Temperature Sensors — Overview
Comparison of temperature sensor ICs used in cryptocurrency mining hardware — LM75A, TMP451, and NCT218 specifications, I2C addresses, and roles in thermal management.
Overview
Temperature sensors are critical safety components on hash boards. They monitor ASIC chip temperatures and provide the data that firmware uses to regulate fan speed, adjust operating frequency, and trigger emergency shutdowns when temperatures exceed safe limits. A failed or misreading temperature sensor can lead to thermal damage (if the miner cannot detect overheating) or reduced performance (if the miner throttles based on incorrect readings).
Every hash board contains between 2 and 8 temperature sensors, depending on the model and manufacturer. These sensors communicate with the control board over the I2C bus.
Sensor Comparison
| Parameter | LM75A | TMP451 | NCT218 |
|---|---|---|---|
| Manufacturer | NXP / TI / multiple | Texas Instruments | ON Semiconductor |
| Sensing Type | Local (on-die) | Local + Remote diode | Local + Remote diode |
| Temperature Range | -55°C to +125°C | -40°C to +127°C (local), up to +150°C (remote) | -40°C to +127°C (local), up to +204°C (remote) |
| Accuracy | ±2°C | ±1°C (local), ±1°C (remote) | ±1°C (local), ±1°C (remote) |
| Resolution | 0.5°C (9-bit) | 0.0625°C (12-bit) | 0.0625°C (12-bit) |
| I2C Address | 0x48-0x4F (A0-A2 configurable) | 0x4C default | 0x4C default |
| Package | SOIC-8 or SOT-23-5 | SOT-23-6 | SOT-23-6 |
| Alert Output | Yes (OS pin, open-drain) | Yes (ALERT pin) | Yes (ALERT pin) |
| Used In | Antminer S19 series, older models | Whatsminer M50, M60 | Antminer S21, T21 |
How Temperature Sensors Are Used in Miners
Thermal Monitoring Loop
The control board continuously reads temperature data from all sensors on each hash board:
Control Board → I2C Bus → Temp Sensor → Temperature Reading
↓
Firmware compares against thresholds
↓
Adjusts fan speed / frequency / shutdownThreshold Actions
| Temperature Range | Firmware Action |
|---|---|
| Below 40°C | Minimum fan speed |
| 40°C to 70°C | Proportional fan speed increase |
| 70°C to 80°C | Full fan speed, frequency may be reduced |
| 80°C to 90°C | Aggressive throttling (frequency reduction) |
| Above 90°C | Emergency shutdown to prevent damage |
If a temperature sensor fails and reports 0°C or a stuck value, the miner may run the fans at minimum speed while the chips overheat. Always verify temperature sensor readings when investigating thermal shutdowns or abnormally high chip temperatures.
I2C Bus Architecture
Temperature sensors share the I2C bus with other components on the hash board:
- AT24C02 EEPROM at address 0x50-0x57
- PIC16F1704 bridge at address 0x20-0x27 (Antminer only)
- Temperature sensors at 0x48-0x4F (LM75A) or 0x4C (TMP451/NCT218)
In Antminer hash boards, the temperature sensors are not directly accessible on the main I2C bus — they sit behind the PIC16F1704 I2C bridge and must be read using the bridge's CMD 0x3C (sensor read) command.
Sensor Placement on Hash Boards
Temperature sensors are placed at strategic locations on the hash board to capture the thermal profile:
- Near the center of the chip array — where temperatures are highest
- Near the board edges — to detect uneven cooling or airflow problems
- Close to specific voltage domains — to monitor regulator and MOSFET temperatures
A typical Antminer S19 hash board has 4 temperature sensors (LM75A) placed at roughly equal intervals along the chip chain. The firmware uses the highest reading from all sensors for thermal management decisions.
Common Failure Modes
1. Sensor Not Responding on I2C
Symptoms: Miner log shows "temp sensor error" or temperature reads as 0°C / -1°C.
Causes:
- Failed sensor IC
- Broken I2C trace (SDA or SCL line)
- Missing 3.3V power to the sensor
- I2C bus contention from another failed device
2. Inaccurate Readings
Symptoms: Reported temperature is significantly different from actual board temperature.
Causes:
- Poor thermal contact between the sensor and the area being monitored
- Sensor operating outside its rated voltage range
- Solder joint degradation causing intermittent contact
3. Stuck Reading
Symptoms: Temperature reading does not change despite varying load or ambient conditions.
Causes:
- Sensor in a fault state (latched register)
- I2C communication partially broken (reads stale data from a cache)
- Failed remote diode (for TMP451/NCT218 — local temp may still work)
Testing Procedures
Verify Sensor Power
Measure the VCC pin of each temperature sensor:
- Expected: 3.0V to 3.6V (typically 3.3V)
- If 0V: Check the 3.3V LDO regulator supplying this section
Check I2C Bus Signals
With an oscilloscope or logic analyzer, verify:
- SCL (Clock): Square wave present during communication (typically 100kHz or 400kHz)
- SDA (Data): Active data transitions during sensor reads
- Both lines should idle HIGH (pulled up to 3.3V by pull-up resistors)
- If either line is stuck LOW, a device on the bus has failed and is holding the bus
I2C Address Scan
If you have access to an I2C scanner tool (or the miner's diagnostic interface):
- Scan the I2C bus for responding devices
- Verify each temperature sensor responds at its expected address
- Compare the number of detected sensors against the expected count for the board model
Read Temperature Values
Read the temperature register from each sensor and compare:
- All sensors on the same board should read within 5-10°C of each other (ambient conditions)
- Values should change when you apply heat (hot air) or cold (compressed air) to the sensor area
- A sensor that reads exactly 0°C or 255°C is likely failed
Replacement Notes
- Temperature sensors in SOT-23 packages are small but straightforward to replace with a soldering iron
- SOIC-8 packages (older LM75A) are even easier
- Pay attention to the I2C address configuration — replacement sensors must have the same A0-A2 pin connections as the original
- After replacement, verify the sensor responds on I2C and reads a reasonable temperature
Used In
- Antminer S19 Series — LM75A (4 per board)
- Antminer S21 — NCT218 (4 per board)
- Whatsminer M50 — TMP451 (2-4 per board)
- Whatsminer M30S — TMP451 (2 per board)
Related Pages
LDO Regulators — I/O & Auxiliary Power Rails
LDO voltage regulators used in cryptocurrency miners for VDDIO, 3.3V control logic, and auxiliary power rails — including AMS1117 specifications and testing procedures.
LM75A Temperature Sensor
LM75A digital temperature sensor specifications, I2C interface, register map, and testing procedures for cryptocurrency mining hash board repair.