Temperature “sensor” that works like a fuse

Discussion in 'General Electronics Chat' started by kender, Apr 16, 2007.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    Folks,

    Is anyone aware of a small, PCB-mounted (preferably SMT) device that acts like a temperature fuse? Something on the PCB that will break (or may be make) a fusable link if the temperature of the PCB exceeds a threshold temperature somewhere between 40C and 100C ? If such exposure takes place, it will last for hours. I have a suspicion that these devices might be either nontrivial or nonexistent, because they would need to survive the soldering temperatures that are higher than their threshold. On the other hand, there are fire sprinklers that have this type of behavior.

    Unfortunately, the arrangements such as thermistor-comparator-flipflop-FET will not work, because the circuit doesn’t have power.

    I know that there are “permanent” temperature labels that change the color with temperature and retain the color of the maximum temperature (like these http://www.omega.com/toc_asp/subsectionSC.asp?subsection=F02&book=Temperature). But they can’t disable the circuit. They are only visual aids.

    Any references, insights or inspiration is appreciated!

    Cheers,
    Nick
     
  2. John Luciani

    Active Member

    Apr 3, 2007
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    The fuse and the temperature sprinkler contain metals with a melting temperature
    that is much higher than 100 DegC. If you had a fuse that would open at a temperature less than <100 DegC you could socket it and plug the fuse after soldering.

    The only thing I can think of would be a bi-metalic strip that mechanical latches
    off at a high temperature. This would be big and custom.

    (* jcl *)

    ---
    www.luciani.org
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you can get some indium based solder, it may melt at a low enough temp to be useful. Straight indium melts around 160 C, but the solder alloy may have a lower temp. If so, you might arrange for a springy wire to come off the board and open up a circuit.
     
  4. Gadget

    Distinguished Member

    Jan 10, 2006
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    A microtemp. Not SMD, but used extensively in heaters, inside transformers, jugs, and sandwiched between 2 resistors inside electric blanket controllers to blow if current thru resistors is excessive.
    Lots of values from 60 odd degrees up to 240 (degrees C).
    I think they use a wax compound to hold internal contacts together until the wax melts.
    Normally used as a failsafe.... blows when all other safetys/thermostats etc fail. A bastard to replace.
    Most common way of attaching is to crimp, but can also be soldered by freezing with spray, tinning, refreezing, soldering, refreezing.

    http://www.4most.co.uk/thermal_fuses.htm
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    I googled thermal fuse and came across this link for what it is worth.

    hgmjr
     
  6. antseezee

    Active Member

    Sep 16, 2006
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    You could probably design one manually if you wanted to save costs (unless the components were extremely out of hand). Perhaps by using a LM34 or LM35 temperature sensor, and then designing some sort of conditional circuit that has the physical specifications to latch off when the desired temperature is met. Obviously, the conditions of the layout would affect what components are useable.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  8. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    Not quite. The device I was looking for is called "thermal cutoff" or "thermal cutout". The 2 key features of thermal cutoffs: temperature - as opposed to current - trips them, most of them are non-resettable. Here's a datasheet for one of them: http://media.digikey.com/pdf/Data Sheets/Cantherm PDFs/SDJ1,SDJ2.pdf. There's a figure on p.2 that illustrates the principle of operation. There's a pair of spring-loaded contacts that are held together by a piece of paraffin. Paraffin is formulated so that it will melt at the trip temperature. Once paraffin melts, the spring separated the contacts.
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    These are meant to be in physical contact with whatever might overheat. Your spec:
    kind excludes everything I can think of. Maybe you can explain you total project. That might lead to other approaches.

    It's often more productive to help you solve your problem, than help solve you solution. ;)

    Ken
     
  10. Kermit2

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    Feb 5, 2010
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  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Kermit2,

    Did you see kender's link in post #9?

    Ken
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    No,

    I'm blind


    and I have this large foot shaped thing in my mouth too. Makes it kinda hard to type. :)
     
  13. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    The purpose if the sought component was* to act as a sentry for overheating of the instrument during shipping.

    By itself, the instrument had a material that would start to degrade at moderately warm temperatures around 50°C. In warmer climes, temperature can climb above that level inside a container. If that were to happen, the hazards would be worse than if the instrument refused to operate at all. Another requirement was that the instrument shall be powered down completely during shipping.

    Of course, temperature-controlled shipping would be arranged. Thermal cutoff acted as additional reassurance.

    * Past tens, because it's a 3 year old thread, as yall may have noticed.
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    3 year old thread and people are still trying to help you find a solution.

    ;)
     
  15. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I would be embarrassed :(...if kender hadn't responded. ;)

    Ken
     
  16. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    I appreciate it a lot, folks.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I have a few that have the "bullet" shape from your first link.

    I haven't seen any mounted to a PCB, a DigiKey search doesn't show any either.

    I've mostly seen them on coffee machines to prevent fire, as well as on electric dryers for the same reason.

    I know the thread is old, but I'm curious as to what the actual use of a Surface mount version would be, compared to a thermistor or other temp sensing device.
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Last edited: Dec 26, 2010
  19. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    I haven't seen board mounted or SMT thermal cutoffs either. At the same time, I don't know how compatible "bullet" shape thermal cutoffs are with board soldering processes. I've seen datasheets that specify spot welding.
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That's the only way I've seen them used, either spot welded or crimped. I suppose you could coat them with that heat absorbing paste they sell at most plumbing shops and wash it back off later, that stuff works like a charm. An alternate method would be like we did in the old days, use a pair of hemostats on each lead pin to direct the heat away from the actual device.
     
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