Siemens LOGO! Output Expansion: Paralleling Contactors Safely

Overview

Field technicians regularly hit the I/O ceiling on a Siemens LOGO! base module before the mechanical scope of the machine is finished. The classic symptom is a fully populated LOGO! 8 (6ED1052-xxx08-0BA8) or older 0BA6/0BA5 controller with all 16 digital outputs assigned and a new conveyor, light stack, or solenoid valve still to add. The hardware manual confirms the absolute limits in the LOGO! system manual: a maximum of 24 digital inputs and 20 digital outputs when expansion modules are stacked, but in practice most base units ship with 4 outputs and grow to 16 or 20 through DM8/DM16 modules (LOGO! 8 System Manual, entry ID 109741041).

When the controller cannot be replaced, cannot be networked, and budget rules out a SIMATIC S7-1200, the only practical option is to multiply the load capacity of an existing output. The cleanest way to do this on a relay-output LOGO! is to drive multiple low-power DC contactor coils from a single dry contact, then use the contactor's own auxiliary contacts to provide status feedback to free LOGO! inputs. The remainder of this article is the engineering reference for that fix: rating math, inrush analysis, wiring topology, safety interlock strategy, and verification.

Prerequisites

Before touching the panel, confirm the following:

• Current LOGO! base module part number (e.g. 6ED1052-1MD08-0BA8 = LOGO! 8 12/24 RCE with relay outputs).
• Whether outputs are relay (R) or transistor (T). Relay outputs are required for this technique, since you are switching multiple DC coils in parallel.
• Coil voltage of every contactor on the new conveyors. Mixing 24 VDC coils with 230 VAC coils on a single dry contact is a fault and must not be attempted.
• Steady-state and inrush current of every coil, taken from the manufacturer datasheet, not estimated.
• Available free LOGO! inputs for the safety and overload feedback path. If none are free, the technique is not viable without an expansion DM8 module.

LOGO! Output Ratings: Base vs. Expansion

The contact rating of a LOGO! relay output is not uniform across the system. The base unit and each expansion module type carry different UL/IEC ratings for resistive, inductive (AC-15 / DC-13), and motor loads. Pulling a single number from the front of the manual is a common field error.

For the parallel-contactor fix the only number that matters is the resistive rating of the relay you are actually using, because DC contactor coils look substantially resistive once saturated. Always derate from the published figure by 20% for sustained service and 50% if the contact drives a mixed cluster of inrush-heavy loads.

Steady-State Current Calculation

The driving problem, as posted on the source thread, is four new spine conveyors each fed by a 24 VDC contactor, with the contactors currently intended to be tied to individual LOGO! outputs. Because the LOGO! has only 16 outputs and 12-14 are already in service, the proposal is to drive 3 or 4 contactors in parallel from a single output.

Per-coil data given by the integrator:

• Coil voltage: 24 VDC
• Steady-state current: 197 mA per coil

Aggregate steady-state current for the four-coil cluster:

I_total = N_coils x I_coil
I_total = 4 x 0.197 A
I_total = 0.788 A

This is well under the 10 A base-module rating. The cluster is using only 7.88% of the contact's continuous capacity. Headroom is therefore not the concern; inrush and fault discrimination are.

Inrush Current Analysis

DC contactor coils present a transient load that is significantly higher than steady state at the moment the contact closes. The coil is a wound inductor; at turn-on the only opposition to current is the winding's DC resistance (R_cu), because the inductance opposes the rate of change of current, not the current itself. The current climbs exponentially toward V_coil / R_cu with a time constant of tau = L / R.

A typical inrush multiplier for a 24 VDC contactor coil is 2x to 4x steady state, settling in 30-150 ms depending on coil construction. For a conservative field estimate, assume 3x:

I_inrush_per_coil = 3 x 0.197 A = 0.591 A
I_inrush_cluster  = 4 x 0.591 A = 2.36 A

Even with the worst-case 4x multiplier, total inrush is 3.15 A. The LOGO! relay handles this comfortably; the contact's I^2t let-through is the more relevant limit for sub-10 ms surges, and a 3 A surge on a 10 A rated AgSnO₂ contact is in spec.

Wiring Topologies: Parallel vs. Cascade

Two field-proven topologies are viable. Pick based on whether each contactor must be individually controllable, or whether the cluster is genuinely all-on / all-off.

Topology A: Direct Parallel

All contactor coils wired coil-to-coil across the LOGO! output. The single dry contact closes, all coils energize.

   +24 VDC                          +24 VDC
     |                                 |
     +--[A1 K1]--[A1 K2]--[A1 K3]--[A1 K4]
     |        |        |        |
     |       [K1]     [K2]     [K3]     [K4]   (coils)
     |        |        |        |        |
     +--[A2 K1]--[A2 K2]--[A2 K3]--[A2 K4]
                       |
                  LOGO! Qx (relay COM)

Pros: simplest, lowest component count, no interlock logic. Cons: loss of one coil disables fault discrimination. If any one contactor fails to pull in, the LOGO! cannot tell which one.

Topology B: Cascade Through Auxiliary Contact

Each downstream contactor is fed through the normally-open (N/O) auxiliary contact of the previous contactor. The first contactor is fed directly by the LOGO!; subsequent contactors pull in only when the upstream one is sealed in.

LOGO! Qx ---- [A1 K1 A2]
            |
            +-- (K1 N/O aux) ---- [A1 K2 A2]
                                |
                                +-- (K2 N/O aux) ---- [A1 K3 A2]
                                                    |
                                                    +-- (K3 N/O aux) ---- [A1 K4 A2]
                                                                        |
                                                                   (run feedback to LOGO! Iy)

Pros: built-in sequential start reduces simultaneous inrush by up to 4x. Provides a free run feedback input wired from the last contactor's auxiliary. Cons: cascade is sequential, so the fourth contactor is delayed by the cumulative pickup time (typically 20-40 ms per stage — usually acceptable for conveyor start-up).

For the integrator's case of 4 spine conveyors on a common run command, Topology B is preferred. It trades 100 ms of start latency for a single wire back to a free LOGO! input that confirms the entire chain is sealed in.

Safety Interlock Strategy

Dropping a single output that drives 4 contactors means dropping 4 conveyor segments at once. The most important consequence is the emergency-stop response and the individual motor overload (O/L) response. Treat both as hard requirements, not as nice-to-haves.

Emergency Stop Wiring

The E-stop must open the LOGO! output command path, but the LOGO! scan time and the relay drop-out time (typically 15-25 ms) is acceptable because the contactor's own N/C auxiliary wired in series with the coil provides a hardware-grade disconnect independent of the controller. Recommended:

1. Wire one N/C pole of the contactor in series with the coil it controls. This guarantees that if the contactor welds, downstream power is broken.
2. Route an additional N/O pole of the last contactor back to a free LOGO! input, configured in the program as the "spine run" permissive. If the input goes low without a Qx command going low, the LOGO! raises a fault.
3. Bring the E-stop pushbutton into the LOGO! as a hardwired NC loop through a safety relay or directly into the LOGO! input as called out in the program — never rely on the LOGO! relay contact alone for category-1 stop.

Overload Feedback to Free Inputs

If the panel already has individual thermal overload relays on each conveyor motor, the O/L's N/C "trip" contact is the perfect feedback source. Wire one O/L trip output per spine conveyor to a free LOGO! input, and let the program latch the specific fault. This is the "individual O/L outputs for fault indication" requirement raised in the field discussion.

If free inputs are exhausted, replace one DM8 expansion module with a DM16 and reclaim 8 inputs, or use a wired-OR diode matrix on a single input at the cost of fault resolution. The diode matrix is a serviceable field expedient but never a substitute for individual signaling in a maintenance-heavy plant.

Step-by-Step Implementation

1. Inventory and derate. List every coil that will share the output, with steady-state and inrush current. Verify aggregate inrush < 50% of the relay rating.
2. Verify mixed-voltage compatibility. All coils on one output must share the same nominal voltage. Mixing 24 VDC and 230 VAC is not allowed.
3. Verify free inputs. Confirm at least one free LOGO! input exists for the cascade run-feedback signal. Confirm additional inputs for O/L trip feedback if fault discrimination is required.
4. Stop the program and isolate power. Lock out the 24 VDC logic supply and the contactor-coil supply. Verify zero energy.
5. Wire the first contactor from the LOGO! output (Q1..Q16). Use the relay's common (COM) and normally-open (NO) terminals, not a solid-state output, and observe the polarity of the suppression diode if the coil is DC.
6. Cascade the remaining contactors through the upstream N/O auxiliary blocks. Confirm auxiliary contact rating ≥ coil current; most contactor auxiliaries are 10 A thermal / 1-3 A inductive and are fine.
7. Wire run feedback from the last contactor's N/O auxiliary to a free LOGO! input, configured with a 100 ms debounce filter in LOGO! Soft Comfort.
8. Wire O/L trip contacts to additional free inputs, one per conveyor. Use NC contacts from each O/L relay.
9. Update the program in LOGO! Soft Comfort to treat the spine run input as a permissive: if Qx is high and Iy is low for > 500 ms, latch a fault bit. If any O/L input opens, latch a specific fault word identifying the conveyor.
10. Add a 1 A or 2 A fuse per coil in the +24 V feed. This isolates a shorted coil from the cluster and prevents total loss of the parallel run.
11. Re-energize and download the program.

Verification & Commissioning

Commissioning the parallel cluster requires three passes, each one verifying something the previous step cannot.

Pass 1: Cold Visual and Continuity

• Confirm no coil A1/A2 is reversed, no diode is backwards, no fuse is missing.
• Continuity-check the cascade path: with Qx off, all downstream contactors must show open coil resistance only at the first contactor.

Pass 2: Energized Functional

• Force Qx low. All four contactors should release within 30 ms.
• Force Qx high. All four contactors should seal in, in cascade order, within 200 ms total. Run feedback input should go high.
• Pull one O/L contactor manually (or via a test pushbutton wired to the O/L N/C). The corresponding LOGO! input should drop within one scan. The program should latch the fault.

Pass 3: Dynamic Inrush

• With a clamp meter on the LOGO! output wire in inrush mode, cycle Qx 20 times. Peak inrush should be consistent cycle-to-cycle; a trend upward indicates a failing contact.
• Measure the 24 VDC rail during inrush. A sag of more than 10% indicates the power supply is undersized for the cluster; reduce the simultaneous-start population or add a soft-start NTC.

Alternatives Worth Considering

The decision tree in practice: if the LOGO! is undersized for the long term, the second LOGO! or the S7-1200 is the right answer. If it is a 3- to 6-month capacity blip, the parallel-contactor technique documented above is the cheapest correct fix.

Troubleshooting Matrix

Design Checklist Before Energizing

• All coils on the shared output share the same nominal voltage.
• Aggregate inrush ≤ 50% of relay rating.
• Per-coil fuse installed.
• Run feedback input wired and configured with debounce.
• O/L trip contacts wired to individual inputs (or to a diode-OR matrix with documented compromise).
• Free diodes (1N4007 or equivalent) across each DC coil, cathode to +24 V.
• LOGO! Soft Comfort program updated and downloaded with fault-latch logic.
• E-stop response verified under no-load and loaded conditions.

FAQ