Definition
Undervolting (sometimes shortened to UV, or grouped under “voltage tuning”) is the practice of deliberately lowering the supply voltage fed to a miner’s hashing chips below the factory default, in order to cut power draw and heat. Because the dynamic power a chip burns scales roughly with the square of its voltage, even a modest voltage reduction returns a disproportionately large power saving — at the cost of timing margin, so push it too far and the chips start producing hardware errors or drop offline.
Undervolting almost never happens in isolation. On a real ASIC it is paired with a frequency change, because voltage and clock are two halves of the same operating point. Stock firmware ships these pairings as a fixed ladder. Bitmain’s own factory tables for the S19j Pro hashboard, for example, walk a tight band of roughly 1320–1380 mV across frequency steps from 545 MHz down to 465 MHz — the lower-frequency, lower-voltage rows are the eco/quiet presets, while a hydro/immersion board variant pushes voltage higher at lower clocks to chase heat density instead of efficiency. Newer-generation chips run a different band entirely (the BM1366 family sits around 1530–1600 mV), which is why an undervolt figure that is safe on one model is meaningless on another.
How undervolting actually works on an Antminer
It helps to know what “the voltage” physically refers to, because it is easy to assume you can dial in any number you like per chip. You cannot. On a typical S19-class machine the PSU delivers a single shared DC rail to all the hashboards — on the S19j Pro that rail sits in the ~12–15.2 V window. A per-board DC-DC converter, commanded by a small dsPIC controller, then steps that rail down to the chip-supply target (around 13.7 V on that platform). The chips on each board are wired in series across voltage domains, so the granularity of voltage control is the board/domain — not the individual ASIC. That is the single most common misconception about undervolting: there is no true per-chip voltage knob on these miners.
This is why undervolting is fundamentally a firmware exercise. Stock firmware only lets you select preset rows from the factory ladder. Aftermarket firmware exposes the voltage and frequency targets directly and, on many builds, runs an autotuner that searches for the lowest stable voltage at a target clock at runtime rather than reading a baked-in value. Either way, the firmware is what issues the SetVoltage command down to the board controllers; you are not turning a potentiometer.
Why operators undervolt
- Efficiency. Dropping voltage and clock together lowers watts faster than it lowers hashrate, improving the miner’s efficiency (J/TH). This is the core lever behind making an older miner profitable again when electricity is the binding constraint.
- Heat and noise. Less power in means less heat out and lower fan speeds, which is what makes an ASIC tolerable — or a usable heat source — in a home-mining setup.
- Chip longevity. Running silicon cooler and at lower stress generally extends its working life.
The trade-off is real: drop the voltage below what the chip needs to switch reliably at its current clock and you see rising rejected shares, then chip drop-outs, then a board that won’t initialize. The “right” undervolt is the lowest voltage that still hashes cleanly at your chosen frequency, which is exactly the point the firmware autotuner is hunting for. It is the inverse operation of overclocking, and it usually travels with underclocking.
Undervolting is also where the small open-source machines shine: a single-chip Bitaxe exposes its voltage and frequency targets openly, making it the cleanest place to learn the relationship hands-on before you touch a 3-kW production unit.
If you want to go further than the factory presets allow — tighter efficiency, quieter operation, or running an older miner as a heater — that lives in the firmware and power-delivery layer. Explore D-Central’s open-source mining hardware and our broader shop for the gear that makes responsible voltage tuning practical, and treat any undervolt as something you verify with real error-rate and hashrate data rather than trust on faith.
In Simple Terms
Lowering chip voltage to reduce power consumption and heat, improving efficiency for home mining.
Undervolting (sometimes shortened to UV, or grouped under "voltage tuning") is the practice of deliberately lowering the supply voltage fed to a miner's hashing chips below the factory default, in order to cut power draw and heat. Because the dynamic power a chip burns scales roughly with the square of its voltage, even a modest voltage reduction returns a disproportionately large power saving — at the cost of timing margin, so push it too far and the chips start producing hardware errors or drop offline.
Undervolting almost never happens in isolation. On a real ASIC it is paired with a frequency change, because voltage and clock are two halves of the same operating point. Stock firmware ships these pairings as a fixed ladder. Bitmain's own factory tables for the S19j Pro hashboard, for example, walk a tight band of roughly 1320–1380 mV across frequency steps from 545 MHz down to 465 MHz — the lower-frequency, lower-voltage rows are the eco/quiet presets, while a hydro/immersion board variant pushes voltage higher at lower clocks to chase heat density instead of efficiency. Newer-generation chips run a different band entirely (the BM1366 family sits around 1530–1600 mV), which is why an undervolt figure that is safe on one model is meaningless on another.
How undervolting actually works on an Antminer
It helps to know what "the voltage" physically refers to, because it is easy to assume you can dial in any number you like per chip. You cannot. On a typical S19-class machine the PSU delivers a single shared DC rail to all the hashboards — on the S19j Pro that rail sits in the ~12–15.2 V window. A per-board DC-DC converter, commanded by a small dsPIC controller, then steps that rail down to the chip-supply target (around 13.7 V on that platform). The chips on each board are wired in series across voltage domains, so the granularity of voltage control is the board/domain — not the individual ASIC. That is the single most common misconception about undervolting: there is no true per-chip voltage knob on these miners.
This is why undervolting is fundamentally a firmware exercise. Stock firmware only lets you select preset rows from the factory ladder. Aftermarket firmware exposes the voltage and frequency targets directly and, on many builds, runs an autotuner that searches for the lowest stable voltage at a target clock at runtime rather than reading a baked-in value. Either way, the firmware is what issues the SetVoltage command down to the board controllers; you are not turning a potentiometer.
Why operators undervolt
- Efficiency. Dropping voltage and clock together lowers watts faster than it lowers hashrate, improving the miner's efficiency (J/TH). This is the core lever behind making an older miner profitable again when electricity is the binding constraint.
- Heat and noise. Less power in means less heat out and lower fan speeds, which is what makes an ASIC tolerable — or a usable heat source — in a home-mining setup.
- Chip longevity. Running silicon cooler and at lower stress generally extends its working life.
The trade-off is real: drop the voltage below what the chip needs to switch reliably at its current clock and you see rising rejected shares, then chip drop-outs, then a board that won't initialize. The "right" undervolt is the lowest voltage that still hashes cleanly at your chosen frequency, which is exactly the point the firmware autotuner is hunting for. It is the inverse operation of overclocking, and it usually travels with underclocking.
Undervolting is also where the small open-source machines shine: a single-chip Bitaxe exposes its voltage and frequency targets openly, making it the cleanest place to learn the relationship hands-on before you touch a 3-kW production unit.
If you want to go further than the factory presets allow — tighter efficiency, quieter operation, or running an older miner as a heater — that lives in the firmware and power-delivery layer. Explore D-Central's open-source mining hardware and our broader shop for the gear that makes responsible voltage tuning practical, and treat any undervolt as something you verify with real error-rate and hashrate data rather than trust on faith.