1746-P4 (SLC 500 Power Supply): Specs, Common Failures & Replacement

The 1746-P4 is the highest-capacity standard power supply in the Allen-Bradley SLC 500 family — the module bolted to the left of the chassis that keeps every I/O card and the CPU alive. SLC 500 systems are long discontinued, but they run entire production lines to this day, and when a 1746-P4 fails, the whole rack goes dark. This guide covers the specs, how to tell a dead power supply from a dead CPU, and the fastest way back.

What Is the 1746-P4?

The 1746-P4 is a rack-mounted power supply for the SLC 500 modular hardware platform. It takes 120/240 V class AC mains and converts it to the DC voltages the chassis backplane needs to run the processor and I/O modules. Within the 1746-P family, capacity steps up from P1 through P4 — the P4 is the top of the range, chosen for fully loaded chassis with many modules.

If you are newer to the SLC platform, our PLC basics guide gives useful context on how power supplies, CPUs, and I/O fit together in a rack system.

1746-P4 Key Facts

Dead Rack: Is It the 1746-P4 or the CPU?

When an SLC 500 rack goes dark, the power supply is the prime suspect — but not the only one. Run this sequence before ordering anything:

1. Check the mains first. Verify incoming voltage at the power supply terminals. Tripped breakers and loose terminals cause more “dead PSU” calls than dead PSUs.
2. Listen and look. A healthy 1746-P4 hums faintly and its LED indicator is lit. Silent, dark, or smelling of heat means internal failure.
3. Measure the backplane rails if you can. No DC output with good mains in equals a failed supply. Good rails with a dark CPU points elsewhere.
4. Consider the load. A shorted I/O module can drag a good power supply down. If a replacement supply also fails to start, start pulling modules to find the short.

Getting this right saves real money: we regularly test “failed” 1746-P4 units that are perfectly good — the fault was upstream power or a shorted module downstream.

Common 1746-P4 Failures We See in Our Repair Lab

1. Electrolytic capacitor aging. The number one killer after 20+ years of service. Dried-out capacitors cause reduced output, ripple, and eventually a supply that will not start. Routine repair for a properly equipped lab.
2. Blown internal fuse or inrush components. Usually following a mains surge or lightning event. The fuse itself is cheap — but it often blows for a reason, so we test the whole primary circuit before returning a unit.
3. Heat damage. A fully loaded chassis in a hot cabinet cooks the supply over time. Discolored components and cracked solder joints are the telltale signs.
4. Intermittent output. The maddening one: the rack randomly resets or faults, then works for weeks. Aging capacitors again — we soak-test under load to reproduce it before repairing.

SLC 500 rack down and not sure it is the 1746-P4? Tell us the LED states and what the mains voltage reads at the terminals — our engineers will help you isolate it before you order. [email protected]

Compatibility: Can a P4 Replace a P1, P2, or P3?

• Stepping up is usually fine. A 1746-P4 has more capacity than a P1, P2, or P3, and oversizing a power supply is generally acceptable — as long as the physical mounting and input voltage match your chassis.
• Stepping down is not. Replacing a P4 with a lower-capacity supply risks overload on a fully loaded chassis. Match capacity or exceed it.
• Check the input rating. Confirm whether your panel feeds 120 V or 240 V class power and that the supply is configured accordingly.

Repair or Replace: What Makes Sense?

Given the age of every SLC 500 in service, our standing advice is to keep one tested spare on the shelf. A power supply sitting in stores is far cheaper than a production line waiting on international shipping.

1746 Series In Stock at OUKE

We support the full SLC 500 platform — power supplies, processors, and I/O like the 1746-NI4 analog input module. See our Allen-Bradley product overview for the wider range. Every unit is bench-tested, ships from our Dongguan warehouse within 48 hours, and carries a minimum 1-year warranty with no MOQ.

Repairs run 3–5 working days standard, with a 99.5% first-pass success rate.

FAQ

Is the 1746-P4 discontinued?

Yes. The entire SLC 500 platform is discontinued and no longer sold new. Tested surplus, refurbished, and repaired units from specialist suppliers like OUKE are the standard way to keep these systems running.

What is the difference between the 1746-P1, P2, P3, and P4?

Capacity. They are all SLC 500 rack power supplies on the same platform, stepping up in available output from P1 to P4. The P4 is the highest-capacity standard model, used on heavily loaded chassis.

My SLC 500 rack is completely dead. Is it the power supply?

Most likely, but verify first: check incoming mains at the terminals, then check for DC output from the supply. Good mains in and no DC out confirms a failed supply. Also consider a shorted I/O module dragging the supply down.

Can I use a 1746-P4 in place of a smaller P-series supply?

Generally yes — oversizing the supply is acceptable as long as mounting and input voltage match. Never go the other direction on a heavily loaded chassis.

How long does 1746-P4 repair take?

Our standard repair turnaround is 3–5 working days with a minimum 1-year warranty. If downtime is critical, a tested unit ships from stock within 48 hours.

Get Your 1746-P4 Back Running Fast

Need a tested 1746-P4 or a repair quote for your failed supply? We have 100,000+ SKUs in stock, all tested, shipped within 48 hours — no MOQ. Send us your part number: [email protected] or WhatsApp +86 15107352558.