Murrelektronik 857781 Migration-Ready PSU for Legacy Systems

Overview

Murrelektronik 857781 Migration-Ready PSU for Legacy Control Systems

The Murrelektronik 857781 is a compact, high-reliability 24 VDC DIN rail power supply from the MPS series, purpose-built for industrial control environments where legacy infrastructure must be maintained, upgraded, or replaced without disrupting production continuity. As original equipment manufacturers phase out older PSU models and control cabinet integrators face increasing pressure to source verified replacements, the 857781 has become a trusted drop-in solution across a wide range of retrofit and modernization projects.

Whether you are replacing an end-of-life power supply in a Siemens S7-300 control cabinet, upgrading the power rail in a Schneider Electric Modicon TSX rack, or restoring a legacy Allen-Bradley SLC 500 panel, the 857781 delivers the stable 24 VDC output required to power PLCs, I/O modules, communication gateways, and HMI panels without requiring structural changes to the existing cabinet layout. Its slim DIN rail form factor fits standard 35 mm TS35 rails, making it compatible with the majority of control enclosures manufactured over the past three decades.

Before integrating the 857781 into an existing system, engineers should verify several key parameters. First, confirm the total connected load — sum the current draw of all 24 VDC consumers including the CPU module, digital I/O cards such as the SM 321 or SM 322, analog input modules, and any 24 VDC-powered field devices. The 857781’s rated output current must meet or exceed this total with a recommended 20% headroom. Second, inspect the existing terminal block wiring: the output terminals accept standard ferrule-terminated conductors, and the input terminals support both 115 VAC and 230 VAC ranges, eliminating the need for a separate voltage selector in mixed-voltage facilities.

For projects involving communication protocol migration — for example, transitioning from PROFIBUS DP to PROFINET IO — the 857781 provides the clean, low-ripple 24 VDC supply that PROFINET-capable gateways and managed Ethernet switches require for stable operation. When installing a new PN/DP coupler or replacing a CP 342-5 communications processor with a CP 343-1 Advanced, the power budget of the existing PSU must be recalculated. The 857781 simplifies this step by offering a clearly rated, predictable output that integrates directly into the updated power distribution scheme.

Backplane and rack compatibility is another critical consideration during retrofit planning. In systems using modular I/O racks — such as the Murrelektronik MVK Metal or third-party distributed I/O stations — the 857781 can serve as the centralized 24 VDC source feeding the rack’s power bus. Engineers should confirm that the backplane bus voltage matches the PSU output and that the inrush current at startup does not exceed the PSU’s peak current rating. For systems with high capacitive loads, a soft-start or staged power-up sequence may be required to prevent nuisance tripping.

HMI panel compatibility is equally important. Many legacy operator panels — including older Siemens TP177 or MP277 units — require a stable 24 VDC supply with minimal voltage drop under load. When replacing a failed PSU in a cabinet that also powers an HMI, verify that the 857781’s output voltage remains within the HMI’s specified input range (typically 20.4–28.8 VDC) under full load conditions. If the HMI and PLC share a common 24 VDC bus, consider adding a decoupling diode or a secondary PSU to isolate HMI power from PLC logic power, reducing the risk of HMI resets caused by PLC-induced load transients.

Installation space confirmation is a practical step that is often overlooked during the planning phase. The 857781’s compact housing is designed to minimize DIN rail footprint, but the available space in the control cabinet must be measured before ordering. Confirm the available rail length, the clearance above and below the unit for convection cooling, and the proximity to heat-generating components such as motor starters or frequency inverters. Adequate spacing ensures the PSU operates within its thermal derating curve and achieves its rated service life.

Firmware and program compatibility are not directly affected by a PSU replacement, but the power-up sequence can influence PLC initialization behavior. In systems where the CPU performs a cold restart if power is interrupted for more than a defined period, replacing the PSU may alter the restart behavior if the new unit has a different hold-up time. Verify the hold-up time specification of the 857781 against the PLC’s power-fail detection threshold to ensure the CPU performs a warm restart rather than a cold restart, preserving the current program state and output values.

Every 857781 unit supplied by KNMKS undergoes pre-shipment functional testing, including output voltage verification under load, ripple measurement, and visual inspection of terminals and housing. Units are dispatched with a support terms confirmed by quotation covering manufacturing defects and premature failure under normal operating conditions. Stock is maintained for immediate dispatch, supporting urgent maintenance requirements and minimizing unplanned downtime in critical production environments.

Migration Compatibility Table

Retrofit Planning for Existing Automation Systems

A successful retrofit begins with a complete power audit of the control cabinet. List every 24 VDC consumer: the PLC CPU (e.g., a Siemens CPU 315-2 DP or CPU 317-2 PN/DP), all installed I/O modules including SM 321 digital input cards and SM 332 analog output modules, any installed CP communications processors, and field-side devices powered from the cabinet’s 24 VDC bus. Add the inrush current of contactors and relay coils if they share the same supply rail.

When the cabinet also houses a Murrelektronik MVK Metal distributed I/O node or a third-party remote I/O station connected via PROFIBUS or PROFINET, the 24 VDC supply to the fieldbus coupler must be stable and free of voltage dips during module insertion or removal. The 857781’s regulated output and fast transient response make it well-suited for this application. If the system includes a Murrelektronik 9000-41034-0300300 or similar signal conditioner for analog loop isolation, confirm that the conditioner’s 24 VDC input tolerance is met under all load conditions.

For cabinets undergoing a full modernization — replacing an older Siemens S5-115U rack with a current-generation S7-1500 system — the 857781 can serve as the interim 24 VDC supply during the transition phase, powering the new CPU and I/O modules while the legacy system is progressively decommissioned. This phased approach reduces the risk of a full production stoppage and allows the commissioning engineer to validate each new module’s operation before the next legacy component is removed. A programming cable such as a PC Adapter USB A2 or a SIMATIC NET CP 5711 may be required to transfer and adapt the existing STEP 7 program to TIA Portal format during this phase.

Downtime Control During System Migration

Minimizing unplanned downtime during a PSU replacement or control system upgrade requires careful pre-migration preparation. Before removing the existing power supply, document all terminal wiring using photographs and a wiring schedule. Label every conductor at both ends with permanent markers or cable labels. If the cabinet uses a common 24 VDC bus bar, confirm that the bus bar can be de-energized independently of the main incoming supply, allowing the PSU to be replaced without shutting down the entire panel.

Where a hot-swap or live replacement is not possible, schedule the migration during a planned maintenance window. Prepare the 857781 for installation in advance: mount it on a short section of spare DIN rail, pre-terminate the input and output conductors, and verify the output voltage with a calibrated multimeter before installing it in the cabinet. This pre-assembly approach reduces the time the cabinet is de-energized to the minimum required for the physical swap and reconnection.

After energizing the new PSU, monitor the output voltage under load for a minimum of five minutes before re-enabling the PLC CPU. Check for any fault LEDs on I/O modules, communication processors, or HMI panels. If the PLC performs a cold restart, reload the program from the memory card or programming device and verify that all output states are correct before releasing the system to production. Document the completed migration in the cabinet’s maintenance log, including the date, the replaced component’s serial number, and the new PSU’s installation date for support terms tracking purposes.

Retrofit Support FAQ

Q: Is the Murrelektronik 857781 a direct replacement for other MPS series power supplies?
A: The 857781 is compatible with standard MPS series mounting positions and terminal configurations. Verify the output current rating of the unit being replaced against the connected load before substitution. If the original unit had a higher current rating, confirm that the 857781’s rated output is sufficient for the application.

Q: What commissioning steps are required after installing the 857781?
A: After installation, verify the output voltage at the PSU terminals and at the most distant load point in the cabinet. Check for voltage drop across the 24 VDC bus wiring. Confirm that all connected modules power up without fault indications. If the PLC CPU performs an unexpected cold restart, check the hold-up time specification and compare it to the CPU’s power-fail detection threshold.

Q: Can the 857781 power both PLC logic and field devices from the same output?
A: While technically possible, best practice is to separate PLC logic power from field device power using a decoupling diode or a dedicated secondary PSU. This prevents field-side load transients — such as solenoid valve switching — from causing voltage dips on the PLC logic supply, which can trigger CPU faults or I/O module errors.

Q: What support terms and testing does KNMKS provide with the 857781?
A: Every unit is pre-tested before dispatch, including output voltage verification under load and ripple measurement. A support terms confirmed by quotation is provided from the dispatch date, covering manufacturing defects and premature failure under normal operating conditions. Stock is available for immediate dispatch to support urgent maintenance requirements.

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