I used to be shocked not too long ago when a (almost) two-decade-old design thought of mine for take-back-half temperature management received a query from reader John Louis Waugaman. John says he wants a approach to management the temperature of batches of rising bread dough. I’m glad he could be contemplating making use of my TBH circuit to his drawback, but it surely actually is form of overkill in that context. So, I started pondering whether or not an easier topology may resolve his dough incubation drawback in addition to the TBH circuit would whereas saving some price and energy. Plus, there’s a backstory.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
For a protracted whereas I’ve been keen on a very elegant, correct, and (very!) cheap technique (∆Vbe) for sensing and controlling temperature utilizing abnormal, uncalibrated bipolar transistors. I first noticed it defined in an software observe by famed analog guru Jim Williams (see web page 7 for ∆Vbe idea).
I’m all the time expecting alternatives to make use of ∆Vbe and this chance of offering low-cost, correct, and calibration-free temperature management in a cool culinary context was too good to let move by. Determine 1 exhibits the brand new circuit I cooked up, (loosely) based mostly on Jim’s recipe.
Determine 1 Delta-Vbe sensor Q1 is programmed through R1 for desired setpoint temperature in Kelvin = R1/100 = 312oOkay = 39oC for R1 = 31.2k. Asterisked Rs needs to be precision varieties (1% or higher).
The R2 R3 D5 D6 community drives Q1 within the magic 10:1 present ratio ∆Vbe measurement cycle described by Williams. Observe that absolutely the currents provided to Q1 aren’t any extra correct than the uncooked unregulated 60-Hz line voltage that creates them, however that doesn’t have an effect on ∆Vbe accuracy. All that counts is their 10:1 ratio which is ready unbiased of line voltage variation solely by the precision of (R2/R3 + 1) = 10.
This makes Q1 generate a PTAT (proportional to absolute temperature) AC sign equal to oOkay/5050 volts peak-to-peak that follows the 120-Hz log(|sine|) waveshape proven in Determine 2.
Determine 2 Q1’s ∆Vbe PTAT log(|sin(r)|) oOkay/5050 waveform. (Yaxis = volts, Xaxis = radians Purple = common worth = AC baseline)
The PTAT sign Vpp is boosted by A1a’s acquire = –2,742,160/R1, then in contrast by A2 to its precision (2.50 V +0.4%) inside shunt reference (thanks once more, Konstantin Kim, for locating the versatile AP4310A!).
A2’s output stays at zero, holding Q2 off, whereas Q1’s temperature and the PTAT sign are beneath setpoint. This permits 120-Hz pulses coupled by way of C3 to succeed in Q3’s gate, swap it on, and apply energy to the heater. When the heater warms Q1 (and presumably the dough) to the programmed temperature, then the PTAT waveform rises above A2’s reference voltage. This makes A2 begin turning Q2 on which diverts the TRIAC gate pulses to floor. That turns Q3 and the heater off permitting Q1 to chill, and so on., and so on. The ensuing biking completes a thermostasis suggestions loop to make the dough develop.
About Q3: Despite the fact that heater drive is unipolar, I chosen a TRIAC as a substitute of an SCR for Q3. This wasn’t to get bipolar functionality however fairly as a result of the TRIAC has a better max gate present score. This makes Q3 in a position to shrug off the 2-A inrush doable at power-up, which could vaporize an SCR gate.
D7 offers a path to floor for C3 return present, stopping it from false triggering Q3.
For heater obligation, John urged an incandescent gentle bulb. I agree radiant heating ought to work effectively. Since Q3’s most obligation issue is 50%, a 100-W bulb could be nearly good for a most heating energy of ~60 W. Plus, a bonus good thing about decreased voltage could be a decrease filament temperature. This could make an abnormal tungsten bulb final many 1000’s of hours.
It could be simply what John kneads.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Temperature controller has “take-back-half” convergence algorithm
- Take-back-half thermostat makes use of ∆Vbe transistor sensor
- Take-Again-Half precision diode cost pump
- 20MHz VFC with take-back-half cost pump
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