In latest EDN design concepts, we’ve seen thermostat designs that meld the features of sensor and heater right into a single system: FET, BJT, or perhaps a easy size of advantageous gauge copper wire. A advantage inherent in thermostat designs that use a transistor as mixed sensor and heater is that impartial of whether or not it’s operated in linear or pulse mode, excessive effectivity is nearly assured.
This occurs just because when the ability go system and heater are mixed, energy dissipated isn’t wasted. As an alternative, by definition, it’s merely extra warmth. Consequence: close to 100% effectivity is inevitable! Sadly, life isn’t so easy for a hotwire thermostat. Whereas it too melds sensor and heater, they continue to be separate from the go system. The ability that it dissipates by working in linear mode subsequently contributes nothing to heating. It’s completely wasted, thus eroding effectivity. The potential for avoiding this inefficiency makes swap mode an attention-grabbing chance.
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Determine 1 exhibits a design concept that achieves it.
Determine 1 A swap mode thermostat effectively heats melded copper wire sensor/heater.
Determine 1 shares a lot in widespread with a linear sibling: “Hotwire thermostat: Utilizing advantageous copper wire as built-in sensor and heater for temperature management” whose schematic is present in Determine 2.
Determine 2 Linear mode scorching wire thermostat that makes use of 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 basically an identical. The place they differ is the way in which op-amp A1a controls Q1.
In Determine 2, temperature dependent voltage variations between R1 and R5+R6 are linearly amplified by A1a and utilized to Q1’s gate to linearly drive hotwire heating to match the setpoint dialed in on R5. The result’s good temperature management, but in addition as much as 10 W of dissipation on Q1.
In Determine 1, against this, constructive suggestions round A1a through R7 forces the amplifier to latch Q1 totally ON or OFF in response to the identical error indicators. This easy distinction improves heating effectivity sufficient that, not like Determine 2, Determine 1’s Q1 wants no heatsink and the general circuit runs from solely half the provision voltage.
Heating effectivity will depend on hotwire size, and ranges from 83% for five ft, to 94% for 15. These numbers evaluate properly to the linear model, that maxes out at about 50%.
In the meantime, the calibration sequence stays the identical for each switcher and linear:
- Earlier than first energy up, permit sensor/heater to completely equilibrate to room temperature.
- Set R4 and R5 totally counter-clockwise (CCW).
- Push and maintain the CAL NC pushbutton.
- Flip energy on.
- Slowly flip R4 clockwise till LED first sparkles on.
- Launch CAL.
Thanks for the suggestion, Konstantin Kim!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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