Thermal Fuse

[electronbean]

I am currently facing an issue with my rice cooker, where the thermal fuse has been triggered twice, leading to a disruption in its normal operation. I am reaching out to seek your expertise and advice on identifying the root cause of this problem.

To provide some context, the thermal fuse serves as a safety device by opening circuits when the temperature exceeds a certain threshold. I understand that it can detect heat caused by overcurrent due to a short circuit or component breakdown. In my rice cooker, I have identified three key components: the Heating Coil (for cooking), the High Resistance Coil (for temperature maintenance), and the Magnetic Temperature Switch (which opens when the set temperature point is reached).

I believe the internal configuration of my rice cooker matches the diagram shown in this video: https://youtu.be/dZxdVCHAAsA?t=124
Additionally, I have attached an image of my rice cooker, highlighting these three components for your reference. : https://ibb.co/ZBfJcsK

The thermal fuse's specifications are as follows: SEFUSE - SF188E - JET192ºC - (PS)E10A - CCC 250V - C1632. You can find more information about it: https://media.schott.com/api/public/content/ff224fc379ec4eb09a770a547ac50b04?v=37aaf3af#page=6&zoom=180,-102,564

Despite changing the thermal fuse twice, the issue persists. The rice cooker operates normally for approximately 15 minutes, and just before the Magnetic Temperature Switch opens (indicating that the set temperature point is reached), the thermal fuse is triggered.

I am eager to troubleshoot and identify the specific component causing the thermal fuse to trip. Could you please provide guidance on tests or diagnostics that I can perform to isolate the problematic component?

[max.wwwang]

Looks like this is the relatively simple (so original in terms of its working principles) type of rice cooker. I recently repaired one for a friend, which was a little bit 'advanced' I guess (Panasonic SR-DF101), because it did not have the magnetic mechanism. I was even able to find the schematics of it online. By replacing one of the faulty TF's (temperature fuse), it worked just fine. The new components (among which I needed only one) cost virtually nothing from AliExpress, which is amazing.

At the time, the workings of rice cooker were new to me, and I found this video very helpful (and it's more relevant to your case).

[pickle9000]

Thermal fuses are sensitive to vibration when close to the trigger point. Basically give the cooker a wack or drop the lid while the element is on and that can trigger it.

[Ian.M]

The thermal fuse's specifications are as follows: SEFUSE - SF188E - JET192ºC - (PS)E10A - CCC 250V - C1632. You can find more information about it: https://media.schott.com/api/public/content/ff224fc379ec4eb09a770a547ac50b04?v=37aaf3af#page=6&zoom=180,-102,564

Despite changing the thermal fuse twice, the issue persists. The rice cooker operates normally for approximately 15 minutes, and just before the Magnetic Temperature Switch opens (indicating that the set temperature point is reached), the thermal fuse is triggered.

So why did you buy 172ºC thermal fuses?  That's 20ºC less than what was there, which may be why it blows before the temperature switch opens!

Edit: I was wrong, see below.

[floobydust]

But either fuse temperature seems way too high, above any normal operating point.
I thought rice cookers (thermostat) max temperature is around 100°C for steaming and ~65°C for warming. Assuming the water boiling is regulating temps until that dries up. I have an egg cooker that simmers until the water boils off, which then causes the heating plate temps to jump up (no thermal load) and then it trips the temp switch.

I wonder if OP has a lazy magnetic temperature switch that takes it too high?

[pickle9000]

The fuse temp is related to mounting location. The side away from the rice will get substantially hotter.

[pickle9000]

With a bad control running away the contact points or driver will be shorted. For mechanical points that means welded together.

Also note the thermal fuses are crimped not soldered as soldering would make the fuse go open.

[inse]

A crimp connector is visible in the photo of the thermal fuse

[Haenk]

(folks, the exploded view with 172oC fuse has nothing to do with the original issue, different forum members, different machines)

I can only assume the thermo switch hangs (therefor needs to be replaced) and the thermo fuse kicks in as a safety device. At least *that* works.

[Ian.M]

Yes,  In general for any heated appliance, a correctly rated thermal fuse can weaken over time, and blow, or blow due to abuse, but if its replacement blows repeatedly on the bench, it means that power to the heating element is not being properly controlled (or in fan cooled devices, the fan may have failed).

[soldar]

A crimp connector is visible in the photo of the thermal fuse

Thermal fuses HAVE to be crimped and cannot be soldered. I hope the reason is obvious.

[pickle9000]

Yes,  In general for any heated appliance, a correctly rated thermal fuse can weaken over time, and blow, or blow due to abuse, but if its replacement blows repeatedly on the bench, it means that power to the heating element is not being properly controlled (or in fan cooled devices, the fan may have failed).

This is an excellent description of what you need to keep in mind.

[johansen]

A crimp connector is visible in the photo of the thermal fuse

Thermal fuses HAVE to be crimped and cannot be soldered. I hope the reason is obvious.

Yes people are too slow and fumble around, over heating the joint with an 800F unregulated oxidized soldering iron, then wonder why the thermal fuse failed.

Im over here having no problems soldering tabs on lithium ion batteries.....

[Ian.M]

Its not impossible to solder a thermal fuse, if you can keep its leads and body cold enough through the whole process.  However the difficulty of  doing so is high so in most circumstances you'd be crazy not to use a crimped or clamped connection.    Consider this when removing the failed fuse, as if it isn't crimped in wire to wire, you may need to keep its leads to  have something to crimp to.

[johansen]

Its not impossible to solder a thermal fuse, if you can keep its leads and body cold enough through the whole process.  However the difficulty of  doing so is high so in most circumstances you'd be crazy not to use a crimped or clamped connection.    Consider this when removing the failed fuse, as if it isn't crimped in wire to wire, you may need to keep its leads to  have something to crimp to.

Dude in 0.3 seconds you can solder two 20 gauge wires together, and 1 inch away you would measure maybe 5C temperature rise.

[Jwillis]

Theirs a PTC thermistor in circuit. The video called it  resistor coil. If that starts to fails it can't control the current to the heating element correctly. So the unit goes into an over temp condition which trips the thermal switch. The thermal switch is only there as a safety device and not to control temperature. The Thermistor needs replacing.

[Ian.M]

@johansen,
Sure, if they are properly prepared and there is no large thermal mass.  However, in that case, you should be crimping them as a solder joint alone is not regarded as a reliable means of connection for mains wiring, especially at elevated temperatures.  The joint must be mechanically sound *before* it is soldered, and if you crimp there is no need to solder.

However the repair I am thinking of originally had a thermal fuse spot-welded to what was effectively a bus bar (IIRC the other end was crimped to a wire) and due to the thermal mass of the bus bar, I had to heatsink and chill the fuse to let me make a satisfactory solder joint without blowing it.

[johansen]

@johansen,
Sure, if they are properly prepared and there is no large thermal mass.  However, in that case, you should be crimping them as a solder joint alone is not regarded as a reliable means of connection for mains wiring, especially at elevated temperatures.  The joint must be mechanically sound *before* it is soldered, and if you crimp there is no need to solder.

However the repair I am thinking of originally had a thermal fuse spot-welded to what was effectively a bus bar (IIRC the other end was crimped to a wire) and due to the thermal mass of the bus bar, I had to heatsink and chill the fuse to let me make a satisfactory solder joint without blowing it.

Its a rice cooker

[Ian.M]

Yes, And?

The repair I just referred to was not a rice cooker, as back then they weren't a 'thing' here except in certain ethnic minority communities.  It was a small kitchen appliance, I forget exactly what.  The boss dumped it on my bench, having failed to solder in a new thermal fuse without blowing it.

However, are you telling me that no rice cooker in the history of electric rice cookers has ever had its thermal fuse spot-welded or crimped direct to a much larger metal part that would be difficult to solder?

[electronbean]

Thank you all for your responses.

The Thermal Fuse activates before all the water inside the pot has completely evaporated. From my perspective, this suggests that the thermal fuse may not be appropriately sized for this specific environment/location. Let's consider the scenario where the Magnetic Temperature Switch, designed to open when a set temperature point is reached, loses its ferromagnetic properties upon reaching its Curie Temperature (as explained here: https://youtu.be/RSTNhvDGbYI?t=419). This Curie Temperature is at 100ºC, which is the temperature anticipated at the top of the Magnetic Temperature Switch within the pot.

Observing the thermal dynamics in this video (https://youtu.be/RSTNhvDGbYI?t=309), it's evident that the Thermal Sensor is positioned near the bottom of the Heating Coil, the component responsible for heating. I'm assuming that the temperature in this region reaches up to 192ºC. Here's my question and suggestion: Is it possible that the temperature of the labeled part, specified as 240V 700W CFXB-40, exceed normally 192ºC, and is there potential to increase its rated functioning temperature of the thermal sensor?

Which temperature this piece, labeled with 240V 700W CFXB-40 can reach? https://imgbb.com/ZBfJcsK. Does anyone have the datasheet or spare list of this equipment?

[electronbean]

Theirs a PTC thermistor in circuit. The video called it  resistor coil. If that starts to fails it can't control the current to the heating element correctly. So the unit goes into an over temp condition which trips the thermal switch. The thermal switch is only there as a safety device and not to control temperature. The Thermistor needs replacing.

Where are you seeing this "PTC thermistor in circuit"?

[Ian.M]

Assuming you correctly identified the original thermal fuse p/n, this may indicate it has over time developed poor thermal contact between the main heating element and the pot, resulting in the heating element overheating before the liquid has boiled away. Another possibility is that you failed to refit the thermal fuse in exactly the same manor so it is now getting hotter than the original did.

[max.wwwang]

Thermal fuses HAVE to be crimped and cannot be soldered. I hope the reason is obvious.

From the response it seems far from obvious.

[Jwillis]

Theirs a PTC thermistor in circuit. The video called it  resistor coil. If that starts to fails it can't control the current to the heating element correctly. So the unit goes into an over temp condition which trips the thermal switch. The thermal switch is only there as a safety device and not to control temperature. The Thermistor needs replacing.

Where are you seeing this "PTC thermistor in circuit"?

The Coil of wire. Usually it's in it's own insulated enclosure to prevent shorting, It is made of a material that has a specific resistance that increases with temperature. A PTC resistor can be made of any resistive material that has a positive temperature coefficient including a coil of wire.