Sizing 24 VDC Power Supply for LOGO! 8 Base Module and DM16 24R

Overview

Siemens LOGO! 8 logic modules are typically deployed in small automation, building-services, and lighting-control cabinets where a single 24 VDC rail feeds both the base module (BM) and the digital expansion modules (DM). A common engineering question when commissioning a LOGO! 8 stack is: how much current does the BM + DM16 24R combination actually draw, and what size 24 VDC power supply is safe to specify? This reference walks through module identification, current calculation, fusing strategy, and the field-proven practice of buffering internal relay outputs with external interposing relays so that the BM relay contacts are never exposed to inrush, capacitive, or inductive stress from real-world loads.

For a stack of one LOGO! 8 base module and two DM16 24R relay-output expansion modules, the engineering rule of thumb is to specify a 24 VDC power supply rated at 1.5 A minimum, sized to the sum of:

• BM quiescent / operating current at 24 VDC
• Per-DM internal power consumption at 24 VDC
• External relay coil current (if 24 VDC coils are sourced from the same rail)
• Field-input sensor current (4 mA–20 mA loops, illuminated pushbuttons, etc.)
• 20–30 % engineering margin

The remainder of this article derives each of those terms in detail, shows the working calculation for the specific BM + 2×DM16 24R stack, and documents the fusing layout used in field cabinets.

Prerequisites

Before sizing the supply, gather the following data from the project documentation:

1. Base module part number (e.g. 6ED1052-2MD08-0BA1 for LOGO! 12/24RCEo) — visible on the front label.
2. Expansion module part numbers (e.g. 6ED1055-1NB10-0BA0 for DM16 24R).
3. Worst-case field input list — count of active inputs and their sensor type (mechanical contact, PNP, illuminated pushbutton).
4. Output load list — what each DM16 24R relay contact switches, including coil current of any external interposing relay.
5. Source of 24 VDC — a regulated Class 2 / SELV power supply is mandatory; a generic unregulated wall adapter is not acceptable for industrial cabinets.
6. Local electrical code for conductor sizing, fuse type, and cabinet labelling.

System Architecture and Module Identification

A LOGO! 8 stack consists of one Base Module (BM) and up to eight digital or analog expansion modules. The expansion bus is internal — there is no external data cable between the BM and DMs. Power, however, is not bus-fed; each module draws its own supply current from the same external 24 VDC rail.

Each DM16 24R expands the system with 8 inputs (24 VDC, PNP/sourcing) and 8 relay outputs (normally-open contacts rated 5 A at 250 VAC / 30 VDC). The total reachable I/O for this stack is 24 digital inputs and 20 relay outputs.

Power Consumption Specifications

Power consumption figures for Siemens LOGO! 8 modules are published in the LOGO! 8 Product Information Manual (Edition 03/2023). The relevant figures for a 24 VDC-fed system are:

Distinguishing Input Current from Module Consumption

The Siemens datasheet lists two distinct current parameters, and they are often confused:

• I_input at "1" — the current that a single digital input draws from the field when driven to a logic-high state. For the DM16 24R this is typically 2 mA per active input. With all 8 inputs high: 8 × 2 mA = 16 mA. This current is sourced by the field device or by the BM's internal 24 V sensor supply, not from the DM's main power terminals.
• I_24V module consumption — the current the module itself draws from the 24 V power terminals to run its microcontroller, bus interface, and relay coils. For the DM16 24R this is 30–115 mA depending on coil count.

For supply sizing, use the module consumption figure, not the input-current figure. The input current is relevant only when calculating the capacity of the BM's sensor-supply output (max 200 mA on a LOGO! 8 BM) or a separate 24 VDC feed for sensors.

Step-by-Step Sizing Procedure

1. List every module on the 24 V rail. For the reference stack: 1× BM + 2× DM16 24R.
2. Look up worst-case current for each module from the datasheet:
   • BM: 165 mA
   • Each DM16 24R: 115 mA (all relays energised)
3. Sum the module currents:
   I_logic = 165 mA + (2 × 115 mA) = 395 mA
4. Add external loads sourced from the same 24 V rail — e.g. Finder 55.34 relay coils at 24 VDC draw ≈ 21 mA each. If 20 output channels each drive an external relay coil:
   I_coils = 20 × 21 mA = 420 mA
5. Add field-sensor current fed from the BM's 24 V sensor output (capped at 200 mA on a LOGO! 8 BM):
   I_sensors = 200 mA (worst case)
6. Sum total load:
   I_total = 395 + 420 + 200 = 1015 mA ≈ 1.0 A
7. Apply engineering margin (25 %):
   I_supply = 1.0 A × 1.25 = 1.25 A
8. Round up to next standard supply size (1.25 A, 1.5 A, 2.0 A, 2.5 A, 3.0 A, 5.0 A):
   I_supply = 1.5 A (24 VDC)
9. Verify thermal headroom: the supply must deliver rated current at 60 °C ambient (or per its own derating curve) and survive the inrush of all relay coils energising simultaneously.

Working Calculation for the Reference Stack

For the canonical "1× BM + 2× DM16 24R, all outputs driving Finder-type interposing relays, BM sensor supply feeding 200 mA of field inputs" configuration:

If the cabinet is hot (≥ 45 °C ambient) or the supply shares a small enclosure, step up to a 2.5 A / 24 VDC unit. LOGO! 8 modules are specified for 0 °C to +55 °C horizontal mounting and 0 °C to +40 °C vertical mounting, but a derated supply will run cooler and last longer.

Fusing Strategy

A well-built LOGO! cabinet separates the protection into at least three fused circuits. This is the field-proven layout used when the BM and DMs are inside the cabinet and the 230 VAC load wiring is in the field:

External Relay Protection Strategy

The internal relay contacts of a LOGO! 8 BM or DM16 24R are rated 5 A resistive / 2 A inductive at 250 VAC, with a mechanical life of 10 million cycles and an electrical life of 100 000 cycles at full load. The cheapest way to burn those contacts is to switch:

• LED drivers (high inrush C, 50–200 A sub-cycle spike)
• Transformers (magnetising inrush 10–20× rated)
• Long cable runs (high capacitance → high closing current)
• DC inductive loads (no zero-crossing → severe arcing)

The field-proven mitigation is to fit an external interposing relay at each LOGO! output, sized for the actual load. The Finder 55.34 (8 A contact, 24 VDC coil) is a popular choice for lighting contactor duty. The DM16 24R then switches a 21 mA coil instead of a 6 A load — well within the module's rating, and the external relay contactor absorbs all the inrush and arcing.

LOGO! DM16 24R output contact
    L (24 VDC from rail)
    |--[contact]--
    |         A1
    |     [Finder 55.34 coil, 21 mA]
    |         A2
    N (24 VDC return)

Finder 55.34 contact (10 A rated)
    L (230 VAC phase)
    |--[contact]--> lighting load
    N (230 VAC neutral)

This arrangement also gives a clean failure mode: a stuck external relay contact is a €5 replacement instead of a BM swap. It is the single most common upgrade seen on field service calls.

Power Supply Selection

For a LOGO! 8 stack drawing 1.0–1.3 A at 24 VDC, a regulated industrial supply in the 1.5 A to 2.5 A class is appropriate. Examples that drop straight into a DIN-rail cabinet:

Select the supply based on three criteria: (1) continuous rated current at 60 °C ambient, (2) peak / surge rating for inrush, and (3) diagnostic output if the LOGO! should be told (via a digital input) that the rail is healthy.

Wiring Topology

Figure 1 — Reference 24 VDC distribution: PSU → F1 → BM → DM16 24R (each module on its own terminal tap). Field input loop is fused separately at F2.

Verification and Commissioning

After wiring, before powering the load circuit:

1. Measure the 24 VDC rail at the BM power terminals with no field wiring connected. Confirm 24.0 V ± 0.5 V.
2. Apply the 24 VDC rail only (F3 open). Measure quiescent current at the PSU. Expect ≈ 215 mA (1× BM 50 mA no-display + 2× DM16 30 mA all coils off) — or up to 395 mA if the BM display is fitted and lit.
3. Force every output ON in LOGO! Soft Comfort and re-measure. Expect 1015 mA worst case (all relay coils energised, sensor supply fully loaded).
4. Verify PSU current limit is not engaged. A good 1.5 A supply should be at ≈ 70 % load at worst case, well inside its continuous rating.
5. Close F3 (load circuit). Verify each external interposing relay picks up cleanly with no chatter.
6. Monitor the rail voltage under full load. If it sags below 22.8 V (the LOGO! 8 lower spec limit at 24 V nominal), the supply is undersized.
7. Log voltage and current for 24 hours to catch thermal drift.

Troubleshooting Matrix

Notes on Cable Sizing

The 24 VDC rail must be sized to limit voltage drop to under 3 % at full load. For copper conductors at 70 °C:

V_drop = 2 × L × I × ρ / A (round trip)

where ρ = 0.0221 Ω·mm²/m for copper at 70 °C, L = one-way length in metres, I = load in amps, A = conductor cross-section in mm². For the reference 1.0 A load over 5 m of 1.5 mm² cable:

V_drop = 2 × 5 × 1.0 × 0.0221 / 1.5 = 0.147 V (0.6 %) — well within spec.

Over 20 m of 1.5 mm² at 1.0 A, the drop becomes 0.59 V (2.5 %), still acceptable. Beyond that, step up to 2.5 mm² or install the PSU closer to the cabinet.

FAQ