I have given it a lot of thought, but some professional opinions will help a lot. I am building a coffee machine, one of the problems is to boil my water. I am converting a normal kettle to be able to switch of using electronics. I browsed a lot, trying to figure out what solution would be best.

I thought of water proofing a lm35, but logicly thinking, I don't know if it would be the best choice? Any opinions?​

 

How about attaching sensor to outside of kettel? A diode and comparator also makes a useable temp detector, another comparator can give over temp warning.

 

I've used the LM35 on a previous project. It's not a very GOOD temperature sensor, but it will get you in the ballpark.

The BEST choice depends on the over all system and what your requirements are. Is this going to be an analog-only feedback system, or are you planning on using some sort of MPU?

 

I've used the LM35 on a previous project. It's not a very GOOD temperature sensor, but it will get you in the ballpark.

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What do you mean "It's not a very GOOD temperature sensor.."? How good do you need?

 

I am building a coffee machine, one of the problems is to boil my water. I am converting a normal kettle to be able to switch off using electronics.

It's unclear whether you have control over the heater and where the power for your circuit will come from. My coffee machine has a built in heater to heat the water and another one to keep the carafe hot. There's no "kettle". My "normal kettle" has no power or electronics and sits passively on a burner on the stove.

An LM35+comparator thermostat circuit would be just fine for this purpose.

 

I'm sure an LM35 has all the accuracy you need for a kettle. But a thermistor would probably be good enough, and cost less. I'd guess that the most crucial issue is placing the sensor relative to the heating element and the water in the container. What if the user tries to run the device with no water, is it safe? And is it also safe and usable if it's filled absolutely full?

Unfortunately bimetal strips are cheap and can switch an A.C. load directly, whereas with electronics you need a processor and a triac (or something). It'll be hard to beat the conventional way of doing this.

 

Not to get defensive here, but an LM35 would never be used in real instrumentation where you need accurate and repeatable temperature sensing. Something like platinum resistance temperature sensors are far superior but they require more electronics.

That being said, if all you want to do is sense when you get around 100C, and LM35 is probably totally sufficient.

I also think the thermal cutout idea that JohnP mentions is a better and safer solution - probably cheaper, too.

You have not said what the rest of the system looks like. Will you use AC line voltage to run heaters, or will you use safer, low voltage heaters? Is it all analog, or will you monitor the temperature with an MPU?

Good luck.

 

No defense taken, but the terms accurate and repeatable are relative terms. An LM35 (or bimetal strip) would give plenty of both for the application at hand, which is not instrumentation. I agree that platinum RTD probes are the de facto standard in industrial instrumentation.

Until the OP can enlighten us on the questions I raised above, we've gone about as far as we can. We're like the blind men describing an elephant.

 

It is basically a cheap plastic kettle bought only for this project. I made a outlet pipe just above the element, meaning water will never go lower than the element. I want to monitor the heat, so that after the water has boiled, the MPU can initiate other processes.
Forgot to say, I am switching the kettle on and of using a relay- Might be unsafe?

I actually really want to use the bimetal strip, but is it possible to sense when the power has switched of for the kettle? Like an inverse relay?

 

Switching with a relay is actually safer than a triac, because there's complete isolation between the high voltage and low voltage sides. A solid-state relay solves this by controlling the triac via an opto-isolator.

The idea of a sensor for the heater current should work--you could wind a few turns of wire around a ferrite rod, or use a toroid with a slot cut in it, and put a Hall-effect sensor in there to measure magnetic field. A kettle draws a lot of current, so you should easily see a 60Hz output from the sensor when the heater is on.