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Electronics => Beginners => Topic started by: Cliff Matthews on March 04, 2021, 07:40:41 pm

Title: What's a cheap reliable way to sense 80C to 190C ?
Post by: Cliff Matthews on March 04, 2021, 07:40:41 pm
Replacing a missing mechanical thermostat on an old heat press. I'll use a comparator to control the existing SSR and 1500 watt heater for regulating temps from 80 to 190C. I've tested various 10k ohm NTC's taken off heatsinks in ATX PSU's and resistance always becomes erratic past ~150C. I was thinking about crimping together some glass diode's next. I have lot's of old parts but it seems few idea's.
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Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: jesuscf on March 04, 2021, 07:55:14 pm
Old good Thermocouple wire?  You just need a difference amplifier to convert to voltage.  The op-amp must have very small input offset; I've had good results with the OP-07.  If you want to go fancy, add cold junction compensation.  Lots of information on the web.  Here is the link to application note AN844 from Microchip that could be useful.

http://ww1.microchip.com/downloads/en/Appnotes/AN844-Simplified-Thermocouple-Analog-Solutions-DS0000844B.pdf (http://ww1.microchip.com/downloads/en/Appnotes/AN844-Simplified-Thermocouple-Analog-Solutions-DS0000844B.pdf)


Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: Gyro on March 04, 2021, 08:10:39 pm
Yes, thermocouple or PT100 RTD, but thermocouple is cheaper.

You could maybe try the thermistors sold for 3D printer heads - I don't know what they top out at?
Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: floobydust on March 04, 2021, 08:10:54 pm
At 190C you are past the limit for sensor plastics, wiring included. There are NTC thermistors in a glass diode package, (https://www.littelfuse.com/~/media/electronics/datasheets/leaded_thermistors/littelfuse_leaded_thermistors_glass_encapsulated_thermistors_do_35_standard_datasheet.pdf.pdf) good to 300C but need linearization. An Arduino can do the job then.

For your time and money, why reinvent the thermostat. I would use a cheap REX-C100 kit with thermocouple, under $20 from eBay.

Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: Cliff Matthews on March 04, 2021, 08:38:04 pm
Quote from: floobydust on March 04, 2021, 08:10:54 pm
At 190C you are past the limit for sensor plastics, wiring included.

For your time and money, why reinvent the thermostat. I would use a cheap REX-C100 kit with thermocouple, under $20 from eBay.
Quite agree, thanks! Ordering:
https://www.ebay.ca/itm/REX-C100-Digital-PID-Thermostat-temperature-Controller-SSR-output-K-Thermocouple/164397486095 (https://www.ebay.ca/itm/REX-C100-Digital-PID-Thermostat-temperature-Controller-SSR-output-K-Thermocouple/164397486095)
Looks easy enough.
https://www.youtube.com/watch?v=yBQzHq-sxYw (https://www.youtube.com/watch?v=yBQzHq-sxYw)
Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: jpanhalt on March 04, 2021, 09:27:25 pm
The MAX31855 (analog) and MAX31856 (digital) are single chip TC amplifiers.  You can choose the TC you want.  SparkFun, Mikroelectronics, and Adafruit (maybe others) offer breakout boards with those chips.  If you want analog, then of course, you would be using some sort of comparator.

I have been using the MAX31856 for a few years.  Works great.
Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: jmelson on March 05, 2021, 02:49:21 am
You can get thermocouple controllers on eBay for about $50, some even come with the thermocouple.
I have used a couple of these.

Jon
Title: Re: What's a cheap reliable way to sense 80C to 190C ?
Post by: perieanuo on March 05, 2021, 08:34:31 am
buy encapsulated thermister with M3/4/5/6/8 (whatever you like and fits well) screw head, that's one pro approach, or some encapsulated thermocouple with any attachment type you like
https://www.amazon.fr/DollaTek-Temp%C3%A9rature-3x10x1000mm-Thermocouple-Imprimante/dp/B08HQ5DX4R/ref=sr_1_11?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-11 (https://www.amazon.fr/DollaTek-Temp%C3%A9rature-3x10x1000mm-Thermocouple-Imprimante/dp/B08HQ5DX4R/ref=sr_1_11?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-11)
https://www.amazon.fr/TOPINCN-Thermocouple-Chauffage-Remplacement-Accessoires/dp/B07QYN6DQX/ref=sr_1_28?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-28 (https://www.amazon.fr/TOPINCN-Thermocouple-Chauffage-Remplacement-Accessoires/dp/B07QYN6DQX/ref=sr_1_28?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-28)
https://www.amazon.fr/Max6675-thermocouple-Capteur-temp%C3%A9rature-Raspberry/dp/B06XCFKL2Q/ref=sr_1_37?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-37 (https://www.amazon.fr/Max6675-thermocouple-Capteur-temp%C3%A9rature-Raspberry/dp/B06XCFKL2Q/ref=sr_1_37?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=thermocouple&qid=1614933009&sr=8-37)

the links are for making example, buy from farnell :) or some pro seller
I prefer the screw-headed ones, just put some threaded hole in your plate and it's rock solid
for air temp sensing, buy simple thermocouple and leave it in the controlled air flow