How can i build a simple circuit that can generate heat and control temperature?

Thread Starter

Barb5525

Joined Jul 10, 2017
8
I am a design engineer with a very limited knowledge on electrical engineering, however, i am working on a project that requires building a simple circuit that can generate heat and provide a narrow range temperature controlling mechanism. My basic knowledge tells me that i can just use a resistor, an LED to serve as an indicator, a heating coil, and thermistor. i also researched some more and was able to come up with the schematic below. i need assistance to verify if this circuit is feasible. Thank you.

upload_2017-7-10_13-41-14.png
 

MaxHeadRoom

Joined Jul 18, 2013
23,595
What voltage/power? What application?
In some cases a simple bi-metal device would do it, if precise control you may need a PID loop of some kind.
Max.
 

crutschow

Joined Mar 14, 2008
27,696
The big question is how accurate do you need to maintain the temperature?
The circuit you posted will tend to have poor regulation.
 

MaxHeadRoom

Joined Jul 18, 2013
23,595
Yes. 2n7000 good for 200ma
According to the OP you have a 1k load? Or?
I believe you actually need a comparitor that will snap over at a certain temp.
LM311 etc.
Max.
 

BR-549

Joined Sep 22, 2013
4,938
How is the coil connected? So does that mean that when the temp drops to 37.......you want to heat coil to 50 and then off?

What is pulling heat from coil? And that rate?

These simple questions are the best.
 

WBahn

Joined Mar 31, 2012
26,398
How much mass do you need to heat? What is the thermal capacity of the mass? What the thermal conductivity of the mass to the surroundings? How quickly do you need to bring the mass from its starting temperature up to the desired temperature? How long do you need to maintain it?

3 AA batteries aren't going to provide much energy.

An AA alkaline battery has a capacity of about 2 Ah, or about 10 kJ. Three of them give you about 30 kJ to work with.

Raising a 12 oz can of soda (water) 25 °C to 50 °C would require somewhat more than that, assuming no losses.
 

Thread Starter

Barb5525

Joined Jul 10, 2017
8
How is the coil connected? So does that mean that when the temp drops to 37.......you want to heat coil to 50 and then off?

What is pulling heat from coil? And that rate?

These simple questions are the best.
@BR-549 Thank you for your response and the questions. it helps me think through the project deeper. i was thinking of connecting one end of the wire to the 10k resistor and the base of the transistor and the other end to the emitter. The thermistor will be mounted on the heating coil. all the connections will be done through soldering. Please is this something that is doable?
 

WBahn

Joined Mar 31, 2012
26,398
I don't know that you want to mount the thermistor on the heating coil (unless it's the coil temperature you want to regulate). Imagine an electric space heater in your home -- do you want the heating element to only get up to the temperature that you want the room at?

What item is it that you are trying to control the temperature of?
 

Thread Starter

Barb5525

Joined Jul 10, 2017
8
How much mass do you need to heat? What is the thermal capacity of the mass? What the thermal conductivity of the mass to the surroundings? How quickly do you need to bring the mass from its starting temperature up to the desired temperature? How long do you need to maintain it?

3 AA batteries aren't going to provide much energy.

An AA alkaline battery has a capacity of about 2 Ah, or about 10 kJ. Three of them give you about 30 kJ to work with.

Raising a 12 oz can of soda (water) 25 °C to 50 °C would require somewhat more than that, assuming no losses.
@WBahn the i am making a first hand design for a scope warmer which is a one time use device and needs to operate for about 20 - 30mins. The circuit will be wrapped around an aluminum nozzle which weights about 8g. thank you for your response.

the main purpose of the nozzle(chamber) is to heat a surgical scope. so i am trying to control the temperature of the nozzle or the chamber.
thank you.
 

BR-549

Joined Sep 22, 2013
4,938
Barb.........not even close. Every detail makes a difference. Without all the details......we are only guessing.......and that will perplex you.

Every step of the solution depends on all the other details.
 

MaxHeadRoom

Joined Jul 18, 2013
23,595
The problem is that we are asked to design something with absolutely no, or very little info.
When trying to control the heat of something It all depends on how precise you need the control to be, e.g. If using the relatively primitive method you show, there is typically overshoot and undershoot which if undesirable, requires a more sophisticated controller.
Max.
 

wayneh

Joined Sep 9, 2010
17,153
I suspect a better solution would be to use a commercially available, temperature-regulated water bath and to pipe the water to whatever needs to be stabilized. The details of the mechanical arrangement depend on things we haven't been told yet, but trying to control something with little size and thermal mass is a lot more challenging than "simple plumbing".
 

BR-549

Joined Sep 22, 2013
4,938
From what I can tell......I would also recommend a water bath. Use sterile plastic sheaf/bag for non contamination. It should be quicker and more even than a blanket structure.
 
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