5 min read · Apr 21, 2022
In the repair café where I repair stuff on a regular basis, we have a whole drawer of thermal fuses. They get used very frequently, last Tuesday I fixed two devices which had a blown thermal fuse. This made me think about these little devices and whether they are bad and how we could solve the problem with them.
A thermal fuse is a small device that interrupts the flow of electricity if a certain temperature is reached. This is a very important safety feature in many different tools and appliances. It is usually installed close to a “thing” that produces heat. When the “thing” that is protected produces too much heat, it cuts the circuit, and the device no longer works. The “thing” could be a heating element or a motor. This is done to protect the device from damage from overheating and in the worst case from catching fire.
Thermal fuses are very cheap devices that cost way less than 0.5 €/$. They can be easily installed in almost all devices. A fuse has a specified temperature at which it fails. Once it has tripped, it has to be replaced, before the device starts to work again. This can usually be done within a few minutes.
They are everywhere! Thermal fuses can be found in devices that produce heat such as hair dryers, hot air guns, rice cookers, sandwich makers or all kind of ovens. Let me give you an example of why they are there. If the air intake of a hair dryer is blocked by hair and dust, no air can be propelled through the heating element. The element will overheat, start to melt the plastic housing and maybe light the whole dryer on fire. Therefore the thermal fuse kicks in at a temperature way below these events and cuts the current to the heating element (and often the motor as well).
Another place where they are frequently used, are devices with (bigger) motors such as blenders, vacuum cleaners or power tools. In these devices, they prevent the motor from overheating when it is blocked by something. If you stuff too many things in your blender and it has problems turning, it can easily happen, that this is enough for the fuse to trip.
A last place I would like to mention is in power supplies or dimmers. I’ve just replaced one thermal fuse in a dimmable lamp. If the power supply overheats for whatever reason, the fuse blows, and the power supply no longer delivers electricity.
From what we’ve seen so far, they are very important safety features. And I would be the last person wanting to reduce the safety of a device. The problem is that they tend to trip quite easily in different situations. The device is then broken and cannot be repaired by the normal user.
Let’s have a look at the air dryer I mentioned before. As kids we loved to close the aforementioned air intake and let the dryer heat up. We never destroyed one, but I don’t know how close we got. This fault (hands on intake) is very temporal and is immediately gone once you remove your hands. If you trip the thermal fuse, the hair dryer is gone. For most people this means they have to throw it away and get a new one.
Thermal fuses often trip without any obvious cause. This can be caused by wrongly chosen temperatures by the manufacturer, bad/cheap fuses, number of power cycles, age and probably some other reasons. Once repaired, the devices will work safely for many years more.
One could argue, that is done to increase sales of devices (so called planned obsolescence). It is probably more a case of “this is the cheapest solution we have, let’s do that”. I never worked for a manufacturer so any discussion about motivation is pure speculation from my part. There are other solutions that could be implemented but they are more complex and/or more costly.
As always, there is no one-fits-all solution that can be applied to every device. The following approaches could be implemented in many devices and thus reduce e-waste and stop people from having to buy a new device because of such a small thing as a fuse.
Thermal switches are an obvious alternative to thermal fuses. A thermal switch opens when it detects an overtemperature. The user can then simply push the switch to reset it. Unfortunately, they are much more expensive than thermal fuses. They cost around 2–3 €/$. Furthermore, they cannot be installed in any device or require a redesign of the case because they are bigger and the switch part has to be on the outside of the device. User unaware of the consequences could reset a device every time it overheats and maybe cause other problems.
So called self-resetting fuses could be used in some devices. They are similar to the normal thermal fuses, but they reset themselves when they are cooled down again and then let the current pass once again as before. The disadvantage of these is the fact, that if the device really has a problem, the resettable fuse gets triggered, cools down, then the device overheats again and so forth. This can cause other failures in the device which can lead to other safety issues. In some types of devices, they could immediately be used as a drop-in replacement for a thermal fuse.
A way to circumvent the disadvantages of the options mentioned before would be a combination of one of those protection mechanism and normal thermal fuse. The resetting device would protect at a lower temperature and a normal thermal fuse rated at a higher temperature could take over if something really bad happens.
A last option I could see are user replaceable thermal fuses. I am currently not aware of such devices, but in principle the manufacturer could design a fuse holder which contains the thermal fuse (similar to a regular fuse). It certainly would be a design challenge to place this fuse holder close to the heating part but for some devices I think this should be feasible.
If all this is not possible, a last “solution” would be an error indicator on the device that shows that such a fuse has blown. An indicator/lamp could show that there is a problem, and the manual could inform the owner, that this is a fixable state, maybe with directions on where to get it repaired.
Thermal fuses, even though very important safety wise, are a major nuisance and create a lot of electronic waste. A device with a blown fuse has to be considered broken for most people. The motivation of the manufacturers is probably just the price and ease of use and not malice (but this is pure speculation).
There are solutions that are more complex and expensive, but manufacturers should start implementing them. We throw away way too much perfectly good devices which can be repaired in a few minutes with minimal tools. In the meantime, we just have to keep fixing them. In one of the next articles, I will write about how you can fix a thermal fuse yourself, stay tuned.
