Design of heater circuit using resistors - Page 1
Forum section: electronics / projects

author: astroasma
author_rank: Contributor
timestamp: July 26, 2022, 07:51:22 pm
content: Hello everyone!I am a space engineering student at TU Berlin and I am stuck on something.I am working on designing a small (10cm x 10cm x 10cm) microbiology incubator that is intended to be sent to the ISS for future plantation of fungi.The thing is, the incubator needs to maintain a temperature of 30 degrees at all times, the temperature on board the ISS is 20-24 degrees C. To increase the T, I am planning to design a circuit composed of solely resistors.Resistors lose heat as a form of energy and this is how I calculated how much heat is lost:Given a certain voltage V, a current I which can be obtained depending on the resistance in use, and for a heating time t:Heat Produced =(PotentialDifference)×(Current)×(time)The heat produced is thermal energy in joules, which can be converted to degrees C.Now, the problem is the following:I only need a circuit of resistors to increase the T by a maximum of 10 degrees C. I set my heating time to one minute.But I am having a problem figuring out how this is related to the volume, I need this increase of 10 degrees to cover all the cube of 10^{3}cm.Please help if you know how this is solved.Kind regards,Asma

author: wraper
author_rank: Unknown
timestamp: July 26, 2022, 08:04:18 pm
content: How are you supposed to calculate how much energy you need to heat a certain amount of volume at certain amount of time without knowing thermal mass and thermal conductivity to ambient?

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2022, 08:11:26 pm
content: You are going about this the wrong way, I think.For a temperature rise to a precision fixed temperature, you need a closed loop system.What does this mean?It means you need a precision temperature sensor in your incubator that will give feedback to your controller.Based on this feedback, the controller will regulate the power dissipated by the resistors.The resistors just need to be powerful enough (or even more powerful) to achieve this goal. The closed-loop controller will do the rest.Team up with someone who knows about PID loops. Getting the system stable is not trivial.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 26, 2022, 08:26:05 pm
content: Yes, before considering the controller and power needed etc you will need to know :Convection to air from surface [ Watt/m^2.K ] expect 15 to 30 W/m^2.KSurface Emissivity [dimensionless ref black body =1 ] , for shiny metal like aluminum ~ 0.1, for black coated metal ~ 0.9Heat flow via any mounting attachments etc.With those the steady state temperatures and heat flow can be calculated.To do the transient heating, the specific heat, dimensions and positions of all masses and the resistors will be needed.If I were to do this I would certainly use 2D transient heat analysis.It might seem overkill, but it adds discipline and necessitates all parameters to be entered, saves approximations on the box corners,moreover you get results and archives in a formal format.And do a transient at the outset because steady state is in there too saving another model.

author: astroasma
author_rank: Contributor
timestamp: July 26, 2022, 08:38:24 pm
content: Thank you so much for your reply!Sorry I forgot to mention that I do have a closed loop system!The whole circuit will be controlled via a microcontroller that will supply power to the circuit once the T sensor indicated a drop below 30, and supplies power to the resistors until the incubator's ambiant T is 30 again.So this is not the problem.

author: astroasma
author_rank: Contributor
timestamp: July 26, 2022, 08:41:34 pm
content: Yes this is my question! I don't know how to get there..I just know how to calculate the thermal energy dissipated for a certain amount of resistance and I couldn't do the rest.

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2022, 08:44:32 pm
content: Just heat the chamber experimentally with your resistors, it doesn't have to be exact, it just needs to have "enough" power.Trying to do all the thermal maths on this is a fool's errand.The resistor power dissipation is just that: P = U x I.

author: astroasma
author_rank: Contributor
timestamp: July 26, 2022, 08:49:34 pm
content: Thank you for your reply!The material used to build the incubator will be plexiglass, the circuit will be mounted on one of the panels along with a small fan to help distribute the T equally on all sides.So the heat flow will be really from air to air as there will be no structure to cover the circuit. The microcontroller along with the T sensor will help regulate the T once it drops below 30.Is there any tool that you recommend to do the 2D transient heat analysis? thanks

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 26, 2022, 09:11:07 pm
content: The plexiglass might have lower convection to external air than my post #3 because I assumed metal.And the radiation will now depend on emissivities of objects inside, through the (assumed) transparent plexiglass.Does your Institution have FEM? If so there should be data available for the materials, and maybe someone familiar to assistwith setting up thermal FEM. Maybe there is already FEM package with transient heat coupled to fluid flow to model air inside the container.That will help to identify / avoid stagnation of air flow, hot spots etc which might upset the experiment.Workflow for a job like this:Draw (solid) all the parts they way they need to be. Collect all properties by search if necessaryModelOptimiseRefine model....

author: planet12
author_rank: Regular Contributor
timestamp: July 26, 2022, 09:51:18 pm
content: Quote from: astroasma on July 26, 2022, 08:38:24 pmIf you're just doing simple on/off control, you're going to have overshoot and/or oscillation around the setpoint.Overshoot will be due to thermal inertia - you'll pump a bunch of energy into the heating element, and it will take a while to heat the air - and will continue heating the air for a while after it's switched off due to the heat still in the element.Oscillation will happen if you have a single setpoint with no hysteresis.Another user suggested PID control - this is by far the best option if you need a stable temperature, using PWM to do proportional control of the heater, temperature sensor to measure, and the PID to close the loop.For thermal FEM analysis, if your organisation doesn't already have a package for it, FreeCAD can do it - although you're probably better off to just build it and measure.

author: james_s
author_rank: Super Contributor
timestamp: July 26, 2022, 11:11:48 pm
content: This is exactly what I was going to suggest, you need a PID controller, that is a solved problem, there is already code out there to do what you want, or analog circuits that do the same. It sounds like what you are proposing is bang-bang control and that is not going to be accurate at all. Also instead of ordinary resistors I would suggest a film style heating element. They come in a wide variety of sizes and shapes, many with adhesive backing so they will stick on whatever you are trying to heat. IIRC they were one of the developments that came out of the space race.

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2022, 11:27:31 pm
content: It seems we're all on the same page.But don't ignore my suggestion about getting someone on the team who knows how to tune a PID system. It's not trivial.

author: wraper
author_rank: Unknown
timestamp: July 27, 2022, 06:47:15 am
content: Quote from: astroasma on July 26, 2022, 08:38:24 pmThat in not a quite correct way to do it unless you don't really care about temperature stability or have extremely low thermal mass. You will have significant temperature cycling since temperature will continue to rise after you switch off heating and will not raise immediately once you switch it on.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: July 27, 2022, 01:29:35 pm
content: I made an 1.3 gram analog temperature controller with adjustment pots. It controls the PWM duty cycle to an external resistive heater, for high efficiency. It uses proportional control only so it's not as exact as a full PID controller, but certainly better than the suggested on-off scheme.The manual can be helpful also for other implementations:https://sparv.io/Heating%20controller%20manual%20v1.pdfPM me if you want to use the controller itself.A general note: If the heating is uneven throughout the volume, the location of the feedback temperature sensor is crucial to the accuracy of the temperature control. Your plan to use a fan is good.General note #2: I think a simulation is overkill. Just build the box and measure where the power draw settles in equilibrium. Make sure the power supply has some margin. If the power draw is too high, add insulation around the box.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 27, 2022, 10:57:21 pm
content: Here is a suggestion;Use a mosfet on a heat sink as the heater.Use an op. amp as the drive to the gate of the mosfet.Regulate the drive from the op amp with the temp sensor, probably a temp sensitive resistor that is in a proper spot. Or your Controller.....The resistance of the source to drain of the mosfet will create heat when the mosfet is turned on. You can add resistance from drain to ground in a different spot if you find it necessary to spread out the heat more than the heat sink will do.Turning it on and off smoothly will be the biggest challenge.You might actually want to measure the ON source to drain resistance of the mosfet you use rather than take the specifications for granted.Do they actually want a fan??? Fungi are sensitive to humidity, what will be humidity control???

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 27, 2022, 11:04:02 pm
content: It might seem over kill, but an elementary model consists of drawing in about 12 lines and loading 9 properties !https://app.box.com/s/r2z8z89x0htem83l00b2mwdlnk4ryud6I can not do fluid air here and recommend that for the real job because some moving air may be necessary.I thought there must be soil or something for the fungus.So this model consists of a 100 by 100 by 100 mm plexiglass box with an irregular triangular pile of soil at the bottom.The static air above the soil has a thermal conductivity of 0.030 W/m.K from a paper by NIST.I embedded a wide copper plate in the soil and applied power to it.At 2.6 Watt the soil and air immediately over it reaches about 35 C with air outside the plexi box ( boundary condition) at 20 C.The thermal time constant of the soil near the wide heating plate is aproximately 220 second.The whole box aproximates steady state after about 2100 second.

author: cellularmitosis
author_rank: Unknown
timestamp: July 28, 2022, 07:00:56 am
content: A while back I built a resistor-based heater for an aluminum Hammond enclosure (the resistors were epoxied to the aluminum using thermal epoxy), driven by an Arduino running a PID loop (if I recall, the resistors were controlled via an op amp and a small transistor -- the target temp was 25C, so only just above ambient).I kept logs of the whole process of tuning the PID, so you can replay all of my trial-and-error:https://github.com/cellularmitosis/logs/tree/master/20180124-25c-chamberhttps://github.com/cellularmitosis/logs/tree/master/20180126-25c-chamber-tuningI eventually got it to regulate within +/- 0.01C, which I was pretty happy with.

author: cellularmitosis
author_rank: Unknown
timestamp: July 28, 2022, 07:16:18 am
content: Quote from: astroasma on July 26, 2022, 08:49:34 pmJust to check my understanding, there will be a plexiglass box, and inside of the box will be the resistors with a fan blowing over them, and the temperature sensor will also be in the box (probably on the opposite side)?

author: astroasma
author_rank: Contributor
timestamp: July 28, 2022, 11:15:33 am
content: Hii, thank you so much for your input.I checked what you did and I think it's pretty awesome, I will try to do something with similar components but I need to design a circuit board (which will be directly placed on one of the panels of the box), and I only need an increase by 10 degrees so I need to find out how many resistors will I need for that.Yes the box will be in plexiglass. I don't know where to place the T sensor and the fan yet, I will need to simulate how the heat distributes best and go accordingly.

author: cellularmitosis
author_rank: Unknown
timestamp: July 28, 2022, 02:56:27 pm
content: Quote from: astroasma on July 28, 2022, 11:15:33 amGreat, the good thing is that plexiglass and cardboard are both insulators, so you could implement the entire project in cardboard and then the final translation to plexiglass will only require a small change to the PID constants.If you are fast with simulations, that sounds like a good approach. For someone like me, it would be faster to simple implement it and just see how it behaves / tweak it. I.e. implement the control circuit, and then have a separate arduino which just monitors a few thermistors placed within the box to see how well the heat distributes. Once the project is working satisfactorily, the second arduino would be removed.If the resistor approach isn’t a requirement, you could potentially simplify the project just a bit by having the heater be a single mosfet attached to a heat sink / fan combo.

author: astroasma
author_rank: Contributor
timestamp: July 29, 2022, 09:52:34 am
content: I am not required to use resistors. I actually thought it would be the simplest idea, I didn't think MOSFETs could be a solution too (I have very basic knowledge in electronics).I just found out I can to use a coil as well for heat induction, check this out if you want (:But the T increase in using that circuit looks a lot higher than what I need, what do you think I need to tweak in my circuit to get a less T increase? Thank youuuuu

author: exmadscientist
author_rank: Frequent Contributor
timestamp: July 29, 2022, 11:02:16 am
content: It might be worth noting that it is common in professional equipment to use the flexible heater strips (widely available at Digi-Key and friends). These are just a resistor made of a copper trace printed on a polyimide backing. They bend well and spread out the heat well. They are not so easy to control as a FET gate, but it isn't too hard (especially if you go with PWM). The ease of getting the heat out that they provide may or may not be important to you.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 29, 2022, 12:10:55 pm
content: I suggested the Mosfet previously.You need something to run the higher heater current through anyway, so taking advantage of the heat the mosfet gives off seems quite reasonable.I have seen the Mosfet approach in commercial oven controlled Xtal oscillators at much higher temperatures.You could probably just use an aluminum sheet metal piece cut to match one side of the box as your heat sinkHere is another suggestion: Use a thermoelectric cooler in reverse.These things are solid state heat transfer devicesThese things work well enough to make small refrigerators, use a small one to transfer heat into your boxYou probably only need maybe a 2x2 cm module for this project.You can control the heating/cooling by changing the voltage.Many modern laboratory grade incubators use this technology. Apparently it is the current teck used.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 29, 2022, 01:49:15 pm
content: There was a comment about a model with soil at the bottom.I think is space there is no gravity "bottom". I suppose the fungus will grow on the plexiglass?

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 29, 2022, 02:23:10 pm
content: Quote from: Wallace Gasiewicz on July 29, 2022, 01:49:15 pmOh Yes !! ....I am sorry.And the fan will become a little sand blaster.

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 03:33:56 pm
content: Quote from: astroasma on July 29, 2022, 09:52:34 amThe T increase in your circuit will be simpler, it be related to how much power is being dumped by the mosfet, which is just volts * amps.I'm sure you could calculate it based on the thermal conductivity of plexiglass and the surface area of the box, but it would be a lot easier to just build the circuit and try it.

author: astroasma
author_rank: Contributor
timestamp: July 29, 2022, 03:50:18 pm
content: Fungi will be grown inside petri dishes that will be attached to one of the panels, they don't need soil

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 04:16:20 pm
content: I would start with this and a cardboard box.

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 04:28:28 pm
content: Oops! I made a mistake on the 22k resistor of the voltage divider of the PWM low-pass filter.

author: wraper
author_rank: Unknown
timestamp: July 29, 2022, 07:14:10 pm
content: MOSFET in linear mode approach IMHO is overcomplicating things with no benefit whatsoever. Also it needs some resistive load anyway to prevent shorting the power rails. Not good for reliability and temperature uniformity either. Why would you bother with digital to analog conversion, current sensing, opamps, running MOSFET in a linear mode most of them are not rated for. When you can just drive a gate of the MOSFET directly from MCU and connect bunch of resistors to its drain. Or just a single larger resistor which would be similar to using a single MOSFET as concentrated heat source. IMHO it only makes sense if the circuit is completely analog to begin with with no MCU involved.

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 07:15:57 pm
content: Quote from: wraper on July 29, 2022, 07:14:10 pmSounds great. Post a schematic which he can implement.

author: IanB
author_rank: Super Contributor
timestamp: July 29, 2022, 07:23:13 pm
content: Quote from: mag_therm on July 26, 2022, 08:26:05 pmNote that natural convection is a gravitational phenomenon. Since the ISS is a zero gravity environment, normal expectations of convective heat transfer won't apply. This is probably a good thing, since ambient cooling will be less.

author: wraper
author_rank: Unknown
timestamp: July 29, 2022, 08:37:04 pm
content: Quote from: cellularmitosis on July 29, 2022, 07:15:57 pm

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 08:50:49 pm
content: Oh, now I see, you're suggesting bang-bang control. Yeah that's probably fine for fungi anyway.Edit: oh, unless you mean PWM'ing the entire load directly, that would allow for PID control

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 29, 2022, 08:53:48 pm
content: The Oven Xtal Osc I was working on had the LOAD resistor between the source and ground. The resistor was actually 3-4 small resistors in parallel with a total resistance of just a few ohms. I think the resistors in my case were to develop a signal to indicate active heating.I had to rebuild the circuit and did put a bigger resistor in the place of the parallel resistors. I think it was 5 ohms. Don't remember. I really do not think it matters. I do not think you need an external resistor to limit current if the gate drive does it correctly.The mosfet with its flat mounting, is a really good way to transfer heat to the heat sink and hence the xtal in my case.I did not even use the original mosfet type, just picked a cheap big one out of my junk stuff.You are just using the mosfet as a variable resistor in this case and it simplifies the design IMHO.

author: wraper
author_rank: Unknown
timestamp: July 29, 2022, 09:00:34 pm
content: Quote from: cellularmitosis on July 29, 2022, 08:50:49 pmIn what way PWMing the load prevents you from using PID?

author: cellularmitosis
author_rank: Unknown
timestamp: July 29, 2022, 09:10:32 pm
content: Quote from: wraper on July 29, 2022, 09:00:34 pmmaybe I didn't phrase that correctly. we both agree that PWM'ing the load allows for PID.

author: wraper
author_rank: Unknown
timestamp: July 30, 2022, 07:33:40 am
content: Quote from: cellularmitosis on July 29, 2022, 09:10:32 pmWell, you can use PID regardless in what way you decide to control the output - PWM, PFM, PDM or whatever.

author: wraper
author_rank: Unknown
timestamp: July 30, 2022, 08:07:47 am
content: Quote from: Wallace Gasiewicz on July 29, 2022, 08:53:48 pmIn what way it supposedly simplifies design when you literally complicate the circuit a lot? You cannot just use MOSFET as a variable resistor, you need a feedback loop which will keep the current through the MOSFET in check. Then you need to form an analog setpoint from MCU, which is another complication. It only makes practical sense if you put a temperature sensor into that analog feedback loop and there is no analog to digital and digital to analog conversion in the system whatsoever. Otherwise analog heater control is completely redundant. The only reason I can think this analog heating can be useable in combination with digital control is when you want to put a heating element right into a super sensitive circuit to minimize the noise.

author: astroasma
author_rank: Contributor
timestamp: July 30, 2022, 04:04:38 pm
content: I am thinking of using a raspberry Pi, easier no?I can connect the T sensor to the Pi and control the circuit based on the output of the sensor

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2022, 04:31:19 pm
content: Quote from: cellularmitosis on July 29, 2022, 08:50:49 pmLots of PID controllers cycle the heater on at a low frequency, 1Hz or less, I guess technically it's PWM, as long as the thermal time constant is sufficient all that really matters is the average on time. That isn't what bang-bang control means.

author: cellularmitosis
author_rank: Unknown
timestamp: July 30, 2022, 04:44:29 pm
content: Quote from: astroasma on July 30, 2022, 04:04:38 pmIf you are more familiar with the Pi than the Arduino, that's a good way to go.

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2022, 04:50:35 pm
content: Using a Raspberry Pi as a simple temperature controller is a bit like using a sledge hammer to pound in a tack. This is clearly a job for either a very basic microcontroller or an analog circuit. The analog circuit is probably simpler, you can even just copy one used in a crystal oscillator.

author: astroasma
author_rank: Contributor
timestamp: July 30, 2022, 05:00:15 pm
content: I am not familiar with neither. I am doing the electric part of this project because we are just two students working on it. so I am literally discovering my way through this on the go! In other words, I have no idea what I'm doing

author: cellularmitosis
author_rank: Unknown
timestamp: July 30, 2022, 05:01:46 pm
content: Quote from: astroasma on July 30, 2022, 05:00:15 pmThe Arduino is more popular for this sort of project, so you're more likely to find support on forums. If you have no opinion either way, I'd probably pick the Arduino.

author: astroasma
author_rank: Contributor
timestamp: July 30, 2022, 05:03:04 pm
content: Even if I use raspberry pi pico?

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2022, 07:54:03 pm
content: A Raspberry Pi pico is completely different from a Raspberry Pi. I'm really not sure why they recycled the same name.

author: cellularmitosis
author_rank: Unknown
timestamp: July 30, 2022, 10:36:17 pm
content: Pi pico is relatively new on the scene. There will be fewer tutorials, source code libraries, and forum help available. That being said, if you are ok with blazing your own path, you could give it a shot.

author: RJSV
author_rank: Super Contributor
timestamp: July 31, 2022, 04:35:55 am
content: Reading through this, (I'm not expert):Nobody mentions available power (restrictions), and, as trivial as it might be, no one has mentioned the emitted SOUND, from that FAN running.Running even a miniature fan has perhaps, too much power drain, at 12 volts, and another possible source of failure. Plus that power, from fan, is yet another heat source. At, 100 mA, 12 V comes to 1.2 watts, right there.I would consider a slight restriction on fan speed, like a 20 ohm resistor, maybe, THAT would drop your fan to 10 volts in the example (100 mA), but can get the emitted sound and vibration down a lot.I bet that project has a bunch of restrictions on total power used, and acoustic emissions (if even allowed).Perhaps, even restrictions on any 'paints' that would be incorporated (on outsides).Nice subject.

author: pcprogrammer
author_rank: Super Contributor
timestamp: August 01, 2022, 07:48:25 am
content: Quote from: RJHayward on July 31, 2022, 04:35:55 amA simple solution for the fan would be to use one that has speed control like in computers. Simple PWM with a frequency feedback. Easy to do with the arduino.https://create.arduino.cc/projecthub/Monstermotte/make-a-fan-controller-with-arduino-b5436dAbout the heat loss from the enclosure, the original post mentions an ambient temperature of 20-24 degrees Celsius. So we are talking about a 6 - 10 degrees Celsius of difference which is not that much. With this in mind and the heat loss properties of the enclosure it should be possible to calculate the needed amount of power to sustain this difference, and also the initial amount of power to get the box up to temperature.Also important to know is what the allowed delta temperature is. If a +/-1 degree is fine, a simple on/off strategy might do. No need to trim the parameters of a tricky PID system. Just a simple set point. Reading above, turn the heater of, reading below, turn the heater on. A simple experiment can be done to test this. Sure it won't be like in the space station, because as someone mentioned there is no gravitation, but there is air and maybe that still behaves in a similar way as here on earth?Someone else already mentioned this, knowing the amount of available power from the space station is important. Things like supply voltage and max allowed current matter in making a proper design. Also mentioned in her other thread about hooking things up to an arduino it is mentioned that there may be restrictions on the EMC field. Also there is something like space radiation that can raise the question, will a standard MCU do. Probably yes, because there are also humans there, but always good to consider.