oskars
Posts: 33
Joined: Thu Feb 05, 2015 7:23 am

Controlling a 2kW heat plate

Mon Mar 23, 2015 11:30 am

Hey!

I have a cheap heat plate which internal thermostat sucks and I wanna replace it with a Raspberry Pi!

I get how it is supposed to work and what parts I need:

* RPi B+
* Relay board
* Temperature sensors

Also:
* 30l pot
* 2kW heat plate

My heat plate has a thermo fuse that goes off at 214 degrees C.

Should the sensor who determines if the RPi-thermostat should be ON or OFF be placed into the water/pot or should it be placed in contact with the heating element of the heat plate? Or do I need a sensor both in the water, and inside the heat plate?

The most logical must be to have the sensor into the water… But is there no risk for the heat plate to fry before the water reaches higher temperatures?

Need to get some directions on where to go! :D

Thanks!

/ O

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Richard-TX
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Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 12:14 pm

if I were you I would first get rid of the mechanical relay and use a solid state relay. Normal relays will drive you nuts with the constant click-click-click.
Richard
Doing Unix since 1985.
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oskars
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Joined: Thu Feb 05, 2015 7:23 am

Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 12:19 pm

Okey, good advice! :D But I will go with the mechanical ones for now because I already have one.

cacti
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Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 12:33 pm

Theoretically water will never reach 214 degrees (boiling point is 212). I think you will have to connect the sensor to the element.

Also I would use an Arduino for This type of project.

oskars
Posts: 33
Joined: Thu Feb 05, 2015 7:23 am

Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 12:34 pm

Okey. Why not a RPi?

Mark_T
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Joined: Sat Dec 27, 2014 10:54 am

Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 1:01 pm

Water can easily reach > 100C - often happens in a microwave oven for example, then
suddenly the cup/bowl undergoes rapid boiling, throwing the hot water out. This is called
superheating. Its less likely to happen with ordinary stoves as the heat from below causes
dissolved air to bubble out from the pan surfaces which nucleate boiling as soon as the
water gets to 100C.

But more inportantly the hot plate will be a lot hotter than the water since without that
temperature difference it can't cause heat to flow into the water... You are unlikely
to boil any water before the fuse goes.

oskars
Posts: 33
Joined: Thu Feb 05, 2015 7:23 am

Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 1:17 pm

Yes, that is the problem. I can not get 25l water to an boil because the heat plates thermostat sucks… So I wanna replace it. I could just remove the thermostat and run without one… But I wanna minimize the risk of frying the heat plate!

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DougieLawson
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Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 1:43 pm

cacti wrote:Theoretically water will never reach 214 degrees (boiling point is 212). I think you will have to connect the sensor to the element.
Please stop using deprecated and obsolete temperature units. The OP clearly stated 214°C not 214°F (101.1°C).
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oskars
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Re: Controlling a 2kW heat plate

Mon Mar 23, 2015 1:51 pm

:)

These are the guts of my heat plate:
Image

It consists of a mechanical thermostat + 1pcs of thermo fuse (I think it is SF214E). The thermo fuse is the overheating protection.

At the time the thermostat acts so stupid that when the water is about to start boiling, it turns itself off. So frustrating!

That is the problem. :)

I wanna replace it, as I said, with a RPi and relayboard and temperature sensors. But I can not figure out the best way to do it.

cacti
Posts: 56
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Re: Controlling a 2kW heat plate

Wed Mar 25, 2015 7:06 pm

DougieLawson wrote:
cacti wrote:Theoretically water will never reach 214 degrees (boiling point is 212). I think you will have to connect the sensor to the element.
Please stop using deprecated and obsolete temperature units. The OP clearly stated 214°C not 214°F (101.1°C).
My bad... But degrees F is not deprecated in the US ;) .... Crazy but true :shock:

The point however remains that the sensor must be connected to the element and not the water.

I suggest using the Arduino since there are many libraries for measuring temperature and it also has a built in ADC.

cacti
Posts: 56
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Re: Controlling a 2kW heat plate

Wed Mar 25, 2015 7:21 pm

You will require a K-type probe and an amplifier, something like the MAX31855. There is a tutorial here: https://learn.adafruit.com/thermocouple?view=all

I have used a simular circuit to build a reflow over that switches multiple elements to achieve a near perfect temprerature profile.

Since the MAX31855 uses SPI a raspberry should work, but the Arduino has a library for the MAX31855.

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