S7-1200 Power Supply: Requirements, Sizing, and Wiring

Overview

The SIMATIC S7-1200 PLC family has two fundamentally different power-input architectures that engineers routinely confuse. The DC-powered CPU variants (CPU 1211C, 1212C, 1214C, 1215C, 1217C with catalog suffix -1xxx30- or -1xxx40- in the 24 V DC input form) expect an external 24 V DC source on the M and L+ terminals. The AC-powered variants, most notably the 6ES7214-1BG40-0XB0 (CPU 1214C AC/DC/RLY), accept 85–264 V AC at 47–63 Hz directly on the line and neutral terminals and internally generate the 5 V and 24 V rails the rest of the system needs.

Because the CPU carries an internal power supply that feeds the backplane, signal modules (SMs), and the 24 V DC sensor-power output, the external 24 V supply is not strictly mandatory in every architecture — but it is mandatory for any sensor or actuator that draws from the CPU's L+ terminal. The Siemens SIMATIC S7-1200 manual collection states explicitly that "all S7-1200 CPUs have an internal power supply that provides power to the CPU, any expansion modules, and the 24 V DC sensor power output." The external supply you choose only needs to refill what the internal supply cannot, and it must stay inside the published 20.4 V to 28.8 V Class 2 window.

S7-1200 Power Architecture: Internal vs External

An S7-1200 system draws power from up to three independent sources:

1. System power (Mains / 24 V input): Powers the CPU logic, backplane, expansion I/O, and the 24 V sensor-power output.
2. Sensor power (24 V DC sensor supply, terminal L+): A limited-budget 24 V output derived from the internal supply. Used to source digital inputs and short-loop sensor loads. Maximum budget is published in the data sheet for each CPU (typically 300 mA to 400 mA for the 1214C/1215C).
3. Load power (24 V DC load supply, terminal 2L+/3L+ on SMs): Required whenever an SM output group needs 24 V to drive a load. This is wired directly to the SM terminals, not to the CPU.

For an AC-powered CPU like the 6ES7214-1BG40-0XB0, the system-power source is the AC mains, and the internal supply creates the 5 V backplane rail and the 24 V sensor supply. The L+ terminal on the AC CPU is therefore not a power input — it is a 24 V output intended to feed connected sensors. Connecting a 24 V supply to that terminal is a wiring error.

PM 1207 Power Supply Modules

Siemens offers the PM 1207 family as the matched, listed, and warranty-supported choice. Two generations are in active service.

The PM 1207 G2 manual describes the unit as a "load power supply module PM 1207 120 W 120 - 240 V AC/DC" that "supplies input and output circuits (load circuits) as well as sensors and actuators of the SIMATIC S7-1200 G2." The G2 variant is mechanically and electrically different from the classic PM 1207 and is not a drop-in replacement for the S7-1200 (first generation) — verify the controller family before ordering.

Power Budget Calculation

The S7-1200 backplane provides 5 V at a fixed per-CPU maximum (commonly 1600 mA on the CPU 1214C, 2200 mA on the 1215C, 2400 mA on the 1217C). The TIA Portal device configuration reports the consumed and remaining budget in the device properties. Use the following procedure to size the external 24 V supply:

1. Sum the 5 V consumption of every SM and SB in the configuration. Each module's data sheet lists 5 V DC current consumption (typ.).
2. Confirm the sum is below the CPU's 5 V DC max rating.
3. Sum the 24 V sensor current of every input that will be sourced from the CPU's L+ terminal.
4. Confirm the sum is below the CPU's Sensor power 24 V DC budget.
5. Add the 24 V load current of every output group fed by an SM, plus the CPU's own 24 V load share.
6. Apply a 25 % derating to the total. This gives the PM 1207 output current requirement.

Worked example for a CPU 1214C DC/DC/DC with three SM 1223 modules, two SM 1221 modules, and ten 24 V PNP sensors drawing 30 mA each:

• 5 V backplane: 3 × 145 mA + 2 × 80 mA = 595 mA (limit 1600 mA) — pass.
• Sensor power (CPU L+): 10 × 30 mA = 300 mA (limit 400 mA on 1214C) — pass.
• 24 V load (SM outputs): 3 × 8 channels × 0.5 A = 12 A peak, but 6 A continuous design current.
• 6 A × 1.25 derating = 7.5 A required from external supply.

A single PM 1207 6EP1332-1SH71 cannot deliver 7.5 A. Either split the load across two 24 V rails, use a 5 A supply, or use a third-party 24 V/10 A DIN-rail supply. For an AC CPU like the 6ES7214-1BG40-0XB0, the AC mains feeds the internal supply; the external 24 V supply only needs to power the SM load groups and any sensor loads that exceed the CPU's sensor budget.

AC-Powered CPU: Wall-Socket Wiring (6ES7214-1BG40-0XB0)

The 6ES7214-1BG40-0XB0 is the CPU 1214C in AC/DC/RLY form factor: 85–264 V AC input, 24 V DC sensor supply output, relay outputs. The terminals on the bottom of the CPU are:

For bench testing the 6ES7214-1BG40-0XB0 directly from a wall socket you may use a standard IEC 60320 C7 (figure-8) cable such as the CUI AC-C7-EU. The female end goes to the CPU's integrated C8 inlet, the male end goes to the wall. The C7/C8 pair is double-insulated and rated to 2.5 A at 250 V AC, which is well within the CPU's ~150 mA typical mains draw.

For permanent cabinet installation, terminate the AC feed on a DIN-rail terminal block upstream of a 1 A slow-blow fuse, then run a short pigtail to the CPU's integrated C8 inlet, or land the conductors directly on a separate terminal-block header if the cabinet builder has provided one. Always bond PE.

Can a Third-Party 24 V Supply Replace PM 1207?

Yes, for the functional behavior of the PLC. The PM 1207 is not load-bearing on the firmware, the backplane protocol, or the I/O scan. Any 24 V DC supply that meets the following is electrically compatible:

• Output voltage: 20.4 V to 28.8 V DC (24 V nominal, ±20 %)
• Output current: ≥ the sum of all connected loads with margin
• Output rating: NEC Class 2 (or SELV-equivalent per IEC 61131-2 for CE-marked machinery)
• Hold-up time / ride-through: ≥ 20 ms at full load (CPU must not brown-out on momentary mains dips)
• Ripple and noise: ≤ 200 mV pk-pk to avoid digital-input bounce on long sensor leads
• Overload behavior: foldback or constant-current, never a hard latch that hangs the I/O

Functional equivalence is one thing; commercial reality is another. If a non-Siemens supply fails after three years on a customer panel, the warranty path to Siemens is closed and the integrator owns the failure analysis. The most common field failure modes for cheap 24 V supplies are electrolytic-capacitor drying (manifesting as 24 V droop under SM inrush) and Y-capacitor failure (manifesting as common-mode noise coupled into analog inputs).

Wiring Topology and Distribution

For a 6ES7214-1BG40-0XB0 with relay outputs and a mixed signal module stack, the recommended distribution is:

AC Mains 85–264 V
   |
[Breaker 1 A] -- [Fuse 1 A slow-blow]
   |
   +--- L1, N to CPU 1214C AC terminals (or C8 inlet)
   |
[PM 1207 24 V DC] ----- 24 V DC bus
   |                          |
   +-- CPU L+ (sensor PWR)   +-- SM 2L+ (output loads)
   |                          +-- SB 1221 2L+ (if used)
   |                          +-- Field devices (fused per channel)
   |
   M (24 V return) ------------ 24 V return bus

Key rules:

• Keep the 24 V return bus and the protective earth bus separate. Bond them only at a single point, typically at the 24 V supply's negative terminal or the cabinet PE bar.
• Use a single common 24 V return for all SMs to avoid ground loops that the digital inputs read as "stuck on."
• For relay-output SMs (SM 1226, SM 1222 RLY), the 2L+/3L+ terminals are typically jumpered to the same 24 V source as the sensor supply. Mixing supplies can produce undefined output behavior on power-up.

Commissioning and Verification

1. With power off, verify the AC mains is within 85–264 V AC and 47–63 Hz on a true-RMS multimeter.
2. Confirm PE continuity from the CPU mounting tab or terminal to the cabinet PE bar (≤ 0.1 Ω).
3. Apply AC power. The CPU's RUN/STOP LED should briefly show the self-test pattern, then settle to STOP (yellow) or RUN (green) per the project state.
4. Measure the 24 V sensor supply at the CPU's L+ and M terminals. The voltage should be 20.4–28.8 V DC, with ripple ≤ 200 mV pk-pk measured at the terminal with a scope or a true-RMS DMM in mV AC mode.
5. From TIA Portal (or an attached HMI), read the Device → Properties → System diagnostics page. No "24 V missing" or "Sensor power overload" diagnostic bits should be active.
6. Exercise one input and one output end-to-end. Confirm that input filtering and output response times match the project configuration.

Troubleshooting Matrix

Safety and Standards

For CE-marked machinery, the S7-1200 power section must be installed per IEC 61131-2 and the system must comply with UL 508A for industrial control panels. Specific obligations:

• The 24 V DC source must be SELV (Safety Extra-Low Voltage) per IEC 61131-2. The PM 1207 is SELV by design; a non-SELV supply voids the system safety analysis.
• Overcurrent protection on the AC feed: a 1 A slow-blow fuse or a 1 A Type B circuit breaker is the standard integrator choice.
• A disconnect device (UL 489 or IEC 60947-2 listed) must be provided within sight of the panel.
• Bond PE on the AC-powered CPU to the cabinet PE bar with a dedicated conductor. Daisy-chained PE is not acceptable for industrial control panels.

Notes for S7-1200 G2 Users

Customers migrating to the G2 controller family should order the G2 PM 1207 (catalog 6EP1333-1SH03 in the published G2 manuals). The classic PM 1207 cannot deliver the wider input range (120–240 V AC/DC) of the G2, and the mechanical mounting clips differ. The G2 manual describes the unit as supplying "input and output circuits (load circuits) as well as sensors and actuators of the SIMATIC S7-1200 G2." Confirm the S7-1200 generation (classic vs G2) before ordering the supply module.