Yes, a small HMI panel needs its own supply. Size it from the datasheet's absolute maximum watts, not the standby figure: divide watts by the rail voltage, double the result, and round up to the next standard DIN-rail rating. A panel rated 18.4 W maximum draws 0.77 A, so a 2 A supply carries it.
A small HMI panel needs its own supply at its rated voltage, and the panel's datasheet decides how big it is. Take the absolute maximum watts, divide by the nominal rail voltage, double the result for headroom, and round up to the next standard DIN-rail rating. A compact panel that draws 0.77 A runs on a 2 A DIN-rail supply with room to spare. After wiring, one measurement settles the install: the voltage at the panel's terminals with the screen at full brightness. This page gives the rule, the wiring, the commissioning check, and a worked example.
Why the panel needs its own supply
An HMI is one of the larger loads in a small cabinet, and its datasheet states the demand in watts on the power consumption line. No serial or USB port on a controller carries that demand: ports budget milliamps, the panel needs whole amps, and a panel fed from a programming cable stays dead. Give the panel its own DIN-rail supply and a short copper branch.
If the temptation is to feed the panel over the controller cable, the outcome is the one HMI panel not powering from a PLC serial port cable describes: the port carries signals, the panel never boots, and the real fix is a supply on the rail.
Read the datasheet's power line first
The power consumption line carries three numbers, and you size from the largest. A compact basic HMI lists 15 W or less when its USB ports feed nothing, 5.9 W or less in standby with the backlight off, and 18.4 W as the maximum. A large performance terminal lists 29 W maximum in its current revision; the earlier revision listed 50 W, which the same datasheet prints as 2.1 A, so the division is already done for you there. A mid-size panel that feeds expansion units through its own terminals lists 26 W or less bare and 45 W or less loaded.
Two traps sit on that line. The standby figure is what the panel draws after the screen has dimmed, so it undersizes the supply the moment the screen lights up. The expansion gap is real load: 26 W bare becomes 45 W loaded, and the supply carries the loaded figure.
| Panel class | Datasheet power line | Current at nominal voltage | Supply rating |
|---|---|---|---|
| Compact basic HMI | 15 W normal, 5.9 W standby, 18.4 W max | 0.77 A | 2 A |
| Performance terminal, current revision | 29 W max | 1.21 A | 2.5 A |
| Performance terminal, earlier revision | 50 W max, printed as 2.1 A | 2.08 A | 5 A |
| Mid-size panel with expansion units | 26 W bare, 45 W loaded | 1.88 A | 5 A |
Current is the maximum divided by the nominal rail voltage. The rating doubles it and rounds up to the next standard size.
What you need
- The HMI datasheet's absolute maximum watts, plus the loaded figure if the panel feeds USB or expansion units.
- A DIN-rail supply at the panel's rated voltage; common ratings step 1.25, 2, 2.5, 3, 5 and 10 A.
- 1.5 mm² copper for the branch and a fuse at or just above the panel's maximum current.
- A multimeter for the commissioning measurement.
The sizing rule
Divide the panel's absolute maximum watts by the nominal rail voltage to get amps. Double the result: the margin covers wiring drop between supply and panel plus the power-up surge, which sizing a separate 24 VDC supply for HMI inrush current works through on its own terms. Round up to the next standard DIN-rail rating.
Doubling means each amp of supply rating buys 12 W of panel (the rail voltage divided by two). So the 2 A rating covers a panel up to 24 W, 2.5 A up to 30 W, 3 A up to 36 W, and 5 A up to 60 W.
Count anything else on the same rail before you round. The panel's own inputs are nearly free: one input design draws 2.1 to 2.6 mA across the input voltage range. A 2-wire sensor is a real load: EN 61131-2 allows up to 30 mA per sensor, so twenty sensors on a shared rail add 0.6 A. Keep the rail at its setpoint once sized, too. A digital input under IEC 61131-2 reads a level above 11 V as on, with an undecided band from 5 to 11 V below that, so a rail that sags far enough puts every input's state in doubt.
Example: a compact basic panel rated 18.4 W maximum
Inputs: the datasheet lists 18.4 W as the absolute maximum, with 15 W normal and 5.9 W standby on the same line; the rail is at nominal voltage; standard DIN-rail ratings step 1.25, 2, 2.5, 3, 5 and 10 A.
Rule in words: maximum watts divided by the nominal rail voltage gives maximum amps; double for headroom; round up to the next standard rating.
Arithmetic: 18.4 W ÷ 24 V = 0.77 A. 0.77 A × 2 = 1.54 A. The next standard rating above 1.54 A is 2 A.
Result: that DIN-rail supply carries this panel, about 2.6 times its maximum current.
Decision: the same supply covers any panel up to 24 W maximum. A 29 W performance terminal works out to 29 ÷ 24 = 1.21 A, doubled to 2.42 A, so it moves up to a 2.5 A unit. The mid-size panel loaded to 45 W works out to 1.88 A, doubled to 3.75 A, so it takes a 5 A unit.
Wiring the branch
Mount the supply on the DIN rail beside the controller and keep the branch short. 1.5 mm² copper measures about 12 milliohms per metre at room temperature, and the out-and-back loop doubles whatever length you run. At 0.77 A, a 10 m loop drops 10 × 0.012 × 0.77 ≈ 0.09 V, which leaves the rail where you set it. The same branch stretched to 50 m drops 0.46 V; that is the point where the wire grows before the supply does.
Fuse the branch at or just above the panel's maximum current so a fault clears before the wiring heats. Set the supply's output trim to 24 V at its own terminals with the load off, before the panel is connected.
Steps
- Copy the absolute maximum watts off the datasheet, and the loaded figure if the panel feeds USB or expansion units.
- Divide by the nominal rail voltage, double, and round up to the next standard rating.
- Add any loads sharing the rail first, at 30 mA per 2-wire sensor.
- Mount the DIN-rail supply, fuse the branch, and run 1.5 mm² copper to the panel's power terminals.
- Set the supply output to the rail voltage at its terminals with the load off.
- Power the panel with the screen at full brightness and any expansion connected, and measure at the panel's power terminals.
- Compare that reading with the one at the supply's output. The difference is wiring drop, and anything over 0.5 V calls for a bigger wire or a shorter run. Confirm the reading sits inside the input range printed on the panel's datasheet.
How to check it worked
A sound install shows three things at once. The terminal voltage holds within 0.5 V of the supply's output with the screen at full brightness and expansion connected. The backlight stays steady while the controller polls. Nothing resets when a contactor or another cabinet load switches on.
If the screen dims instead, the rail is sagging, and HMI screen dim on PLC power: check the supply drop walks that fault. If the picture flickers in step with controller traffic, HMI backlight flickers when polling: check the power supply traces it to the supply. Once the panel holds its voltage, alarms and email events are a software job.
Recommendation: Run a dedicated DIN-rail supply for the HMI, sized at twice the datasheet maximum current and rounded up to the next standard rating: a 2 A unit up to 24 W of panel, 2.5 A up to 30 W, 3 A up to 36 W, 5 A up to 60 W. Share the rail only after adding every sensor's current to the budget, and measure the terminal voltage at full brightness before you sign off the panel.
Questions
Can the HMI share the supply with the PLC?
Yes, if the arithmetic still clears. Convert the controller's datasheet maximum watts to amps at the rail voltage, add the panel's amps, and add 30 mA for every 2-wire sensor on the same rail, then double the total and compare it with the supply rating. If a shared unit needs to be bigger than 5 A for one HMI and one compact controller, give the HMI its own supply anyway; a shared rail makes every fault look like every other fault.
What does an undersized supply look like on the floor?
The panel boots, then misbehaves under load: the backlight dims, the screen flickers when the controller polls it, or the panel resets when a large load elsewhere in the cabinet switches on and pulls the rail down. All three point at the supply and not at the program. Measure the voltage at the panel terminals during the event; a reading that dips while the screen misbehaves settles it.
How do I work out the voltage drop for a longer run?
Multiply the out-and-back length in metres by 0.012 ohms per metre for 1.5 mm² copper, then multiply by the panel current in amps. A 20 m loop at 0.77 A drops 0.18 V; the same loop in 0.75 mm² copper, roughly 24 milliohms per metre, drops 0.37 V. Move up a wire size before you move the supply.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.