Allen-Bradley 1746-P3 24 VDC Input Power Supply Module

Description

Product Introduction

The Allen-Bradley 1746-P3 is a rack-mounted DC power supply engineered for the SLC 500 PLC family. Unlike the 1746-P1 and 1746-P2, which require AC input, the 1746-P3 operates directly from a nominal 24 VDC industrial power source, making it well suited for battery-backed control systems, mobile equipment, and facilities that standardize on DC distribution.

In our field experience, the 1746-P3 is frequently found in water treatment plants, OEM machinery, and remote process stations where maintaining control during brief AC disturbances is important. Before installing a replacement, calculate the total 5 VDC and 24 VDC backplane current requirements. An overloaded power supply often causes intermittent processor faults that resemble CPU or communication failures rather than power problems.

Key Technical Specifications

Application Scenarios & The “Trench” Experience

Power supply failures rarely look like power supply failures.

One shift reports random processor faults. Another shift sees communication errors. A week later the rack suddenly reboots without warning. Everyone starts replacing CPUs, communication modules, and I/O cards.

The actual problem?

The power supply can no longer maintain stable backplane voltage under load.

Typical applications include:

• Water & Wastewater – Supplying 24 VDC-powered SLC 500 racks installed in pumping stations with battery-backed DC systems.
• Oil & Gas – Operating remote PLC panels powered from DC distribution systems where AC utility power is unavailable.
• Machine Building – Supporting OEM equipment designed around centralized 24 VDC control power.
• Power Generation – Powering auxiliary control cabinets connected to station DC systems.
• Transportation Infrastructure – Maintaining PLC operation in tunnel ventilation, signaling, and utility control systems using regulated DC supplies.

Field Case

A pipeline booster station experienced random PLC resets that became more frequent during cold mornings. Technicians initially suspected the processor because diagnostics showed no I/O faults.

After measuring the backplane under load, maintenance found the aging 1746-P3 could no longer maintain stable output during input voltage dips. Replacing the power supply eliminated the resets immediately. No processor replacement was required, and the station returned to continuous service before the next pumping cycle.

1746-P3

1746-P3

1746-P3

Transparency SOP: Quality Assurance & Testing

We don’t just ship boxes; we test them on actual Allen-Bradley SLC 500 racks.

• Inbound Inspection
  • Verify catalog number, hardware revision, and serial labels.
  • Inspect enclosure, terminals, and backplane connector.
  • Screen for counterfeit labels, corrosion, and previous repairs.
• Live Rig Testing
  • Install the unit in a functioning SLC 500 chassis.
  • Verify processor startup and complete rack initialization.
  • Operate under representative backplane loading.
• Electrical Parameter Checks
  • Measure regulated 5 VDC and 24 VDC outputs.
  • Verify operation across the specified 19.2–28.8 VDC input range.
  • Confirm voltage stability during load changes.
• Hardware Verification
  • Inspect the fuse.
  • Record hardware revision.
  • Verify compatibility with supported SLC 500 chassis.
• Final QC & Anti-Static Packaging
  • Complete functional inspection.
  • Clean connectors.
  • Package in ESD-safe materials with protective cushioning.

The Veteran’s Tech Trap Guide

⚠️ Calculate the total chassis current before replacing the supply. The 1746-P3 provides 3.6 A at 5 VDC and 0.87 A at 24 VDC. Adding high-current analog or communication modules without checking the power budget is asking for intermittent faults.

⚠️ Don’t confuse the 1746-P3 with the 1746-P1. They fit the same chassis, but the 1746-P3 requires a 24 VDC input, while the 1746-P1 operates from AC mains. Installing the wrong model prevents the rack from operating.

❗ Measure the incoming DC voltage under load. A healthy power supply cannot compensate for an undersized or failing external 24 VDC supply. Voltage sag during startup often appears as unexplained processor resets.

⚠️ Inspect the fuse before replacing the unit. A blown 1746-F3 fuse may indicate a downstream short circuit rather than a defective power supply. Replacing the power supply without identifying the field fault usually leads to another outage.

Always verify the chassis power budget and field wiring using the official Rockwell installation documentation before energizing the system.

Frequently Asked Questions (FAQ)

Q1. Can the 1746-P3 be replaced while the PLC is powered?

No. Remove the 24 VDC input supply before removing or installing the power supply. Live replacement can damage the backplane or connected modules.

Q2. What input voltage does the 1746-P3 require?

The module accepts 19.2 to 28.8 VDC, with 24 VDC nominal operation. It is intended for DC-powered SLC 500 systems.

Q3. How much backplane current is available?

The 1746-P3 supplies 3.6 A at 5 VDC and 0.87 A at 24 VDC for the SLC 500 backplane. Unlike some other SLC power supplies, it does not provide a separate user 24 VDC output.

Q4. Is the 1746-P3 still manufactured?

No. Rockwell Automation discontinued the 1746-P3 on September 30, 2024, and no direct Rockwell replacement is planned. Most available units today come from verified surplus inventory or professionally refurbished stock.

Q5. Can I replace a 1746-P1 with a 1746-P3?

Only if the control cabinet is designed for 24 VDC input and the power budget has been verified. The two power supplies use different input sources and have different backplane current capacities. Always review the system design before substitution.

Q6. Are refurbished power supplies dependable?

They can be, provided they undergo load testing, output voltage verification, thermal stability testing, and live rack operation. Cosmetic condition alone is not a reliable indicator of remaining service life.

Q7. What warranty is typically offered?

Most reputable industrial automation suppliers provide a 12-month warranty on qualified new surplus and professionally refurbished 1746-P3 power supplies, covering functional defects under normal industrial operating conditions.