Siemens LOGO! DC Motor Reverse: H-Bridge Interlock Timer Guide

Overview: Reversing a 24 VDC Motor from a Siemens LOGO! 12/24RC

The Siemens LOGO! 12/24RC logic module is a popular controller for small machine automation, but driving a brushed DC motor in forward and reverse from a single device routinely catches first-time users off guard. The naive approach of routing two outputs directly to the motor terminals fails on the first direction change: the motor does not slow down instantaneously, the relay contacts have not yet opened, and the second output closes before the first has released. The result is a direct cross-bar short across the 24 V supply through the motor — a dead short that welds the relay contacts, blows the power supply fuse, or destroys the PLC output drivers within a handful of cycles.

This reference documents a field-proven, four-relay H-bridge implementation using the four on-board relay outputs of the LOGO! 12/24RC, governed by Function Block Diagram (FBD) logic with a 500 ms interlock delay on every direction transition. The same interlock pattern applies to any 24 VDC brushed motor — gearmotors, linear actuators without internal limit switches, damper actuators, and small conveyor drives. The article covers wiring, FBD logic, contact ratings, flyback protection, mechanical alternatives, and a commissioning checklist that prevents the classic dead short.

Prerequisites

Before commissioning the reverser, gather the following:

• Siemens LOGO! 12/24RC (catalog 6ED1052-1MD00-0BA8 for the current 8th-generation release, or 6ED1052-1CC01-0BA8 for the legacy 6th-generation unit). Both provide eight 24 V digital inputs, four relay outputs (5 A continuous / 8 A inrush at 250 V AC, 5 A at 30 V DC resistive), and run on 12/24 V DC logic power. Refer to the Siemens LOGO! 8 product manual (109973614) for terminal layout and derating curves.
• LOGO!Soft Comfort V8.x or later for offline program development and simulation. Download the current build from the Siemens LOGO!Soft Comfort V8.4 download page.
• Four single-pole normally-open relay outputs (the on-board outputs are sufficient up to 5 A continuous). Verify the inrush current of the brushed motor under locked-rotor conditions — small gearmotors stay below 5 A, but a 24 V motor rated above ~80 W should be switched by external contactors driven from the LOGO! outputs.
• 24 VDC power supply sized for 1.5× the motor's full-load current to absorb locked-rotor transients, plus 200 mA for the LOGO! logic power.
• Free-wheeling diodes (1N5408 or Schottky equivalent) across each motor terminal, and a TVS (P6KE33A or similar) across the 24 V bus.
• Three-position selector switch or two maintained pushbuttons (FWD / STOP / REV) wired to LOGO! inputs I1 and I2.

H-Bridge Topology: Why Direction Reversal Demands Special Care

An H-bridge is the canonical topology for bidirectional control of a brushed DC motor. Four switching elements arranged in an H-configuration allow the polarity applied to the motor to be inverted without rewiring. The H-bridge article on Wikipedia is the standard reference for the topology. The forbidden condition is conduction of the two switches on the same leg of the bridge simultaneously — this creates a short-circuit path across the supply that bypasses the motor entirely. In a relay-based H-bridge, this condition occurs whenever the forward and reverse contact sets are closed at the same instant, even for a few milliseconds.

When a PLC user maps Q1 to forward and Q2 to reverse and toggles between them, the PLC scan is fast enough that both relays are pulled in during the same scan cycle. The de-energized relay on the previous direction typically needs 5–15 ms to drop out, during which its contacts are still closed while the new direction's relay has already engaged. The motor's own back-EMF keeps current flowing and can momentarily re-energize a slowly-dropping relay. A single missed transition is enough to spot-weld the silver alloy contacts and progressively destroy the output module.

Topology Options Compared

Three practical topologies exist for reversing a 24 VDC motor from a LOGO! 12/24RC. The choice is dictated by motor current, the number of available outputs, and the cost of contact replacement.

The 4-output on-board H-bridge is the lowest component-count solution and is the focus of the remainder of this article. For motors exceeding the LOGO! 5 A relay rating, the third option — two LOGO! outputs driving a Finder 55.34 mechanical interlock relay — is the conservative choice. The Finder 55.34 series has a built-in mechanical interlock and mirror contacts that prevent both directions from being energized simultaneously, providing hardware-grade safety even if the LOGO! program fails.

Wiring the Four-Relay H-Bridge

The LOGO! 12/24RC has four relay outputs (Q1–Q4) with one common terminal per output. Wire the H-bridge as follows. The schematic shows the four relay contacts as single-pole switches; in the LOGO! they are organized as four independent SPDT (or SPST-NO, depending on module) outputs that share a common logic reference.

• Q1 and Q4 closed → current flows left-to-right through the motor → forward direction.
• Q2 and Q3 closed → current flows right-to-left through the motor → reverse direction.
• Q1 and Q2 are never closed at the same time. Q3 and Q4 are never closed at the same time.

Place a 1N5408 free-wheeling diode across each motor terminal pair (cathode to the more positive side) to absorb inductive kick when the field collapses on de-energization. Add a 5 A fast-blow fuse (Littelfuse 0314005.HXP or equivalent) in line with the 24 V supply feeding the H-bridge — this fuse is the second line of defense behind the interlock timer and prevents a welded contact from drawing unlimited current from the supply.

Step-by-Step: LOGO! FBD Program with 500 ms Interlock

The FBD logic below uses the input set/reset and the on-delay / off-delay blocks already present in every LOGO! 12/24RC. No special function is required, so the program fits within the smallest LOGO! 0BA8 base module.

1. Map inputs. Wire the FWD maintained pushbutton to I1, the REV pushbutton to I2, and the STOP (NC) pushbutton to I3. Tie I1 to a contactor-closed feedback input if the motor drives a load that may have stalled.
2. Build the forward latch. Use an SR (set/reset) flip-flop block, model B001. Set input is wired through the FWD pushbutton and the NOT of the reverse latch output. Reset is wired through STOP and the NOT of the FWD input through a 500 ms on-delay (B002).
3. Build the reverse latch. Use a second SR flip-flop, model B003. Set input is wired through the REV pushbutton and the NOT of the forward latch output. Reset is wired through STOP and the NOT of the REV input through a second 500 ms on-delay (B004).
4. Generate the interlock delay. Each direction's latch output is routed through its own on-delay timer (B005 for FWD, B006 for REV) with a 500 ms preset. The output of B005 drives Q1 and Q4; the output of B006 drives Q2 and Q3. This is the critical section: the timer ensures that whenever the direction is changed, the previous direction's relays have at least 500 ms of guaranteed drop-out time before the new direction's relays pick up.
5. Add mutual exclusion. AND the FWD output path with the NOT of the REV latch, and AND the REV output path with the NOT of the FWD latch. This is a software interlock that runs in parallel to the timer — defense in depth.
6. Add a stop pulse. When the STOP pushbutton (I3) is pressed, generate a one-shot pulse to force both latches off and start the 500 ms off-delay (B007). Neither Q1/Q4 nor Q2/Q3 may energize until B007 has timed out, preventing a stop-and-immediately-reverse (plug-reverse) maneuver.
7. Test the simulation. In LOGO!Soft Comfort, run the simulation and force I1 on, then I1 off followed immediately by I2 on. The trace should show Q1/Q4 dropping 500 ms before Q2/Q3 rise. If Q1 and Q2 are ever true at the same instant, the interlock is broken — fix before connecting to hardware.

Why 500 ms? Interlock Timing Math

The 500 ms interlock is not arbitrary. The 500 ms value is the result of multiplying four contributing delays by a safety factor:

Adding the worst-case stack of 415 ms and applying a 1.2× safety factor yields ~500 ms. The same value was discovered empirically by field engineers who tried shorter delays (100 ms, 250 ms) and observed the same dead short each time the relay dropped out. If the motor has an exceptionally large armature (a 24 V gearmotor with 5+ mF of effective capacitance on the commutator), extend the timer to 750 ms or 1 s. The cost of an extra 250 ms of dead time is trivial compared to replacing a $400 LOGO! module.

Component Sizing and Protection

Relay contact rating for the LOGO! 12/24RC outputs is 5 A continuous at 30 V DC resistive. For inductive loads (DC motors, solenoid valves, contactor coils), derate to 2 A continuous or 50% of the rated wattage per the LOGO! 8 manual (109973614), page 232, "Relay Output — Inductive Load Derating". A motor drawing 3 A continuous from a 24 V supply is within derating for 24 VDC resistive loading only if no inrush is expected. To stay safe with brushed motors, the practical ceiling is ~1.5 A continuous per on-board relay output, which equates to roughly 36 W of mechanical power at 24 V. Above that, route the LOGO! outputs to external contactors.

External contactor sizing example: a 24 VDC motor drawing 8 A at full load needs contactors rated for 12 A inductive (1.5× service factor). A Eaton DILER-40(24VDC) or a Finder 55.34 with 24 VDC coil and 10 A silver-cadmium contacts is a common match. Drive the contactor coils directly from the LOGO! outputs; the LOGO! can sink the 200 mA pickup current and the 50 mA sealed current without derating.

For reverse-energy protection, place a TVS diode (P6KE33A, breakdown 33 V, peak pulse 13 A for 1 ms) across the 24 V bus at the H-bridge input. A 24 VDC motor regenerating back into a 24 V supply can push the rail above 35 V during a hard reversal; the TVS clamps the spike before the LOGO! 24 V input protection (typically a 36 V TVS internally) becomes the failure mode.

Commissioning and Verification

Before applying power, perform the following bench checks. They take five minutes and prevent the kind of cascading damage that ruins a first commissioning.

1. Continuity check of the H-bridge wiring with the LOGO! powered off. With all four relays de-energized, there must be infinite resistance between the 24 V supply positive terminal and the supply negative terminal through the H-bridge. If the meter reads 0 Ω, two contacts are shorted (likely a wiring error). If it reads the motor resistance (~5–50 Ω), the bridge is wired correctly and the motor is in the path.
2. Simulated direction test with the LOGO! in STOP mode and the motor disconnected. Force FWD in the simulation; verify Q1 and Q4 close. Force REV 600 ms later; verify Q1 and Q4 open and Q2/Q3 close only after the 500 ms interlock. The trace must never show Q1 and Q2 high at the same time, nor Q3 and Q4 high at the same time.
3. No-load mechanical test: with the motor uncoupled from the load, run FWD for 5 s, STOP, wait 1 s, REV for 5 s, STOP. Listen for any click from the relay contactor during the transition — a single soft click is normal (relay dropout), a hard clack followed by a secondary chatter indicates contact bounce or welding.
4. Loaded mechanical test: couple the load, repeat step 3. Measure the 24 V supply rail with a scope at the H-bridge input. The rail must not dip below 21 V (12% sag) during a 10 A inrush event. If it sags below 18 V, the power supply is undersized and may trigger the LOGO!'s brownout reset.
5. Thermal test: run 50 forward-reverse cycles over 5 minutes, with a 2 s dwell in each direction and the full 500 ms interlock. The relay contactor case temperature must remain below 70 °C. If it climbs above 80 °C, the contactor is undersized for the locked-rotor duty cycle.
6. Endurance test: log 1,000 forward-reverse cycles with the motor under load. Inspect the relay contacts at 250, 500, and 1,000 cycles for pitting or transfer material. Any visible pitting at 250 cycles means the inrush current is exceeding the contact rating and the contactor needs to be replaced with a higher-rated unit.

Troubleshooting Matrix

Alternative: External Mechanical Interlock Relay

For installations where a software-only interlock is unacceptable (medical devices, food-grade conveyors, AGV wheel drives), use a mechanically interlocked relay. The Finder 55.34 series and the Omron MY4K-N1 (with mechanical interlock accessory) provide a hard-wired guarantee that forward and reverse cannot be energized simultaneously. The LOGO! drives only the two interlock relay coils (Q1 → forward coil, Q2 → reverse coil); the interlock relay handles the high-current switching.

Wire the auxiliary NC contact of the forward contactor in series with the reverse contactor coil, and vice versa. Even if the LOGO! outputs both close (a software bug), the auxiliary contact prevents the second contactor from pulling in. The Finder 55.34 has a mechanical flag that physically blocks the second contactor from moving; this is the highest-reliability interlock available in a 14-pin industrial package and is the recommended pattern for SIL-2 systems.

Alternative: Solid-State H-Bridge Driver

For motors under 5 A continuous, a solid-state H-bridge module eliminates the contact-wear failure mode entirely. The Pololu brushed DC motor driver family includes the DRV8870 (3.6 A continuous, 6.5 V–45 V), the TB6612FNG (1.2 A continuous, 4.5 V–13.5 V), and the VNH5019 (30 A continuous, 5.5 V–24 V). These accept two logic-level direction inputs (INA, INB) and a PWM input for speed control. Wire the LOGO! Q1 and Q2 to INA and INB; the DRV8870 handles shoot-through protection internally with a typical 220 ns dead time. No 500 ms interlock is required, though a 50 ms debounce on the direction change is recommended to prevent audible PWM glitches. Solid-state drivers are the right choice for high-cycle applications (>10,000 reversals per day) where mechanical contact wear is the limiting factor.

Field-Proven Lessons

The original discovery that prompted this reference was an engineer who wired a LOGO! 12/24RC to a 24 VDC gearmotor using two on-board outputs and observed a dead short every time direction changed. After extensive wiring review he added a 500 ms delay on each open and close input to the relay and the problem disappeared. The 500 ms value is now the de facto default in panels where on-board relay H-bridges are used. Two takeaways are reinforced by this experience:

• Always scope the relay contact waveform during the first commissioning. A 1 ms overlap is invisible to a multimeter but catastrophic to the contact. A $30 USB oscilloscope (Hantek 6022BE or PicoScope 2204) saves $400 of LOGO! replacement.
• Keep the FBD simulation in LOGO!Soft Comfort as the canonical reference. If a future engineer cannot explain why the interlock is 500 ms, the program will get shortened to 100 ms during a "performance optimization" and the contacts will weld. Document the timer in a comment block inside the FBD file so the rationale travels with the program.

Frequently Asked Questions