Temperature-control with arduino / MAX31855 / NXP KTY84

Question

Hello Community, I'm a software engineer so I don't have much experience with HW. My goal is to create a small temperature-control with an arduino and I already found some parts that might work together, but I'm not sure about that and it would be great if someone more experienced could help me out. Basically I want to measure temperatures up to about 200-250 °C that are created by a e-cigarette-heating element. If the temperature exceeds a certain limit, I want to turn off the heating element. If the temperature drops below a threshold I want to turn the heating element back on. The control is done by an arduino UNO. Because the heating element needs a lot of power (well not THAT much, but propably more than the arduino can handle), I would use a MOSFET controlled by the arduino to switch it on and off. The measurement could be done using a MAX31855 (Type K) connected via SPI to the arduino. As a thermocoupler I would use a NXP KTY84 which has a tempeture-range from -40°C to +300°C which should be good. So here are the questions: - Will a MAX31855 work with the NXP KTY84? (I think so, because it has a K in its label. ) - What kind of MOSFET do you recommend? (I have no idea, but is needs to be activated with the 5V of the Arduino. Here are some links to datasheets. They are also attached to this topic: MAX31855: https://www.adafruit.com/datasheets/MAX31855.pdf NXP KTY84: http://www.produktinfo.conrad.com/datenblaetter/175000-199999/183458-da-01-en-Temperatursensor_KTY84.pdf Thanks in advance for your help! Marc 
 Attachments: 183458-da-01-en-Temperatursensor_KTY84.pdf MAX31855.pdf

shabaz · Accepted Answer

Hi Marc,

The NXP device is not a thermocouple, according to the datasheet it's resistance varies with temperature. A thermocouple generates a voltage with temperature. So the MAX.. device is not compatible. The resistance of the NXP device could be measured using a constant current source and an ADC. I've not done the math to know if the ADC on an Arduino is sufficient for your needs, you'd need to go through the NXP data sheet and the data sheet for your ADC. You probably need to enlist the help of a hardware engineer at this stage though, I don't think a prototype for such a solution can be constructed just from existing Arduino shields.

Maybe a bi-metallic strip is an alternative idea? I know nothing about them though.

Robert Peter Oakes · Answer

I have recently posted a few YouTube videos on how to drive MOSFETS and higher power loads, also how to read from none micro-controller voltage levels, you may find it useful www.youtube.com/user/thebreadboardca regards Peter

jw0752 · Answer

Hi Marc - Sorry I didn't look closer at the data sheet. You are probably right. As far as the resistors to use with the NXP as long as you are using 5 volts for the voltage feed to your resistive divider you will be fine. All you will have to do is map the voltage difference at your analog input and constrain it. This will give you 1023 increments of temperature between low and high that you can use in your program to approximate the temperature of the sensor. If you are going to use a feed voltage greater than 5 volts you will have to make sure that your resistors in the divider are appropriate to keep the analog input to less than 5 volts max. I have done soething similiar to this using the TMP 36 sensors but they will not read as high in temperature as you want to go. Don't feel bad about thinking the NXP was a thermocouple. I have a box of parts that I have accumulated from similiar acquisitions. It just proves the addage that one can't learn without making mistakes. This is certainly true for me at least. Good luck on your project. John