Temperature “sensor” that works like a fuse

Thread Starter

kender

Joined Jan 17, 2007
264
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
 

John Luciani

Joined Apr 3, 2007
477
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.
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
 

beenthere

Joined Apr 20, 2004
15,819
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.
 

Gadget

Joined Jan 10, 2006
614
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
 

antseezee

Joined Sep 16, 2006
45
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.
 

Thread Starter

kender

Joined Jan 17, 2007
264
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.
 

KMoffett

Joined Dec 19, 2007
2,784
These are meant to be in physical contact with whatever might overheat. Your spec:
small, PCB-mounted (preferably SMT) device that acts like a temperature fuse? .... fusable link if the temperature of the PCB exceeds a threshold temperature somewhere between 40C and 100C...would need to survive the soldering temperatures that are higher than their threshold.,


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

Thread Starter

kender

Joined Jan 17, 2007
264
Kind excludes everything I can think of. Maybe you can explain you total project. That might lead to other approaches.
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.
 

thatoneguy

Joined Feb 19, 2009
6,359
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.
 

marshallf3

Joined Jul 26, 2010
2,358
Last edited:

Thread Starter

kender

Joined Jan 17, 2007
264
I haven't seen any mounted to a PCB, a DigiKey search doesn't show any either.

[...] 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.
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.
 

marshallf3

Joined Jul 26, 2010
2,358
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.
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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