Thermistor Temp Controller - Page 1
Forum section: electronics / projects

author: echen1024
author_rank: Super Contributor
timestamp: March 09, 2014, 02:37:59 am
content: Well,Vishay kindly donated 3 of their latest and greatest HZ series resistors, which are the world's most accurate at 0.001%. I plan to build 2 calibration boxes, one of which will be sent to forum member "quarks" for measurement with his Fluke 8508A. My question is to construct the calibration standard, I plan to submerge it in mineral oil and surround it with resistive heaters, I was wondering about any circuit designs you guys had for a thermistor based controller, as my current plan is to use an of the shelf design, and drive an inverter which will drive a relay so when the temperature falls below a certain point, the heaters turn on. I would prefer a simple analog solution, but will use an MCU only if needed.

author: tom66
author_rank: Super Contributor
timestamp: March 09, 2014, 02:51:36 am
content: I would use a comparator with hysteresis, the input either being an IC like MCP9701A, or an NTC thermistor if temperature accuracy is not critical (only stability.) the output of the comparator would just drive a transistor, a relay is a little overkill.

author: echen1024
author_rank: Super Contributor
timestamp: March 09, 2014, 02:53:37 am
content: Quote from: tom66 on March 09, 2014, 02:51:36 amThanks for the input. I will look into something like this. Off the top of your head, could you recommend a few chips, as I do not have much confidence in in ability to pick certain components. I have op amps, logic, and voltage regulators mastered, but never really used a comparator before.

author: tom66
author_rank: Super Contributor
timestamp: March 09, 2014, 03:00:38 am
content: LM393 is a pretty common jellybean comparator.

author: echen1024
author_rank: Super Contributor
timestamp: March 09, 2014, 03:16:46 am
content: Alright, so I would assume the thermistor would be part of a resistive divider, going into one pin of the comparator, and the other being some reference voltage...

author: tom66
author_rank: Super Contributor
timestamp: March 09, 2014, 09:09:12 am
content: You can derive the reference using a diode, zener, or voltage divider, if your input supply is constant.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 10, 2014, 03:17:55 am
content: Resistive bridge is another common way to get the voltages to compare for thermocouple/RTD.I haven't been having much luck with just a comparator in my circuit because of the lag between switching full power and the temperature responding (oscillation period of 20s) Is there a way to adjust a comparator circuit to work in that circumstance or are more complicated methods (like PID) required?

author: Vgkid
author_rank: Super Contributor
timestamp: March 10, 2014, 03:56:02 am
content: Have you tried this circuit yethttp://www.techlib.com/electronics/ovenckts.htm

author: echen1024
author_rank: Super Contributor
timestamp: March 10, 2014, 04:04:01 am
content: Quote from: Vgkid on March 10, 2014, 03:56:02 amThat design looks really nice. Looking at the second circuit, I assume it will try and keep a constant temperature as far as my understanding of the circuit goes. I will calculate the values to keep the temperature I need. Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2014, 12:22:29 pm
content: I built this,http://seventransistorlabs.com/Images/TCXO.pngWhich seems to do okay. The compensation needs a time constant on the order of however much thermal mass is attached to the heater and sensor. If the sensor is near the heater, it effectively puts a zero in the transfer function and the loop can run a little faster, whereas if it is distant, you have a massive pole, and the loop must be very slow indeed.You should try to minimize thermal mass, not pile it up, because the control loop can then do its job much faster, reducing noise. Shielding from temperature gradients and convection currents is the biggest priority from what I've heard. Gradients can be controlled with independent heaters, say, a copper panel on each wall of a cube (you'll also need matched thermistors, or probably better, RTDs -- lower noise, better <mK stability).But really, what's a few ppm of resistance? Resistors that good hardly need baffles, let alone temperature controlled baths. Do you even have the resources to calibrate and verify something that tight?Tim

author: Vgkid
author_rank: Super Contributor
timestamp: March 10, 2014, 02:55:57 pm
content: It does seem like overkill to me as well, as you have to remember that the meters have tempcos as well.Remember that the SR- 10X series resistors aren't temp controlled, just monitored.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 10, 2014, 04:33:39 pm
content: Thank you for the circuits, Tesla and Vg. Interesting! both of those look like integrators to me. The LTZ1000 datasheet uses something similar for the heater control too. Perhaps I gave up on that too soon; I was having trouble choosing a gain that would both stabilize and still react fast enough to "rapid" ambient temperature changes (when my house heat comes on.)

author: echen1024
author_rank: Super Contributor
timestamp: March 10, 2014, 10:08:41 pm
content: No, I'm just going to go with the simplest design on the one Vgkid posted. Only need a small heater, nothing else.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 18, 2014, 06:04:55 am
content: I see now that T3sl4co1l's circuit is a PI controller. Following the advice of EmmanuelFaure, I revisited my PI controller and determined the highest gain I could set for just P without losing stability. That number turned out to be around 5000. I then added an integrator and started cranking that up until it caused instability. That number turned out to be around 500. I dialed the numbers back a tad, and then converted it to the single-opamp PI controller listed in the schematics above.As Emmanuel suggested, this design is resulting in about a 0.1C temperature range of the LT1021 die. (blue is ambient, 4mV/C, about 2.5C. yellow is VTemp, around 2mV/C.)You recommend exploring PWM to get better regulation? Would be nice to be rid of the extra supply for the TEC.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:51:55 pm
content: I'd been put off analyzing how much error I might be getting in the reference voltage. But last night, I swapped in a voltage calibrator for the reference voltage, and overnight the temperature voltage swing was reduced by a factor of 4! Down from 250uV to 60uV. I'm embarrassed I didn't spend time on that half of the equation much earlier