Power Supply 24V DC Symbol
Definition: The Power Supply 24V DC symbol represents a switched-mode or linear AC-to-DC power supply that converts mains alternating current to a regulated 24 V DC output, depicted in industrial and control-panel wiring diagrams as a labelled rectangular block with AC input terminals AC L (line) and AC N (neutral) at the top and DC output terminals DC+ (positive) and DC− (negative) at the bottom, indicating the primary power conversion stage for PLC systems, relay panels, sensors, and instrumentation.
Also known as: 24V DC power supply, PSU 24V, DIN-rail power supply, SMPS 24V, switched-mode power supply 24V, 24VDC converter.
What the Power Supply 24V DC symbol means
The Power Supply 24V DC symbol represents the AC-to-DC conversion module that forms the foundational power source in most modern industrial control panels. In a panel wiring diagram the symbol identifies the unit that accepts 120 V or 240 V AC from the mains and delivers a regulated, galvanically isolated 24 V DC output for powering PLCs, distributed I/O modules, field sensors, solenoid valves, indicator lights, and relay coils.
The 24 V DC standard emerged from IEC 60204-1 and became the dominant control voltage for industrial automation because it is safe to touch (below the 50 V AC / 120 V DC safe-touch threshold of IEC 61140), is compatible with PLC I/O modules worldwide, and allows long cable runs with manageable voltage drop. The symbol's four terminals — AC L, AC N, DC+, DC− — clearly delineate the AC input side from the isolated DC output side, a critical distinction for safety and troubleshooting.
How to identify the Power Supply 24V DC symbol
The Power Supply 24V DC symbol is a rectangle labelled 'PSU', 'POWER SUPPLY 24V', or '24VDC SUPPLY'. At the top edge, two pins are labelled AC L (line) and AC N (neutral), representing the 100–240 V AC mains input. At the bottom edge, two pins are labelled DC+ (positive 24 V output) and DC− (negative/return output). The symbol may also show a PE (protective earth) terminal. The four-terminal layout — AC input top, DC output bottom — is the defining visual pattern. A diagonal arrow or 'DC 24V' annotation may be added to identify the output voltage.
Function in a circuit
A 24 V DC switched-mode power supply (SMPS) rectifies and filters the AC input, switches it at high frequency (50–200 kHz) through a transformer for galvanic isolation, and regulates the rectified secondary output to 24 V DC using feedback control. Output regulation is typically ±1% line and ±1% load. Protection features include overvoltage protection (OVP), overcurrent/short-circuit protection (SCP), and overtemperature protection (OTP). DIN-rail mounted units (the most common in IEC panels) comply with IEC 61558-2-16 (safety) and EN 55032/EN 61000 (EMC) for CE marking; UL-listed units comply with UL 508 (industrial control equipment) for North American use.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-11 (power conversion circuits) defines power converter symbols as labelled rectangles with input and output terminals. IEC 60204-1 specifies 24 V DC as the preferred control supply voltage for machine electrical equipment. IEC 61558-2-16 covers safety requirements for SMPS units with reinforced isolation. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 / ANSI Y32.2 uses a labelled rectangular block for power supply modules. UL 508 governs industrial control panel power supplies for North American listed equipment. NEMA ICS 2 covers rated output voltages and current. |
| Key difference | IEC and ANSI/IEEE use the same rectangular block convention for the power supply symbol. The primary difference in practice is labelling: IEC panels label terminals L, N, PE, +, −; ANSI/UL panels use L1, L2/N, G, +V, COM. The IEC 60617-11 symbol is identical in visual form to the IEEE 315 representation. |
Terminals / pins
| Pin | Name |
|---|---|
| ac_l | AC L |
| ac_n | AC N |
| dc_pos | DC + |
| dc_neg | DC - |
Typical values
Input voltage: 100–240 V AC (universal) or 85–264 V AC wide-range; output voltage: 24 V DC ±1%; output current: 1.3 A, 2.5 A, 5 A, 10 A, 20 A (common DIN-rail sizes: 30 W, 60 W, 120 W, 240 W, 480 W); ripple: <50 mV p-p; efficiency: 88–94% (SMPS); hold-up time: ≥20 ms at full load; MTBF: >500,000 hours; input protection: integrated fuse or circuit breaker; ambient operating temperature: −25 to +70°C (DIN-rail standard).
Where the Power Supply 24V DC symbol is used
- Industrial PLC control panels supplying 24 V DC to CPU modules, I/O racks, and communications modules
- Motor control centre (MCC) panels powering 24 V DC contactor coils, PLC I/O, and HMI touchscreens
- Machine control enclosures supplying field sensors, solenoid valves, and indicator lamps
- Building automation panels powering 24 V DC field devices such as VAV controllers, damper actuators, and thermostats
- Process control rooms distributing 24 V DC to transmitters, positioners, and smart field instruments via marshalling panels
- Safety systems supplying 24 V DC to E-stop relays, light curtains, and safety PLCs per IEC 62061
Example
In a conveyor control panel wiring diagram, a 120 W DIN-rail power supply symbol shows AC L connected to the secondary of the control transformer (120 V AC), AC N to neutral, DC+ to the 24 V DC positive bus rail, and DC− to the panel common (0 V bus). From the DC+ rail, three branches supply a Siemens S7-1200 PLC, a 24 V relay panel, and a run-status pilot light, all referenced to the DC− rail. The symbol communicates the single point of AC-to-DC conversion for the entire control system.
Key facts
- The Power Supply 24V DC symbol represents an AC-to-DC converter with AC L and AC N input pins (100–240 V AC) and DC+ and DC− output pins (24 V DC regulated), following IEC 60617-11 and IEEE 315-1975 block conventions.
- 24 V DC is the preferred control supply voltage per IEC 60204-1 (Safety of machinery) because it falls below the safe-touch threshold of 50 V AC / 120 V DC defined in IEC 61140.
- DIN-rail SMPS units comply with IEC 61558-2-16 (isolation safety), EN 55032/EN 61000 (EMC), and UL 508 (North America) for use in listed industrial control panels.
- Typical output current ratings for DIN-rail 24 V supplies are 1.3 A (30 W), 2.5 A (60 W), 5 A (120 W), 10 A (240 W), and 20 A (480 W).
- SMPS 24 V supplies achieve 88–94% efficiency, far exceeding the 40–60% efficiency of linear regulators at the same power level.
- Hold-up time of ≥20 ms at full load ensures the 24 V output remains stable during brief mains interruptions, preventing PLC faults during momentary voltage dips.
- Galvanic isolation between AC input and DC output (reinforced isolation per IEC 61558-2-16) is mandatory in industrial power supplies to protect field wiring and personnel from mains voltages.
Frequently asked questions
What does the 24V DC power supply symbol look like?
The 24V DC power supply symbol is a labelled rectangle marked 'PSU' or 'POWER SUPPLY 24V' with two AC input pins (AC L and AC N) at the top and two DC output pins (DC+ and DC−) at the bottom. A voltage annotation '24V DC' identifies the output. The four-terminal block with AC input at top and DC output at bottom is the standard industrial representation.
What does the power supply 24V DC symbol mean on a wiring diagram?
The symbol identifies the AC-to-DC conversion point in the control panel. It indicates that mains AC power enters at AC L and AC N and is converted to a galvanically isolated, regulated 24 V DC output at DC+ and DC−, which powers all 24 V DC control devices in the panel.
Why is 24 V DC the standard control voltage in industrial panels?
IEC 60204-1 (Safety of machinery — Electrical equipment) designates 24 V DC as the preferred control supply voltage because it is below the 50 V AC / 120 V DC safe-touch threshold defined in IEC 61140, reducing electric shock risk during panel access and field wiring. It also provides compatibility with the input/output modules of virtually all major PLC manufacturers worldwide.
What is the difference between a DIN-rail power supply and a standard power supply?
A DIN-rail power supply is physically designed to clip onto a 35 mm DIN rail (EN 60715) inside a control panel, has screw-terminal connections for AC input and DC output, and is rated and certified for industrial continuous-duty operation (UL 508, CE/IEC marks). Standard power supplies (desktop, PCB-mount) are not designed for industrial panel installation and may lack the required agency approvals.
What are the AC L and AC N terminals on the power supply symbol?
AC L is the line (hot/live) conductor terminal of the AC mains input; AC N is the neutral conductor terminal. Together they supply the single-phase AC mains voltage (100–240 V) to the power supply. A PE (protective earth) terminal may also be shown for the chassis ground connection.
What protections does a 24V DC industrial power supply include?
Industrial 24 V DC SMPS units include overvoltage protection (OVP, typically 27–29 V trip), overcurrent and short-circuit protection (SCP, either constant-current limiting or hiccup mode), and overtemperature protection (OTP). Better units also provide active power-factor correction (PFC) on the AC input and signalling contacts for remote monitoring of output OK status.
Which standard defines the power supply symbol in industrial diagrams?
IEC 60617-11 (power conversion circuits) defines power converter symbols using labelled rectangular blocks. IEEE 315-1975 / ANSI Y32.2 uses the same convention. IEC 60204-1 specifies the application context (machine control), and IEC 61558-2-16 specifies safety requirements for the physical SMPS unit.
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