Modding rice cooker to increase "keep warm" temperature - bad idea?

[unturned3]

My rice cooker's "keep warm" function holds the temperature at roughly 50˚C, and I'd like to increase it to around 60˚C.

After a bit of searching, it seems like the "keep warm" function is implemented by placing a thermistor (with positive temperature coefficient) in series with the heating element.

The thermistor is a "coil" of resistive wire wrapped around mica sheets. I inserted a length of copper wire underneath a section of the coil, essentially shorting it out, decreasing the effective length (and thus the resistance) of the wire.

The cooker has been running continuously for the past hour or so, holding the temperature at about 64˚C. I did notice that the "coil" in the thermistor is glowing red hot though. Is this a bad idea? Are such wire-wound thermistors meant to glow in normal operation?

[moffy]

To me it looks like the PTC is resistance wire, like in a toaster, whose resistance increases because it gets so hot, glowing like a filament light bulb. If you have shorted out a section of the resistor then the current through the remaining section will be higher than previously quite possibly leading to premature failure. Not recommended, it could also lead to a fire within the device.

[Whales]

The whole cooker unit is designed to run hotter than that in the cooking (non-keep-warm) phase, so presumably your concerns should be localised to near that modified part.  It looks like it's made of mica (like an electric toaster element) but beyond that I don't know.  How brightly did it glow before the mod?  Is it possible to recreate that? 

[amyk]

In the case of incandescent lights, lifetime is inversely proportional to the sixteenth power of voltage. I'm not sure how much that applies to this, but I'd expect it to burn out sooner.

[thm_w]

I'm confused, how did you determine there is a PTC? Are you showing it in the photo? To me it just looks like a nichrome heating wire as moffy says.

The whole cooker unit is designed to run hotter than that in the cooking (non-keep-warm) phase, so presumably your concerns should be localised to near that modified part.  It looks like it's made of mica (like an electric toaster element) but beyond that I don't know.  How brightly did it glow before the mod?  Is it possible to recreate that?

Is the "keep warm" heater the same as the cooking heater though? OP: I assume there are two separate heaters in this thing?
I agree I would go back to the unmodified state and take measurements there first.

If there are two heaters and this is only used for keep warm, the "correct" mod would be to add more heater filament in parallel with the existing heater. This way you know that the filament won't be overloaded. But even then.. I don't know if I would consider it if I can find another cooker for cheap. Apparently to be safe for extended time periods, keep warm should be at 60C, as you are aiming for.

[soldar]

I'm confused, how did you determine there is a PTC? Are you showing it in the photo? To me it just looks like a nichrome heating wire as moffy says.

I have no idea and no opinion on the OP's rice cooker but these days many heating devices use PTC heating elements so that they are sort of self-regulating. Bed warmers, hot glue aplicators, etc.

[zapta]

The answer is NO. 

[soldar]

The answer is NO. 

What's the question?

[unturned3]

The answer is NO. 

What's the question?

Well, I asked "is this a bad idea", so... 

[unturned3]

I'm confused, how did you determine there is a PTC? Are you showing it in the photo? To me it just looks like a nichrome heating wire as moffy says.

Is the "keep warm" heater the same as the cooking heater though? OP: I assume there are two separate heaters in this thing?

I googled how rice cookers work, and this is what came up  The explanation is that the PTC regulates the main heating coils of the cooker to run at a lower power (thus performing the keep warm function).

Which seems reasonable. The "PTC" is not in contact with the pot's metal base, so even if it gets hot it wouldn't heat up the food much.

Btw, the schematics I uploaded is not some random internet image; I drew it based on what I see in my cooker.

[unturned3]

The whole cooker unit is designed to run hotter than that in the cooking (non-keep-warm) phase, so presumably your concerns should be localised to near that modified part.  It looks like it's made of mica (like an electric toaster element) but beyond that I don't know.  How brightly did it glow before the mod?  Is it possible to recreate that?

Yes, the part is made out of mica. I don't know if the coil glowed before the mod, but I took it apart and saw scortch marks that weren't present before, so I guess it was operating somewhat cooler previously.

Guess it's not a good idea after all.

[amyk]

Mica is used because it isn't flammable and can withstand high temperatures.
It looks like it is attached to the base of the pot, and can heat by conduction.
I would be worried about the coil burning out more than anything else.

[NiHaoMike]

Replace it with a light dimmer circuit and adjust it to the desired temperature. If you want more precise control, use a PID controller.

[Doctorandus_P]

I inserted a length of copper wire underneath a section of the coil, essentially shorting it out, decreasing the effective length (and thus the resistance) of the wire.

Yes, this is a very bad Idea.
Those things are highly cost optimized, and they use the minimum amount of material for that 40W "keep warm" heating element. If the designers thought they could have that 40W with a shorter and thinner wire they would have done that. If you short a part of the wire, then the remaining wire gets more current, and thus a much higher Power to area ratio and it turns much hotter.

If you want a higher "keep warm" temperature, then either add some insulation (throw a towel over it?) or replace the heater element with a slightly higher powered one (60W is probably fine. It already has a 900W heating element to do the cooking).

[coppice]

Interestingly, in even the most sophisticated computer controlled rice cookers I have never seen a setting to adjust the keep warm temperature, even though they have full control over the heater and measure the temperature of the cooking pot accurately. It would be a trivial addition to the software. Are there issues with keeping the rice warmer for hours, beyond the potential for eventually drying it out?

[Doctorandus_P]

Interestingly, in even the most sophisticated computer controlled rice cookers I have never seen a setting to adjust the keep warm temperature, even though they have full control over the heater and measure the temperature of the cooking pot accurately. It would be a trivial addition to the software. Are there issues with keeping the rice warmer for hours, beyond the potential for eventually drying it out?

Yes, anything below a certain temperature (I think 70 degrees centigrade) can promote bacterial growth. Especially in a wet and nutrient rich environment.

[ledtester]

The two mods I want to make to my rice cooker are:

1. A signal (audible tone/ indicator light / cell phone alert ...) that goes off a few minutes after the cooker transitions from cook to keep warm mode.

2. Completely cutting the power if the cooker stays in the keep warm mode for more than, say, 30 minutes.

#1 is because I've found that my cooker needs a little extra time (even at the keep warm temp) to make sure that all of the rice is throughly cooked.

#2 is because I've had a couple cookers die due being in the keep warm mode for too long (the thermal safety fuse blows) and also for peace of mind since I often multitask while preparing dinner.

These mods could actually be implemented outside the cooker by monitoring power usage to determine what mode the cooker is in.

[thm_w]

So this thing is not a temperature switch then? Is it just a cutoff fuse.

It looks like a typical thermostat switch.

[ledtester]

So this thing is not a temperature switch then? Is it just a cutoff fuse.

It looks like a typical thermostat switch.

That certainly looks like a temperature limit switch.

In the attached pic I've circled what appears to be the (single use) thermal safety fuse.

[IanB]

Yes, anything below a certain temperature (I think 70 degrees centigrade) can promote bacterial growth. Especially in a wet and nutrient rich environment.

While this is true in general (though the safe temperature is more like 60 C), a lower temperature is not necessarily a problem for a rice cooker. While the rice is cooking it is pretty effectively sterilized by the boiling water and steam, and then as long as the lid stays on it is protected against contamination from the surroundings. So as long as the rice is eaten within a reasonable time (for example within an hour or two) then everything should be safe. But you wouldn't want to keep it warm for 24 hours though.

[coppice]

Yes, anything below a certain temperature (I think 70 degrees centigrade) can promote bacterial growth. Especially in a wet and nutrient rich environment.

While this is true in general (though the safe temperature is more like 60 C), a lower temperature is not necessarily a problem for a rice cooker. While the rice is cooking it is pretty effectively sterilized by the boiling water and steam, and then as long as the lid stays on it is protected against contamination from the surroundings. So as long as the rice is eaten within a reasonable time (for example within an hour or two) then everything should be safe. But you wouldn't want to keep it warm for 24 hours though.

If you forget there's rice in a rice cooker, and leave it on "keep warm" for 24 hours, the rice is still fine. I think the keep warm temperature is chosen to be above the biologically safe line, and below the point where the rice continues to significantly cook and turn into congee.