Non-resettable thermal fuse teardown

non-resettable-thermal-fuse

This component is a one-shot thermal fuse. When the body rises above the specified temperature the two leads stop conducting. They’re useful in applications like motors, where you want to make sure power is cut to an overheating piece of hardware before permanent damage happens. They’re pretty simple, but we still enjoyed taking a look inside thanks to [Fatkuh’s] video.

The metal housing is lined with a ceramic insulator, which you can see sticking out one end in the shape of a cone. It surrounds a spring which connects to both leads and is under a bit of tension. The alloy making the connections has a low melting point — in this case it’s about 70 C — which will melt, allowing the spring to pull away and break the connection. In the clip after the break [Fatkuh] uses his soldering iron to heat the housing past the melting point, tripping the fuse. He then cracks the ceramic cone to show what’s inside.

The only problem with using a fuse like this one is you’ll need to solder in a new component if it’s ever tripped. For applications where you need a fuse that protects against over current (rather than heat) a resettable polyfuse is the way to go.

[via Reddit]

25 thoughts on “Non-resettable thermal fuse teardown

  1. Also used in Espresso/Coffee machines to make them not overheat when left on without water for extended periods of time (my uncle found this out the hard way).

    1. I did too, I had to reorder (and tear down for fun) this exact part for that exact reason last month.

      A warning though! You cant just solder those ones in. They are rated to pop at 240 degrees, which is within the melting range of solder. You wouldn’t want it to unsolder itself and contact something else. They were spot-welded and crimped on the machine I took apart. I used some 18ga steel wire to create a spring-like tube to re-crimp them with.

      1. Soldering these things is common and easy. There are a couple of methods, both use cooling to keep the fuse cold while soldering.

        If the temperature is above 100 degrees but well below 180 (melting point of regular lead solder), such as the overheating protection in regular water heaters (usually around 120 degrees), you can just use a piece of wet paper or cloth to cool it while soldering.

        If the temperature is below 100 degrees you could use some kind of alcohol, or even an ice cube to cool the fuse.

        If the temperature is near or above the melting point of your solder they obviously need to be crimped.

        1. Bleh, forgot to mention that the second method is a cooling clamp, basically a purpose built metal clip that clips on to the component wire and cools it while you are soldering.

          1. its called a heatsink… or “heatsink clip” for those who want to freak out when they see “heatsink” used out of the context of computer processors or ICs.

        2. Common and easy with bodgy repairs, and wrong. I’ve seen where a bodgy repair of an electric kettle resulted in it burning a hole right through a kitchen bench before finally popping the mains switchboard protection and narrowly avoiding a major house fire. Much more good luck than good practice.

          Storage hot water heaters have multiple levels of over-temperature and over-pressure protection, and while it is rare there have been cases of such heaters boiling then exploding and demolishing the house. The Mythbusters did a demo.

          These thermal fuses are used in a wide range of electrical appliances, particularly those that heat, as a secondary or tertiary fire protection after lower temperature bimetal self-reseting cutout(s)have failed, and are a very common cause of total appliance failure. If one of these blows then the thermostat/primary protection is suspect and should be checked for correct operation.

          They are available in a range of temperatures to suit the application, e.g. 77°C, 133°C, 157°C, 192°C, 216°C, 228°C, etc., which is printed on the side. Like conventional fuses they should always be replaced with the *same* temperature value, and they should *never* be soldered in place but *crimped* (as is standard practice for all connections in heating appliances).

          The very good reason for this is that electronic solder has a melting point in the middle of this range at 183°C. If the thermal fuse is for a lower temperature it could be compromised by soldering, and if higher a lead could unsolder itself and the free lead could easily create a dangerous situation, not least that it might bridge the blown thermal fuse.

          A secondary reason is that heating appliances often use wire alloys that don’t take solder very well.

          These are your last line of fire protection – replace them by all means, but please don’t mess with their value and always replace them properly by crimping.

          1. Please explain then, if you are never supposed to solder these thermal fuses, why the lower temperatures (120 degrees and below) are pretty much always soldered…

            If the regular solder melts at significantly higher temperature than the fuse value, soldering is fine since the component can never reach the point where the regular solder melts.

  2. Not to be pedantic, but polyfuses operate based on heat as well. Yes, they are quite different both in operation and intended use, but if you heat up a polyfuse from an external source (i.e. not from the current through it) it will still become high impedance…

    1. You often see this inside paper shredders. The polyfuse is glued to the motor, that’s why the shredder appears to stop working when you’ve used it for too long and its over heated. Then they magically work again 10 minutes later.

  3. These kinds of fuses are often crimped in, since it’s cheap and easy.

    There are also thermal cutouts, which use a bistable spring to cutout at a given temperature, then reset when the system cools down.

  4. Wow! Until now I thought there was a single tiny leg of a flux capacitor in there, and when enough current flowed and the temperature was right, the internals traveled in time, breaking the circuit! Who would have known???

  5. how about those resettable ones that are found in microwaves touching the microwave gun that will turn off if they get too hot?

  6. These are also in older lithium ion battery packs such as laptop packs (maybe new ones too) and portable power tool battery packs.
    I’ve seen them in small ceramic fan heaters.

  7. I replaced the one in my laminator with a high temp version (they’re cheap on eBay) then put a 50K resistor in series with the thermistor that controls temperature.

    Running the laminator at a higher-than-normal temperature is better for toner transfer.

  8. I used one of these to fix the fan speed resistor pack in our car’s AC system (the fan worked only on 0 or 100% which was annoying when you had to defog the windows). The dealer quoted $40. The fix was an 80cent part. The original was spot welded in but I managed to get it to stick with solder.

  9. I actually soldered al lot of them. Even for medical applications. :-)
    Just with a metall-clip and a not-too-long solder-time. Sounds crazy, as most manufacturers will only crimp them, but it works.

  10. “When the body rises above the specified temperature the two leads stop conducting.”

    isn’t it the body that stops conducting?
    the leads should still conduct.

  11. A “Slo-Blo” fuse works in the same way, if it is in a glass capsule, you can see the tension spring near one end and the metal alloy fuse at the other.

  12. What looks like a solder join may be an impulse weld.

    What’s most important is that the melting point of the join must be above that of the thermal fuse. SN05Pb95 has a melting range of 301-314C, Sn10Pb88Ag02 has a melting range of 268-299C (figures are from Kester).

    A compromise method when crimp tubing isn’t available, is to wrap a bare overlap joint with a tight spiral of solid copper wire, and use Sn40Pb60 solder.

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