In recent EDN design ideas, we’ve seen thermostat designs that meld the functions of sensor and heater into a single device: FET, BJT, or even a simple length of fine gauge copper wire. A virtue inherent in thermostat designs that use a transistor as combined sensor and heater is that independent of whether it’s operated in linear or pulse mode, high efficiency is virtually guaranteed.
This happens simply because when the power pass device and heater are combined, power dissipated isn’t wasted. Instead, by definition, it’s simply more heat. Result: near 100% efficiency is inevitable! Sadly, life isn’t so simple for a hotwire thermostat. While it too melds sensor and heater, they remain separate from the pass device. The power that it dissipates by operating in linear mode therefore contributes nothing to heating. It’s totally wasted, thus eroding efficiency. The potential for avoiding this inefficiency makes switch mode an interesting possibility.
Figure 1 shows a design idea that achieves it.
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| Figure 1, | A switch mode thermostat efficiently heats melded copper wire sensor/heater. |
Figure 1 shares much in common with a linear sibling (Reference 1) whose schematic is found in Figure 2.
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| Figure 2, | Linear mode hot wire thermostat that uses the tempco and I2R heating of 40 AWG copper wire as a melded sensor/heater. |
Their respective interfaces with a copper wire melded heater/sensor are essentially identical. Where they differ is the way op-amp A1a controls Q1.
In Figure 2, temperature dependent voltage differences between R1 and R5+R6 are linearly amplified by A1a and applied to Q1’s gate to linearly force hotwire heating to match the setpoint dialed in on R5. The result is good temperature control, but also up to 10 W of dissipation on Q1.
In Figure 1, by contrast, positive feedback around A1a via R7 forces the amplifier to latch Q1 fully ON or OFF in response to the same error signals. This simple difference improves heating efficiency enough that, unlike Figure 2, Figure 1’s Q1 needs no heatsink and the overall circuit runs from only half the supply voltage.
Heating efficiency depends on hotwire length, and ranges from 83% for 5 feet, to 94% for 15. These numbers compare well to the linear version, that maxes out at about 50%.
Meanwhile, the calibration sequence remains the same for both switcher and linear:
- Before first power up, allow sensor/heater to fully equilibrate to room temperature.
- Set R4 and R5 fully counter-clockwise (CCW).
- Push and hold the CAL NC pushbutton.
- Turn power on.
- Slowly turn R4 clockwise until LED first flickers on.
- Release CAL.
Thanks for the suggestion, Konstantin Kim!
Reference
- Woodward, Stephen. "Hotwire thermostat: Using fine copper wire as integrated sensor and heater for temperature control."
