Building a miniature 'Warm Box'....need advice

[jupiter14]

Hi.......I have an insulated 8 inch by 8 inch by 8 inch box that I need to heat to a constant temp of 80 Farenheit. The outside temperature of the box might range anywhere from 25F to 60F.

Is it possible to build a small heating device that: a) runs on 9 v or AA batteries; and b) uses a micro-thermostat similar to this one?

http://thermostat.en.hisupplier.com...thermal-protector-JUC-31F-thermal-switch.html

What could I use for the actual heater? A small lightbulb? Could an LED lightbulb be used?

Is it possible to do this sort of thing? Any advice would be greatly appreciated.....thanks.

 

[CocaCola]

Is it possible to build a small heating device that: a) runs on 9 v or AA batteries; and b) uses a micro-thermostat similar to this one?

No those battery sources will die out way too quickly to be of use...

What could I use for the actual heater? A small lightbulb?

That would be one of my first choices...

Could an LED lightbulb be used?

Likely not, even though some of the high wattage ones do produce heat, a bulb is better...

Is it possible to do this sort of thing?

Sure...

Any advice would be greatly appreciated.....thanks.

There is another thread her about doing this exact thing to grow mushrooms... Lots of ways to go about doing it, it's more a matter of how involved you want to get, how experienced you are with electronics, your budget, among other things...

 

[Harald Kapp]

LEDs are not well suited for heating - they are too efficient. Use a tungsten light bulb.
Two AAs will be drained rather quickly (9V blocke even quicker) if you draw lots of current to power a heater/leight bulb. YOu will have to use a more powerful source, e.g. a wall wart.

The thermostat you linked doesn't seem suitable for your purpose. i see no way how you would adjust the temperature. Something like this kit is more suited.

CocaCola: This time you beat me to the finish line.

 

[Raven Luni]

Coincidentally I came across these yesterday:
http://www.coolcomponents.co.uk/catalog/heating-5x10cm-p-1024.html

Theyre made by sparkfun if I recall (that site resells alot of their stuff).
You could use something like that. As for batteries, the minimum feasable would probably be D cells, maybe even a couple of banks in parallel or a lantern battery (which actually have about half the amp / hour rating of D cells)

I was thinking of getting some for heated animal boxes / blankets for Hessilhead (wildlife rescue centre where I volunteer)

 

[penfold]

Resistors make very efficient heaters.
As the others have said, of course unless your application really demands battery power, you'll be getting through an excessive number of batteries.

There are some nice example circuits dotted around for proportional temperature controllers, typical sort of op-amp feedback circuits. This type of arrangement will avoid the 'bang-bang' control characteristics of using a thermostat (more like a thermal limit switch) you linked in.

 

[jupiter14]

No those battery sources will die out way too quickly to be of use...


What I don't get is how a small Sanyo hand warmer can use 2 AA rechargeable batteries and pump out 110F heat for 2 hours. Are they using some kind of enhanced lithium ion battery?

Also, if the chamber to be used was 6" x 6" x 6", and the temperature instead around 75F, would AA batteries combined with a tiny heating element work?

...here's an example of the Sanyo hand warmer:

http://www.canadacomputers.com/product_info.php?cPath=730&item_id=028540

http://en.ca.sanyo.com/Rechargeable-Hand-Warmers/Palm-sized-rechargeable-hand-warmer

 

[penfold]

Thats a very good point, I'll be entirely honest and state I didn't do any calc's to base my response on.

You could do a good first order approximation from thermal resistance of a material similar to what you've made your box from.

A colleague of mine is currently using a similar sized box made from three layers of 15mm thick expanded polystyrene (measuring losses from a power converter). The internal temperature of the box raises about 5 degrees C per watt of power dissipated in the box. If that puts you in the ball-park of what to expect

 

[jupiter14]

Hi Harold.....thanks for your reply (and same to you Coca Cola).

Again, the example of the Sanyo hand warmer that uses 2 AA batteries makes me wonder if AA batteries would work. Would the batteries maybe work with a small heating element, and not a lightbulb?

Could AA batteries be combined with the MK138 thermostat and a small heater element, if the box was 5" x 5" x 5"? (or 12.7cm x 12.7cm x12.7cm)

If so, what kind of heating element could be used? A thin nichrome wire?

 

[BobK]

In a closed box, a resistor, a light bulb, nichrome wire, or a heating element will all produce the same amount of heat per Watt i.e. 1 Watt of heat for each 1 Watt of electrical power. All of the power is going into heat.

The amount of power needed to maintain a temperature will depend on the amount of thermal insulation, the surface area of the box, and the difference between the inside and outside temperatures.

Bob

 

[CocaCola]

What I don't get is how a small Sanyo hand warmer can use 2 AA rechargeable batteries and pump out 110F heat for 2 hours. Are they using some kind of enhanced lithium ion battery?

Look at how heaters work, if the device is only pumping out 110 degrees it will take a LONG time to raise the ambient temp of of a larger area, also the volume of heat exchange is a huge factor... For example your furnace in your house produces about 1000-3000 degrees F when the flame is burning over say a 16x20 inch heat exchanger with forced air flowing over it, all to maintain your house at a happy 68-74 degrees... When you strike a match it's about give or take 1500 degrees F, how far will that match get at heating something?

Now look back at your 110 degree heater and it;s short comings as a 'heater' should be more clear...

There is also insulation and lose factors to consider, that is why you basically 'brute' force heating and cooling in most cases vs taking the long slow road... A highly isolated never opened 8x8x8 cube will not take much at all to maintain once it reaches a temp, but if there is a lose like opening it or lack of insulation that bleeds the heat, maintaining it can be troublesome... If you want the temp to recover quickly or at all you need to pump the heat in or wait a long time for that to happen, if it happens at all...

 

[penfold]

What is the purpose of this 'warm box'. If it is something that is just going to be run for a long period of time with quite slowly varying external temperature then the answers should be slightly different to if it needed fast recovery or had very rapidly varying temperature extremes.

If you don't require very fast temperature recovery times, then lower temperature heating elements such as resistors would be more beneficial, whereas if you need a very rapid transfer of energy from the heater to the air then a higher temperature (lower thermal mass) element would be required.

Then in terms of selecting the controller, your requirements of accuracy would be useful. If you don't mind a bit of temperature overshoot and undershoot (see bang-bang control) then a very basic bimetallic thermostat would be right up your street, whereas tighter regulation of temperature would require a much more delicate solution.

 

[jupiter14]

Hi Penfold....thank you for raising some excellent questions, and for your previous feedback.

The box will need to maintain a temp of 80F for a maximum of 3 hours. Every 30 minutes, the box will be opened for maximum of 15 seconds and exposed to outside sea-level air that will be anywhere from 32F to 60F. Obviously the influx of cooler air, and the escape of warmer air, during openings will quickly decrease the inside temperature. The aim is to build a unit that will recover the 80F interior temperature within 10 or 15 minutes of closing the lid.

Temperature overshoot/undershoot of 4 or 5 F would be tolerable.

Do you have any extra insights given these parameters?

Any thoughts on this controller, or suggestions for others?

http://www.apogeekits.com/thermostat.htm

Thanks again, Rainier

 

[penfold]

I like the look of the controller you've linked in, it's pretty much what I would suggest. The key to keeping the overshoot minimum with a controller like that is finding a way to keep the thermistor as close the the air temperature as possible (the thermistor temperature will lag that of the air because it takes time for the air to transfer energy to the sensor), I would advise using a small aluminium heatsink, they're as good at taking energy from the ambient as they are at giving it.

The other key to a project like this is to experiment a bit, thermal stuff is very difficult to predict without painfully accurate maths. Try putting a few resistors, perhaps 10 or so 1/4 watt ones, pick a value any value and drive them with enough volts to make the dissipate near their 1/4 watt rating and see what the temperature does. Good practice in ohms law as well!

If things are a bit slow, you need more power.

 

[jupiter14]

Thanks Penfold......this is great guidance to get started.

 

[jupiter14]

Thanks again to all posters who have contributed their thoughts and suggestions for this project. If anyone has any extra recommendations about other micro controllers/thermostats to use, that'd be great too, although I'll most likely start off with the apogeekits model listed above.

Cheers, Rainier