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I would like to warm a glass jar to 27°C and maintain this temperature using a miniature digital thermostat. The thermostat is 12V DC and will support up to a 12V 10A load. Its pre-made and comes with a temperature sensor.

Limitations:

The jar must be warmed slowly and consistently. Ideally the wire would not go over a temperature of 27°C. The jar will be covered in material so I can't flash warm it with on/off bursts of heat. More of a constant warming.

I have in front of me 50ft of Resistor Wire; Model: 6J44; Material: Nickel Copper Alloy

Wire Diameter: 0.2mm; Wire Gauge: 32AWG; Resistance: 2.02 ohm/M

I would like to keep my wire usage under 4m per jar. I haven't chosen a power source yet but would like to run it with a 240V/12V converter plug.

The only part I would like to have to manufacture my self would be the wire coil/jacket that the jar would sit inside of. The material will keep the wire at a snug fit to the jar.

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MushBuddies
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    Now I know what your project is, but I don't actually see a question... – user253751 Jan 26 '18 at 05:10
  • You need to use a question mark somewhere. – Harry Svensson Jan 26 '18 at 05:17
  • What is your tolerance in temp error from top to bottom and variation in ambient? How will you verify it? Without an acceptance criteria, suitable insulation, and verification method, how will you know it works? i.e. invalid requirement. – Tony Stewart EE75 Jan 26 '18 at 05:29
  • set the hysteresis low and resist the wire lead so that it takes, say, 5 mins to click off. – dandavis Jan 26 '18 at 05:43
  • You can make a PID controller with a thermistor, an Arduino and an optional fan. Highly recommended. Usually keeps the temperature within 0.2°C of the set point. I suggested the fan because otherwise, the wire will be very slow to transfer its heat to the air. If you use no fan, I don't recommend heating with more than 0.25 W or so. Otherwise the thermal mass of the wire will bring it way over temperature. But putting something in the jar to help absorb heat would solve that problem. – piojo Jan 26 '18 at 08:12

1 Answers1

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I would like to warm a glass jar to 27°C and maintain this temperature using a miniature digital thermostat.

A thermostat implies an on/off control rather than a proportional control. The thermostat will have hysteresis (usually 0.5 to 2° difference between turn-off and turn-on points. This will affect the tolerance (which you haven't specified) of how tightly you can control the temperature.

The thermostat is 12V DC and will support up to a 12V 10A load.

That means you can switch power of up to 12 x 10 = 120 W.

Ideally the wire would not go over a temperature of 27°C.

The wire will have to go over 27° to bring the jar to 27° in any reasonable time.

The jar will be covered in material so I can't flash warm it with on/off bursts of heat. More of a constant warming.

The jar will have a thermal time constant. Bursts of heat in the order of 1/10 of that time constant will be smoothed out by the thermal inertia of the jar. This is standard industrial temperature control.

Resistance: 2.02 ohm/M.

Resistance: 2.02 Ω/m. (Note small m for meter.)

I would like to keep my wire usage under 4m per jar.

That means 8 Ω if used as one continuous length. We can calculate the power possible as \$ P = \frac {V^2}{ R} = \frac {12^2}{8} = 18\ \mathrm W \$.

Is that enough? We don't know as you haven't given us thermal loss measurement.

You can calculate the heat loss rate by putting a known power into the jar continuously and measure the ΔT that it stabilises at. e.g., If you put 12 W in and the jar stabilises at 37°C with ambient at 21°C then you can calculate your thermal losses as \$ \frac {P}{\Delta T} = \frac {12}{37-21} = 0.75\ \mathrm{W/K} \$ (watt/kelvin). You could then use this a basis for calculating the power required to maintain temperature from any ambient temperature. I would then double that power requirement to ensure that the jar gets from cold to working temperature within a reasonable time.

Usually a PI or PID control loop is required if you wish for fine control and to be able to minimise overshoot.

You haven't asked an actual question. Please edit your post to clarify where you are stuck.

Transistor
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  • Thanks every one for your input. I'm typing from an iPhone. The reason for no actual questions being in my first post is because I didn't really know if what I wanted was reasonably possible. Thanks to you(all). I see that it is. (Can't touch the return key remember). Since my post Iv realised I should have asked the following. How should I control the temperature of a wire that is 4m long and has a resistance of 8ohmes if the power supply I have is 12v dc with a max out put of 10amps. – MushBuddies Jan 27 '18 at 01:30
  • And.... what is a PI or PID control loop? – MushBuddies Jan 27 '18 at 01:36
  • i want to make a user settable but self maintaining incubator jar. To cope with year round temp fluctuations. I understand this will require a power range to cope with different household temps. For this project I wanted to use two layers of 1-2mm neoprene. Stitching the 2ohm/m wire into the inner surface of the outer layer. I had imagined slipping the thermostat probe in between the two layers also. Probably very close to the wire. I'd use the thermal absorption of the inner layer of neoprene and the glass of the jar to smooth the temp spikes. To test. I would have a thermometer in the dirt. – MushBuddies Jan 27 '18 at 01:48
  • See my answer to https://electronics.stackexchange.com/questions/346730/understanding-the-flow-of-a-pi-controller/346759#346759 for some understanding of Proportional-Integral-Derivative control loops. – Transistor Jan 27 '18 at 08:49
  • The neoprene sandwich doesn't sound like a good approach. The hot wire may melt it and a fault could start a fire. I would be trying to find a thermally conductive substance for the inner layer such as oil which is an electrical insulator. That would give even heat application to the glass. I would also be measuring the temperature inside the jar where it matters. – Transistor Jan 27 '18 at 08:52
  • Well the neoprene sandwich wouldn't be a bad idea if I can add some thing so I can vary the voltage or power to the wire. Honestly a 24hr warm up time is fine. What would be a device or circuit to do this. I need a small dimmer switch. I can place a thermometer inside the jar for testing but normally conditions must remain sterile so this would not be ok but yes for testing. Once tested I could set a adjustment screw and set with a glue. Then the thermostat would cut in or out if the users room temperature rose or dropped. – MushBuddies Jan 30 '18 at 03:12