Broken rice cooker: what is wrong with it and what is the flat thing?

We've had our second rice cooker fail (this Black&Decker rice cooker replaced an Aroma rice cooker which failed). What happened was it started to stink and didn't cook well, then finally the thermal cutoff inside failed.

I've already had the exact same problem with the previous Aroma rice cooker... let me explain:
In that Aroma Rice cooker which failed, I replaced the thermal cutoff with a direct connection and it worked! Well sort of...
It actually overheated all the time, but didn't heat the food very much. Inside the connections I had made were melting/burning and it was always stinking, so of course we did not use it anymore.
So this means there was something else wrong with that rice cooker. It overheated for a reason, and removing the overheat protection caused it to overheat.

Rice cookers are very simple, so I have no idea what the problem was (and is). The only suspect is some weird rectangular thing i've seen, where two wires go in; one to the switch and the other to the element.

So with this Black & Decker rice cooker, which has been working for quite a while (maybe a year or two), it finally failed by overheating the same way the old rice cooker did when I removed the overheat protection. It stunk a bit and didn't heat up well, then the thermal cutoff broke and it became dead.

Replacing the thermal cutoff won't fix it because it would just overheat again.

So what is wrong? Is it that rectangular thing?


I opened the rectangular thing... inside there is a sandwich of materials and a wire...
The first layer is the top metal cover which covers it up (and also always becomes rusty). The second layer is a thin papery insulator (or I think it is). The third layer is the same kind of insulator, but cut and it has a thin wire wrapped around it connected to the two wires going to the whole thing. The fourth layer is another insulator like the second. And the fifth layer is the metal frame which all the previous layers sits into, and the frame is bent around them to hold it together, and it has a screwhole to attach it.

What is this thing? Is it some resistor? Does it increase resistance as it heats up, to regulate the temperature? And could this be what failed?

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The rectangular thing you are talking about is a current limiting resistor which works during warm heating or after the rice cooked. It is connected parallel to the mechanical switch which open in a certain peak temperature trigger by the thermostat monitor. Since the rice cooker often overheat, I suspected that the problem is your heating coil.It maybe shorted causing more current. If that so, changing the thermal fuse will not fix the problem. It is not practical to fix it if the problem is the heating coil.

The heating element in a rice cooker is typically on for the full duration of the cooking cycle.

The cycle begins when the user pushes a lever, ka-chunk. (Indicator light turns on. So does the heating element.)

During most of the cooking cycle, there is boiling water in the cooker, so the temperature remains at exactly the boiling point of water.

The cycle ends when the temperature of the rice cooker reaches a preset temperature, set to slightly above the boiling point of water (around 100 C). Also note, reaching this temperature signifies all the liquid water has boiled away, or moved into the rice.

Some thoughtful contributor to Wikipedia was kind enough to draw for you a graph of this change of temperature, and water level, as a function of time, here:
https://en.wikipedia.org/wiki/File:Electric_Rice_C...

part of the article on rice cookers, here:
https://en.wikipedia.org/wiki/Rice_cooker

I know this answer does not address your specific concerns; i.e. What's wrong with my rice cooker? What does this rectangular thing do? Etc. However, I am hopeful my explanation will bring you closer to understanding.

The only temperature regulation in a rice cooker is due to the fact that it contains boiling water, and that keeps the cooker at a temperature equal to the boiling point of water.

The following Youtube video demonstrates this principle. Water in a paper cup can be boiled over an open flame. Without water in the cup, the paper cup quickly overheats, and catches on fire. Here:

The trick to successful rice cooker operation is a switch that can shut off the heating element at a temperature just slightly above the boiling point of water; i.e just before things start burning.

;-)

poiihy (author)  Jack A Lopez2 years ago

Thank you but I already know how a rice cooker works.

After some research, a few articles about how to fix a rice cooker say that the rectangular thing is indeed a resistor, and it should be 20Ω.

Why does the heating element use a resistor?

Speaking generally, a heating element is a resistor. I mean what do you get if you divide the mains voltage squared by 20 ohms? E.g. 110*110/20 = 605 W.

Is that enough power to cook rice with? I dunno. Maybe the rectangular thing is the heating element.

poiihy (author)  Jack A Lopez2 years ago

Yes I know, but that resistor is definitely NOT the heating element. the heating element is the solid circular assembly, and inside it has the actual resistive core going through it.

But the rectangular resistor is wired in series with the heating element resistor, why is that? Makes no sense to me.

As he said the heating element has resistance too it. A resistor is needed in series with the element to get the total resistance to a certain level so the unit only draws a certain amount of current. Much in the same way you use a resistor to limit current to an LED.

If this is a 500W cooker on 120VAC then doing the math the resistance of the heating element will be around 28 Ohms. Give or take a little with the resistance from the rest of the wiring and electronics. So the element would be around 8 Ohms with the 20 Ohm power resistor in series.

As for rusting. These heating elements are using in everything from cookers to hot water heaters. While the thing shouldn't be submersed in water it's not going to kill it. Unless it was still wet on the inside and then turned on. Then you could create a short that kills things. But the problem could be anything from a bad thermal cut off, to a bad heating element, or the resistor being burnt out.

OK. That means I don't know what the resistor thingy is. It probably has a purpose. I mean it would be comforting to me to believe it was put there for a reason, even if I don't yet understand what that reason is.

While your description is nice a picture would be better.

2 years of regular use on a Black and Decker product is pretty much it's full life span. These things are not made to last and not made to be serviceable. They use vary basic and small heating elements that wear out fairly fast. They can easily overheat blowing the thermal cutoff then after that they may not be seated in their plate as they should and won't effectively heat anything but overheat themselves.

poiihy (author)  mpilchfamily2 years ago

Do you think the heating element could've rusted inside? But wouldn't that increase its resistance therefore making less heat?

Or, it could be the fancy button mechanism that controls the whole thing. Maybe that has rusted causing its sensitivity to decrease and causing it to overheat.

poiihy (author)  mpilchfamily2 years ago

I don't have a picture because I already closed the rice cooker up. I don't want to open the Black & Decker rice cooker again because it is difficult to open; it uses security screws that I can only remove with pliers. I'll probably take a picture of the insides of the Aroma cooker later.; it is easy to open.

Cheap and nasty heating systems use the cheapest tech available.
In your case it is a bimetal plate or strip in that box.
At a fixed temp the metal bends and opens the contact, once cooled down enough it pops back, closes the connection and the heat cycle continues.
The standard one are like this Ebay link: http://www.ebay.com.au/itm/NC-Thermostat-Temperatu...

On yours should be a marking for the temp rating, like 90 degree, 95 or such.
Simply order a replacement with the same temp range and connect it instead of the faulty one.
These things only last for a certain number of switches until the contacts burn out or burn together, one causes no heat at all, the other is more common and causes constant heating until the temp fuse kicks in.
You can avoid the sudden death syndrome by adding a relay.
The thermo switch the only controls the relay, which in turn opens and closes the connection between power and heating element.


A better way would be to use and electronic controller with a thermistor.
For example something like this: http://www.ebay.com.au/itm/220-240V-Digital-LCD-Te...

You can get them as a kit too.
Basically you attach the tempearture probe as close as possible to the area you want to control.
For a rice cooker I would use the mount point of the faulty temp switch.
Set the electronics to the right temp and all is good.
For this to work you bridge the connection of the faulty temp switch and leave it "constant on" - the controller switches the whole cooker on and off based on the temp reading from the sensor.
These things either use a small relay or electronic switch, both can be replaced if they fail for very little money.

poiihy (author)  Downunder35m2 years ago

The basic rice cookers don't have a thermal regulator thing like you mentioned. Other cookers do, but rice cookers just operate until they become hotter than boiling water, then they snap off and stay off until you push the lever back down to turn it back on.