1 PC Used Fanuc A20B-1005-0421 In Good Condition A20B10050421 A2OB-1OO5-O421

FANUC A20B-1005-0421 | FS20 / F0I Power Supply PCB — Series 16-C / 18-C / 21-B CNC, Japan Origin

Part Number: A20B-1005-0421

Manufacturer: FANUC Corporation (Japan)

Product Type: Power Supply PCB (Board-Mount Type)

Board Series: A20B-1005

Reference Number: A350-1005-T422

Function: FS20/F0I controller power supply — DC voltage generation for controller boards

Input: 24V DC

Compatible Systems: Series 16-C, 18-C (Type C PSU), Series 21-B (Type B PSU)

Installation: Bolts directly onto the I/O board to form the combined I/O PSU assembly

Status: Legacy (limited OEM availability)

Overview

The A20B-1005-0421 is the power supply PCB for FANUC's FS20 and F0I controller designations — the power board that generates the multiple regulated DC voltages the controller's boards need to operate.

It is a board-mount type power supply: it mounts directly onto the I/O board, physically combining with it to form the I/O PSU assembly that powers the control system. 

  The reference number A350-1005-T422 identifies the same unit in the FANUC parts documentation.

This power supply board served two roles across its application range. In the Series 16-C and 18-C controllers, it functions as the Type C power supply — a smaller capacity PSU matched to the power requirements of the C-type control backplane.

In the Series 21-B controller (including 21-TB and 21-MB variants), it functions as the Type B power supply, powering the compact two-slot backplane configuration of the 21-B series.

The 24V DC input is a defining characteristic of this board. Unlike the earlier FANUC power supply designs that required AC input and performed the AC-to-DC conversion internally, the A20B-1005-0421 takes 24V DC from the cabinet's DC power distribution and converts it to the multiple lower-voltage DC rails the controller needs.

This 24V DC input approach was adopted as controller power consumption decreased with successive generations — the lower power requirements eliminated the need for the relay input stage and the heavier transformer-based AC input supply used in earlier designs.

Key Specifications

Board-Mount PSU Architecture

The A20B-1005-0421 is not a standalone power supply unit in the traditional sense. It is designed to be a sub-assembly component that attaches physically to the I/O board.

The two boards together — the I/O board providing the backplane connection and I/O interface functions, and the PSU board providing the power regulation — form a single functional unit that serves the controller as its combined power and I/O module.

This integrated design was a practical evolution from the earlier approach where the power supply occupied a dedicated slot in the controller rack.

By combining the PSU with the I/O board, the slot count for the controller was reduced, allowing the controller chassis to be more compact.

This suited the 21-B series particularly, which used a minimal two-slot backplane to keep the controller physically small and cost-effective.

The board-mount installation means the PSU board's connectors engage with the I/O board's matching connectors when the two boards are brought together and secured with mounting screws.

The power rails generated by the PSU board are distributed to the controller's other boards through the backplane. 

No external wiring is needed between the two boards once they are assembled.

Multi-Controller Compatibility

The same A20B-1005-0421 board served multiple FANUC controller types across two different functional designations.

This commonality was deliberate: FANUC's engineering rationalised the power supply design across controller variants that had different overall architectures but similar power requirements at the I/O PSU level.

For the 16-C and 18-C systems, the board serves turning centre (TC) and machining centre (MC) configurations: the FS-16TC, FS-18TC, FS-18MC, FS-18GC, and similar controller variants that use the C-type backplane.

For the 21-B systems — the FS-21TB and FS-21MB — the same board provides the complete power supply for the compact two-slot backplane.

The sibling board A20B-1005-0420 serves the B-series variants (16-B, 18-B, 21-B larger capacity configurations) of the same controller generation.

Failure Modes and Repairability

Like all switching power supply boards, the A20B-1005-0421 contains electrolytic capacitors, switching transistors, and magnetic components that are subject to age-related degradation.

Electrolytic capacitors are statistically the most common failure source — they dry out over time, reducing their capacitance and increasing their ESR, which degrades the supply output ripple and regulation.

One practical limitation of this board is that severe failures — where switching transistors have failed short-circuit or where the board has experienced an overvoltage event — can cause burn damage that makes the board effectively non-repairable at component level.

In these cases, replacement is the only viable option. For boards showing the earlier, subtler symptoms of aging — occasional controller resets, unexplained parameter corruption, erratic behaviour at power-on — preventive refurbishment before complete failure is the better strategy.

FAQ

Q1: The controller powers on but some boards in the rack appear not to receive their supply voltages — certain functions are missing or the controller startup stalls at an intermediate point. Could this be the A20B-1005-0421?

Partial power delivery affecting specific boards points to the PSU as a likely cause. The A20B-1005-0421 generates multiple voltage rails; if one rail fails while others remain functional, the boards powered by the failed rail will not initialise while others appear normal.

Measure the output voltages of the PSU against the documented specifications.

A rail that is low, absent, or has excessive ripple confirms the PSU as the fault. Before concluding it is the PSU board, verify the 24V DC input to the board is correct.

Q2: The controller shows no signs of life at power-on — no display, no LEDs, no response. Power was confirmed lost to the machine tool but not the cabinet supply. What should be checked on the PSU?

A completely dark controller with confirmed cabinet supply usually indicates the PSU has failed open-circuit — it is not generating any of its output voltages. Check the PSU board for visible damage.

Check any fuses associated with the 24V DC input path to the PSU.

If the board appears undamaged and fuses are intact, the PSU's internal switching circuit has likely failed. Replace the PSU board.

Q3: After replacing the A20B-1005-0421, the controller appears to operate correctly but parameter data was lost. Was the PSU replacement the cause?

A PSU replacement does not directly cause SRAM parameter loss — the SRAM battery is what maintains parameter data during power-off, not the PSU.

However, if the PSU failure event involved a supply voltage fault that caused the SRAM battery's protection circuit to be overwhelmed, or if the SRAM backup battery was already near depletion and the PSU fault created an extended power-off condition that exhausted it, data loss can follow.

Check the battery voltage after restoring power. If the battery is below its retention threshold, replace it. Restore parameters from backup.

Q4: Is the A20B-1005-0421 interchangeable with the A20B-1005-0420?

These are the two sibling PSU boards in the same family, but they are not interchangeable. They have different power ratings and are matched to different controller configurations. The -0421 is specified for the Type C (16-C/18-C) and Type B (21-B) configurations.

The -0420 is specified for the Type C (16-B/18-B) and higher-capacity 21-B configurations.

Connector positions, power rail capacities, and regulatory circuit designs differ between them. Installing the wrong variant can lead to insufficient power delivery or component stress.

Q5: The controller is a 21-TB that runs a high-cycle production line. How should PSU maintenance be planned to avoid unplanned downtime?

High-cycle production controllers operate their PSU boards at or near thermal capacity continuously.

The electrolytic capacitors in these boards age faster under continuous high-temperature operation than in lower-utilisation applications. 

A preventive replacement schedule based on hours of operation — or a calendar schedule of 5-7 years for continuous production — is more effective than waiting for failure. Keeping a tested spare A20B-1005-0421 on-site provides immediate swap capability when the time comes.

Exchange programs from CNC service suppliers make obtaining a tested replacement straightforward.