Temperature Control

IronOS-powered soldering irons use a modified N-type thermocouple integrated into the tip for temperature measurement. This design creates a temperature sensor at minimal cost but introduces unique measurement challenges.

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Thermocouple Design

The thermocouple is constructed by using different metal types to join one of the rings to the heating coil, effectively creating a free temperature sensor with very low construction difficulty.

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Advantages

• Extremely low cost
• Simple construction
• Integrated into the tip design

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Limitations

1. Non-optimal metals: Uses available materials rather than calibrated thermocouple alloys
2. Non-constant response: Temperature response varies across the range
3. Shared connections: Same connections used for heating current, preventing simultaneous heating and measurement

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How Thermocouples Work

Thermocouples use a junction of two dissimilar metals to generate a small voltage (microvolts). This voltage can be measured and converted to temperature using a known transfer function.

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Cold Junction Compensation

The copper terminations where thermocouple wires connect form additional junctions that also generate voltages. These are called the cold junction.

• If held at 0°C, cold junction effects are negligible
• At room temperature, measured voltage must be compensated
• Every metal-to-metal junction in the circuit acts as a small thermocouple

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Temperature Measurement Implementation

The hardware design (Pinecil, TS100, TS80) uses a consistent measurement approach:

1. Op-amp connected directly across heating connections to the tip
2. Handle temperature sensor for reference measurement
3. ADC sampling when heating is off

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Measurement Process

When the iron is not heating:

1. Microcontroller reads op-amp output via ADC
2. Voltage represents temperature difference (tip - handle)
3. Apply offset compensation (removes ADC and op-amp offsets)
4. Convert to temperature delta in °C/K
5. Add handle temperature to derive tip temperature

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Temperature Control Implications

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Heating vs Reading Tradeoff

Since temperature can only be measured when not heating, there’s a critical tradeoff:

• More frequent measurements = Better temperature control, less heating time
• Fewer measurements = More heating power, slower response

IronOS Solution: Implements a two-speed controller loop:

1. Heat-up mode: Fewer temperature measurements, maximum power delivery
2. Maintenance mode: Double measurement rate for faster reaction times

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Internal vs External Temperature

The thermocouple measures the inside of the tip (near the heater coil), not the outside surface where soldering occurs.

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PID Tuning Philosophy

The PID controller is intentionally tuned to be slightly underdamped (more “jumpy” than typical): Rationale: When internal temperature drops, the external temperature has dropped more significantly. Overcompensation helps the system catch up faster. Result: Temperature display may fluctuate during use, but the thermal mass of the tip smooths these variations for stable soldering performance.

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Temperature Stabilization

With no external cooling, internal and external tip temperatures equalize in 3-5 seconds.

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Measurement Complexity and Accuracy

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Voltage Reference Limitations

The firmware performs best-effort temperature calculation within hardware constraints:

• Voltage reference accuracy: ±1% (affects all measurements)
• No factory calibration on low-cost MCUs
• Cannot compensate for reference drift

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Compensation Model

The firmware accounts for:

1. Cold junction compensation (implemented)
2. Constant offset (implemented)

Not modeled: Linear offset variation with handle temperature

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Handle Temperature Effects

The unmodeled linear offset with handle temperature is generally acceptable because:

• During use, handle temperature stabilizes within ~10°C
• Error becomes relatively constant during normal operation
• Target use case is primarily above 150°C

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Technical Specifications

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Measurement Timing

1. FET switches off (stops heating)
2. Recovery time for tip capacitance discharge
3. Op-amp exits saturation
4. ADC samples 8 times quickly
5. PWM output resumes

This creates a small dead time in the output signal during each measurement cycle.

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Performance Optimization

The firmware balances:

• Frequent sampling: Stable temperature control
• Infrequent sampling: Maximum power delivery

See Power Management for details on power delivery optimization.

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Resources

• Cold Junction Compensation Explained
• Thermocouple Basics Video
• PWM Control Overview