Bitaxe – Undervoltage / VIN Below 4.7V Under Load

Swap to a proper Bitaxe PSU — a real `5V / 4-6A` brick with tight regulation, ideally the D-Central Bitaxe PSU or the OEM supply that shipped with the board. Phone chargers, laptop USB-C ports, and generic universal adapters cause the majority of undervoltage tickets in the D-Central support queue. The PSU is the cheapest upgrade you can make to a Bitaxe and closes more support tickets than every other fix combined. Plug it in directly — no hubs, no splitters, no USB extensions. Give it 5 minutes to stabilize, then watch the VIN reading in AxeOS.

Re-seat the barrel jack firmly. Disconnect power, inspect the barrel tip for dirt or oxidation, push the barrel back into the jack until you feel it click and seat fully. 5.5 x 2.1 mm jacks work loose under cable weight, especially with a PSU dangling off the edge of a desk. Cable-tie the PSU cord to something fixed (chassis, stand, cable channel) so the weight is not pulling on the jack. Consider a locking-collar barrel cable if the miner will run 24/7.

Unplug everything else on the same power strip — especially space heaters, lamps, other Bitaxes, USB hubs, any load sharing the 5V rail or the wall circuit. If undervoltage goes away when you reduce parallel load, the issue is aggregate demand exceeding the brick's or the circuit's capability. Redistribute loads to different outlets, ideally different circuits. One Bitaxe per brick is the house rule.

Verify wall voltage at the outlet under load. A cheap plug-in voltmeter (under CAD $15 at Canadian Tire) tells you whether the wall is holding 115-125V on 120V North American service or sagging into the low 110s at peak evening hours. A sagging wall means every brick in the building outputs low — the fix is electrical (load redistribution or a new circuit), not the Bitaxe.

Give the Bitaxe breathing room. Do not enclose a Bitaxe in a sealed case or stuff it in a cupboard — the TPS546 on Gamma runs warmer when the chassis is hot, and warm regulators are more sensitive to marginal input voltage. Move to an open surface, ideally on the D-Central Mesh Stand with airflow on all sides. This alone has recovered Gammas that presented as undervoltage but were actually thermal stacking.

Capture the AxeOS boot log via serial. Connect USB-C, open a terminal at 115200 baud (`screen /dev/ttyUSB0 115200` on Linux/Mac; PuTTY or Arduino Serial Monitor on Windows). Power the Bitaxe via the barrel jack and capture the first 30 seconds. Look specifically for `Brownout detector was triggered`, `Power Fault Detected`, `TPS546: Status: 0x????`, or `VCORE_init failed`. The exact error string determines which failure mode and which related page applies — save the log regardless.

Multimeter DC at the barrel tip under steady-state load — not at idle, during full hashing. Tip is center-positive, probe the inside pin and the sleeve. Healthy: `4.95V - 5.15V` sustained for 60+ seconds. Marginal: `4.80 - 4.95V`. Failing: below `4.80V`. Record the idle reading too: many failing bricks read fine at idle and collapse under load, which is the exact fingerprint you are hunting.

Probe the VIN rail on the PCB directly. For Gammas, the TPS546 VIN pin is accessible on the side of the package near the barrel. On Supra/Ultra, the 5V test point is usually labeled on the silkscreen. A reading taken on-board is more trustworthy than one at the jack — it accounts for jack contact resistance, which is a common partial failure mode that looks exactly like a bad PSU.

Swap the USB-C-to-barrel trigger cable if you are using one. Cheap PD trigger cables are a documented source of undervoltage on Bitaxe — they request 5V@3A from the brick but fall back to 5V@900mA when the PD controller cuts corners. A quality trigger cable negotiates 5V@3A cleanly with any PD-compliant brick. For Bitaxes running 24/7, skip the trigger cable entirely and use a direct barrel PSU — eliminates an entire failure class.

Verify cable gauge and length. A quality 18 AWG barrel cable delivers 3A over a meter with minimal drop. A thin 24 AWG cable can drop 0.3V-0.5V over the same distance, turning a clean 5.00V brick into a sub-4.70V jack. Length matters — keep it short (under 1m) unless you deliberately specify heavy gauge. If the cable wraps easily around your finger, it is probably too thin for the load.

Tune down the overclock temporarily. On AxeOS, drop frequency and core voltage to lower defaults — e.g. 400 MHz @ 1050 mV on a Gamma, well under stock 485 MHz @ 1100 mV. This reduces steady-state current draw from roughly 3.2A to ~2.5A, which moves the operating point back inside the envelope of a marginal PSU. If the Bitaxe stabilizes at the lower profile, undervoltage is confirmed — fix the power, then tune back up. Diagnostic step, not a permanent setting.

Bench-supply the Bitaxe with current limiting. Set a regulated bench supply to 5.00V current-limited to 5A. Power through the barrel jack and monitor the current-draw waveform through boot. Healthy: momentary peak near 3.5A as VCORE ramps, settling to 3.0-3.3A on a Gamma at stock. If the Bitaxe hashes fine on bench power, the board is good and every subsequent tier is wasted effort — the fault is upstream in the brick or cable.

Reflow the barrel jack. Bitaxes are small and handled roughly; the barrel jack takes mechanical stress that cracks solder flanks over time. Remove the board, flux the jack pads, reflow with an iron or hot air. Let cool, retest. Cracked solder joints are a documented source of mystery undervoltage and the fix is under CAD $5 in consumables. Reflow before considering jack replacement.

Replace input electrolytic caps if bulging, discolored, or measuring out of spec with an LCR meter. They cause ripple that the VRM cannot stabilize. Replace with same-value, same-voltage low-ESR parts. For a Bitaxe the caps are typically 16V polymer or aluminum electrolytic in the 330-680 uF range — verify against the specific board rev's silkscreen or schematic. Source from Digi-Key or Mouser.

Measure the on-board input diode and reverse-polarity protection. Some Bitaxe revs include reverse-polarity protection via a MOSFET or Schottky diode on the input. A partially-failed diode introduces voltage drop that looks identical to undervoltage. Measure forward voltage with a diode-test meter. If it reads above 0.5V forward drop on a Schottky, replace the diode.

Erase NVS and factory-flash AxeOS if the boot sequence is stuck in a loop that looks like undervoltage but VIN at the jack is clean. The ESP32-S3 NVS partition is suspect. With esptool.py and USB-C: `esptool.py --chip esp32s3 erase_region 0x9000 0x6000`. Then flash the current stable factory image from github.com/bitaxeorg/ESP-Miner/releases for your exact board revision. Wrong-board firmware presents with odd power behaviour — always pick the factory .bin that matches your silkscreen.

Stop DIY when: (a) you have tried two verified-good PSUs and a known-good cable and VIN still sags below 4.85V; (b) the input caps or barrel jack show visible damage and you do not have rework capability; (c) the Gamma TPS546 continues to latch Power Fault Detected on a clean supply; or (d) you have burned more than two hours and the bench is now faster and cheaper. Book a D-Central Bitaxe repair slot at d-central.tech/services/asic-repair/.

D-Central bench process: USB-C serial diagnostic first (five minutes, documents the failure precisely), current-draw waveform under bench power, barrel jack inspection under 20x, cap and input-filter audit with LCR, TPS546 swap on Gamma if needed, cable and PSU test on the customer's original power path if shipped with the board, current stable AxeOS flash, 24-hour burn-in at nameplate on D-Central bench stratum before return-ship. Repair notes go back to the customer — you get the diagnostic data, not just a fixed sticker.

Ship safe. Anti-static bag, padded box, at least 3 cm of foam on every side. Include: the exact symptoms observed, the firmware version last running, which tiers you tried, and — this is the big one — ship your PSU and cable too. Half the undervoltage tickets we diagnose are the power path, and we cannot test that if it is sitting on your desk. A Bitaxe plus PSU plus cable fits in a padded flat-rate Canada Post box. Canada-wide, US, and international all welcomed.