How to Modify an APW12 PSU for 120V AC Use: A Comprehensive Guide

Warnings and Disclaimers

The APW12 Bitmain power supply is a robust, high-performance power supply capable of powering most Bitmain X19 and X21 mining rigs. However, in its factory configuration, it requires at least 200V AC input to work properly. This guide provides instructions for modifying the APW12 to allow it to run from 120V AC.

Hashing from 120V AC

Applying the modification steps in this guide to the APW12 PSU will enable it to power most Bitmain X19 and X21 mining rigs from 120V AC. However, extreme care must be taken to not overload the 120V AC circuit (circuit breaker, wiring, outlet, etc.).

Limiting Miner Power Draw

When powering a miner from a standard 120 volt, 15 amp outlet, it’s crucial to limit the miner’s power draw to no more than 1440W MAX (1200W recommended). Here are two options for achieving this:

1. Reducing Hash Voltage and Hash Frequency

• Lower hash voltage (voltage at hash board’s power terminals) and hash frequency generally reduce power draw.
• Avoid lowering hash voltage below the miner’s minimum voltage requirement to prevent ASIC chip failures.
• For multiple hash boards on 120V, run at or close to minimum hash voltage (approximately 12V for most X19 variants, 14V for some) AND underclock significantly.
• Start with low frequency (50 – 100 MHz) and slowly increase until desired power limit is reached.
• When using autotune, start with the lowest power target and increase slowly.
• If lowest autotune setting exceeds 1440 watts, either avoid autotune or remove hash boards.

2. Remove Hash Boards

• Standard X19 miners come with 3 hash boards.
• Each board can draw 350W to 2000W based on voltage, frequency, and temperature.
• Removing one board reduces power draw by about a third, two boards by about two-thirds.
• Removal allows remaining boards to run at higher frequencies within power limits.
• Manage proper airflow through remaining boards – air must flow through heatsink fins.
• Fill gaps to prevent air from bypassing heatsinks.
• Removed boards can be used to create additional 120V mining rigs.
• When powering multiple rigs, distribute them across different circuit breakers.

APW12 120V Unlock Instructions

Follow these steps carefully to modify the minimum required input voltage on the APW12 from 200V AC to 120V AC. Work in a well-ventilated area with protective respiratory gear and a smoke absorber or fan running.

Recommended Tools and Materials:

• #1 Phillips Screwdriver
• Soldering Iron and Solder (70W or greater)
• Sharp Scraping Tool
• 2x 1 MΩ, 0.25 W Axial Resistors

Modification Steps:

Step 1: Power Off and Discharge

Unplug the power supply from the wall and allow the unit to sit unplugged for at least 10 minutes. This is crucial for safety as the high voltage capacitors can remain charged for several minutes after unplugging.

Step 2: Remove Top Cover

After sufficient discharge time, remove the top PSU cover plate by removing the 6 chassis screws with a phillips screwdriver and pulling up on the top cover plate.

Step 3: Remove PCB Mounting Screws

Remove the 10 PCB mounting screws. Note that some screws may be covered by white silicone conformal coating.

Step 4: Access PCB Back Side

Partially remove the PCB from the PSU enclosure by gently lifting up on the center heat sink and PCB edges. The power cables from the input sockets and fan connectors can remain attached.

Step 5: Orient PCB

Orient the PCB top down with the enclosure off to the side, so that the back side of the PCB is easily accessible.

Step 6: Locate Brown Out Detectors

Find both brown out detectors’ high-side resistor networks. Each network has 2 sets of three 0805 surface mount chip resistors labeled “2204”. Note that silkscreen designators may vary between PSU variants.

Step 7: Remove Conformal Coating

Use a sharp scraping tool to gently remove the conformal coating on and around the 4 pads (2 per resistor network) where new resistors will be soldered. This step is crucial to prevent toxic fumes during soldering.

Step 8-9: Prepare and Solder Resistors

Prepare the 1 MΩ resistors by bending and trimming leads to fit between the cleaned pads. Solder one resistor between the two cleaned solder pads in each resistor network (2 resistors total).

Step 10-12: Reassemble PSU

Prepare to reinstall the PCB by ensuring the black plastic insulator is properly aligned in the bottom of the PSU enclosure. This prevents the PCB from making direct contact with the metal enclosure, which could cause damage.

Carefully remount the PCB to the PSU enclosure, aligning screw holes and inserting the 10 mounting screws. Double-check that no part of the PSU is in direct contact with the metal enclosure.

Reattach the top PSU cover plate, aligning screw holes and installing the six screws. Do not overtighten.

Step 13: Test the Modification

With the PSU fully enclosed, power it on by plugging into a 120V AC outlet. If modified correctly, the PSU fans should power on immediately. Test by powering a control board from the PSU’s auxiliary 12V DC, 6-pin ATX connector, or use a multimeter to check for 12V DC at this connector.

Conclusion

Modifying the APW12 PSU to operate on 120V AC can significantly enhance the flexibility of your mining setup, allowing for operation in standard residential settings. However, it’s crucial to approach this modification with caution, adhering to all safety protocols and power management guidelines outlined in this guide.