indicator lights. The supply that produces that 24 V is therefore the one component whose failure stops all of them at once. It is a small box on a DIN rail that gets less attention than it deserves, and the panels that fail worst are the ones where it was chosen by catalog number without a load calculation.
Sizing
- 1
Add up the continuous load
Every device on the supply, from its data sheet, at its worst case: the controller with all modules, every loop-powered transmitter at 22 mA, every relay coil energized, every network device, the HMI at full backlight. The size-a-power-supply page walks through the arithmetic.
- 2
Add margin
25 to 30 percent over the continuous load, for the devices that will be added and for the derating below.
- 3
Check inrush and peak
Some devices draw several times their running current at power-up: drives with 24 V control boards, some HMIs, solenoids, and capacitive loads. The supply must either carry the peak briefly or the devices must be staged. Supplies publish a peak current and a duration; compare them.
- 4
Derate for temperature
Supplies are rated at 40 or 50 °C and derate above. A panel that reaches 55 °C in summer needs a supply rated for more than the load at that temperature.
- 5
Choose the input
Single-phase 120 or 230 V AC, or 480 V for the three-phase supplies used in drive panels; a wide-range input tolerates the sags and swells a remote site sees.
- 6
Choose the protection and monitoring
A DC OK relay contact, an output voltage the controller can read, and a supply that is listed for the panel standard: UL 508 or UL 61010 as a power supply, with the SELV or class 2 output where the wiring method depends on it.
One supply or two
| Arrangement | What it gives | What it costs | Where it fits |
|---|---|---|---|
| Single supply | Simplicity; one device to fuse and monitor | A single point of failure for the panel | Lift stations and small panels with a float backup, where the process survives the panel going dark |
| Two supplies with a redundancy module | Either supply carries the load; the diode module isolates a failed one; the DC OK contacts alarm which one | Two supplies, a module, and each supply sized for the whole load | Plant control panels, SCADA and network panels, anything where a dark panel stops the process |
| Two supplies, split loads | The controller and the network on one, the instruments and relays on the other | Two supplies without a module; a failure still drops half the panel | A compromise; usually worse than true redundancy for the same money |
| Supply plus a DC UPS or a battery module | The 24 V bus rides through an AC outage for minutes to hours | A battery to maintain | Remote sites where the controller and the radio must report the outage; SCADA panels |
Distribution and fusing
A 24 V bus that feeds thirty devices from one terminal block, unfused, is a bus on which a shorted float cable in the wet well takes the controller, the radio, and every instrument down together. The supply output is distributed through branches, each fused or protected for its load and its wire, so that a fault is confined to the branch that has it and the rest of the panel keeps running.
- A branch for the controller and its I/O, so that nothing in the field can drop the processor.
- A branch for instrument loop power, often further split per loop or per group with indicated fuse blocks, so that a shorted transmitter drops one loop.
- A branch for relays and solenoids, which are the loads most likely to fail shorted.
- A branch for network devices and radios.
- A branch for the HMI.
- Protection sized for the branch wire and load, on the ungrounded conductor, with a blown-fuse indication or a status contact where the branch matters.
How a switch-mode supply fails and clears
A transformer-rectifier supply delivers a large fault current into a short; a switch-mode supply does not. It limits its output current to a little above its rating, or it folds back, or it hiccups on and off, depending on the design. A 10 A supply feeding a short through a 10 A fuse may never blow the fuse: it delivers 11 A, the fuse holds, the bus voltage collapses, and every device on the bus resets. The branch protection has to clear on the current the supply can actually deliver. That means fuses well below the supply rating on each branch, so that the supply can deliver several times the branch fuse rating, or electronic circuit protectors that trip on a small overcurrent in milliseconds, or a supply with a defined peak current capability the fuses are chosen against. The supply data sheet gives the overload behavior; the branch fusing is designed from it.
Monitoring
- DC OK contact
- A relay contact in the supply that opens when the output is below tolerance, wired to a controller input and alarmed. On a redundant pair, one per supply, so the failed one is named.
- Output voltage
- The bus voltage on an analog input, trended. A supply that is aging reads low under load before it fails; a bus that sags when the relays pull in is undersized.
- AC input status
- A relay or a voltage monitor on the control power, so that a lost AC feed is distinguished from a failed supply.
- Temperature
- Panel temperature, because the supply is the component most affected by it.
- Load current
- Where the supply provides it or a shunt is fitted; the trend shows the load growing as devices are added.
Installation
- Mounted with the clearance the manufacturer requires for convection, vertically, away from the heat of drives and transformers, and not at the top of the enclosure where the hot air collects.
- Input from a fused or breaker-protected control circuit, on the ungrounded conductor.
- Output negative bonded to the panel ground at one point, where the design calls for a grounded DC system, and nowhere else.
- Output wiring sized for the current and the voltage drop; a long run to a remote rack at 24 V loses volts.
- A label with the output voltage and the branch schedule; a spare supply of the same model on the shelf.
Frequently asked questions
- Why does the whole panel reset when one solenoid shorts?
- The bus is unfused or the fuse cannot clear on the current a switch-mode supply delivers. Distribute the output through branches with protection sized to trip on the limited current, and put the solenoids on their own branch.
- Can I use one supply for the controller and the instruments?
- Yes, and most panels do, through separate fused branches. Where the instruments are in a hazardous area through barriers, or where the instrument loops are especially noisy, a separate supply for the instruments is reasonable. What matters is that a fault on an instrument branch does not reach the controller.
- How much margin is enough?
- 25 to 30 percent over the calculated continuous load, after derating for the panel temperature. More if the panel is likely to grow. A supply running at 95 percent of its rating on a hot day has a short life and no room for the next float.
- Should the 24 V negative be grounded?
- Common practice is to bond the negative to the panel ground at one point, which gives a referenced system that is easier to troubleshoot and that lets ground faults be detected as shorts. Some designs float it deliberately for noise or for ground-fault tolerance, and then a ground fault detector is added. Either way, it is one decision, documented, applied at one point.
Related topics
- How to Size a Control Panel Power SupplyAdd up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
- Control Power SuppliesSizing the 24 VDC supplies that run a controller and its field devices, separating loads that should not share a supply, and keeping the controller alive through a power event.
- Fuses in Control PanelsFuse classes, time-delay and fast-acting types, current limitation and what it does for the panel short-circuit rating, semiconductor fuses for drives, control circuit and DC fusing, and the habits that make a blown fuse a diagnosis instead of a mystery.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
- Circuit Breakers in Control PanelsMiniature, molded-case, and supplementary protectors: what each is listed to do, how a trip curve is read, where the NEC and UL 508A set the rules, and how breaker choice drives the short-circuit current rating of the whole panel.
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