Resistive heating

Discussion in 'The Projects Forum' started by kubeek, Aug 9, 2010.

  1. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    I am starting to build a battery powered heating system (similar to a vaporizer), and need to reach hot air temperatures of about 200-250°C.

    Right now I am not too sure about the heating element that I will use, but I was thinking of using 1W or 2W resistor, do you guys think that it can withstand such temperature without failing? How long do you guesstimate that such resistor could work? I suppose it will be some wire-wiound type. Hope to get some real-world answers, as I am not too experienced in abusing resistors.

    I don´t want to use resistive wire, because the lenght needed to get 10ohm would be too great for the confined space.
    Also do you have any ideas for some thermistor that could withstand these tempereatures?
     
  2. BMorse

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    A sandbar resistor may withstand the heating a bit more than normal resistors...

    B. Morse
     
  3. Wendy

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    So will nichrome wires, though you need to insulate them.
     
  4. BMorse

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    He mentioned already that he does not want to use resistive wire, which I believe covers Nichrome wire... :)
     
  5. marshallf3

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    250* C is pretty darn hot, using common resistors is going to be hit and miss. Whatever you do choose a flameproof one, there are some nice ones in ceramic packages that may take that kind of abuse.

    As mentioned above the sand filled resistors will also take some heat, how much would be in the spec sheets.

    Obviously it's going to have to be a wirewound resistor, and one wrapped externally on a ceramic tube would be best.
     
  6. kubeek

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    Looks like I am into some long-term testing..
    What about the termistor? I want to be able to regulate the temperature, most probably just a simple discrete diff amp made of three transistors. Do you have any ideas for a thermistor that can safely go up to 250°C? And BTW what is the melting point of solder? It won´t be too good for the heater to desolder by itself :)
     
  7. marshallf3

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    I'm not finding any thermistors that claim 250* or higher as an acceptagble operating temperature, once you get above 125* the list almost disappears except for a few expensive industrial devices. Most will jump over to common thermocouples in this range but that's not such a bad thing as they aren't too terribly hard to work with.

    I've seen solder alloys that have melting points around the temperature of boiling water. Common tin/lead solder alloys tend to melt around:
    63/37: melts at 183 °C (361 °F) (eutectic: the only mixture that melts at a point, instead of over a range)
    60/40: melts between 183–190 °C (361–374 °F)
    50/50: melts between 185–215 °C (365–419 °F)

    Silver alloy solder, sometimes contaiing other metals in the alloy such as copper, tend to have a bit higher melting point but will still not maintain any sort of reliability in the range you're dealing with. 250*C is the same as 482* F and the temperatures rated are the fluidic melting points, they material is actually starting to get soft before that. True brazing handles a lot more heat but often by then spot welding or actual mechanical connection is the preference.

    300* C resistors are not going to be much of a problem, connecting to them is another story. A mechanical connection (crimp/clamp/screw) will be the preferred method but at those temperatures you're even pushing the limits of teflon insulated wire so you're going to need to provide some sort of creative mounting arrangement or go with some high-temp asbestos-type insulated wiring or provide a good amount of cooling distance between the heating resistor and the wiring used to supply the current to it.

    It's totally doable but you're going to have to employ some slightly out of the ordinary construction principles.
     
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  8. kubeek

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    Hmm, that´s gonna be fun :cool:

    It may be possible to leave the legs of the resistor long enough to pass through the wall of the heated chamber and thus keep the solder cool. I guess I could use fiber-glass "necks" to insulate it.

    Thermocouple could be ok, but i guess the voltage difference between 25°C and 250°C won´t be high enough to regulate the temperature with a simple discrete comparator.
     
  9. BMorse

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    I don't know if you are familiar with the "Cold Heat" cordless soldering tool (it is not worth a penny if you ask me!) anyway, they use graphite at the tip to generate the heat to solder from 4 AA batteries.... maybe look into some kind of graphite heating element, I know graphite will burn up if too much current is put through it, but if the current is regulated and controlled maybe it could last a while.....

    B. Morse
     
  10. marshallf3

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    300* C resistors turned out to be far too easy to find and there's no problem using a thermocouple to power a comparator circuit. It's all in the construction now, and eventually I guess he'll be able to bake a cake or whatever he plans to do.
     
  11. kubeek

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    Thanks, graphite rod used in trammel with wrapped contacts on the ends could work too.

    As for the sensor, this looks quite promising.
     
  12. kubeek

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    As for the actual thingie, it is a project for evaporating gasses from porous minerals for testing. It should be pretty small, and battery (2-3 AA ni-cd cells) powered, my general idea is about a size of a 2xC battery torch light.

    Here is the circuit I got myself to, rz is a guess for internal resistance of batteries, R is the heater.
    Simulations look good, the only thing I am not sure about is the size and resistance of the heater resistor, because if I use too much power the resistor will be too hot and fail before i reach the desired tempereature. Maybe if i place the sensor close to the resistor, it won´t get over the switch off temperature.
     
  13. kubeek

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    BTW, do you know some kind of epoxy or something that I could use to create the casing that could stand these temperatures?
     
  14. BMorse

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    Here in the US we have something called JB weld, it is a steel epoxy, that some use to repair engine blocks on tractors and stuff.... maybe you can get something similar?

    B. Morse
     
  15. someonesdad

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    Jul 7, 2009
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    I doubt you'll find many epoxies or other plastic type material suitable for 250 °C service -- that's about as hot as your kitchen oven can go. Most plastics' limits are in the 150-200 °C range at best. However (tip of hat to BMorse), JB Weld is rated to 260 °C.

    For thermistors: the first google hit on "thermistors" turned up a candidate. I'd probably prefer a type K thermocouple.

    You can insulate wires with ceramic fish beads; they're used in vacuum systems up to 800-1000 °C or so; we used them 30+ years ago for wires up to molybdenum substrates and tantalum structures that ran near and above those temperatures. Coors Ceramics was one source.
     
  16. marshallf3

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  17. BMorse

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    Kubeek,

    Any success in finding a suitable heater element??
     
  18. kubeek

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    Didn´t have time for it, but today I´m going to buy some resistors and see how well and how long they work.
     
  19. BMorse

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    I found some stainless steel screen that works quite well with a rechargeable NiMh battery, I used about a 3/4" square piece of this screen and it heated right up to around 220+ degrees C, (I still have the blisters on my finger where I touched the screen inadvertently!) The battery power lasted for about 12 to 13 sessions of 35 second heating (approximately 7 1/2 minutes)....

    B. Morse
     
  20. marshallf3

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    Since we know of reliable 300* C resistors, and that a thermocouple (even a tiny one) will work just fine controllng the circuit I was waiting for this question.

    How big as in what dimensions, how sealed must it be and I'm sure it will need access port(s) of some type. If you're planning to run this on small batteries it's going to have to be a very small containment vessel that's extremely well insulated or you'll have no running time at all.

    My guess is something in the ceramics section of http://www.mcmaster.com will fit the bill.
     
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