Help identifying type of temperature sensor - Page 1
Forum section: electronics / beginners

author: kworm
author_rank: Newbie
timestamp: September 20, 2025, 03:07:59 pm
content: Hi,I have a lot of experience with programming and PLC equipment but am a complete beginner on actual electronics. I'm working on a cnc machine that has two temperature sensors that I would like to re-use because of the housing they are in...machine was missing the original control so couldn't see how/what they were connected to originally. They are encapsulated inside a threaded plug so I can't see any mfg part numbers. They have two wires - red and black. I have done the following tests:1) measure resistance between red and black 1 mega ohm; reverse multimeter leads 0L/open2) multimeter in diode mode - open regardless of polarity3) measure mV - starts about 40mV and goes down to .5mV within seconds regardless of polarity4) supply constant current 10-20mA 10V limit - supply goes to 10V with 0A drawn; voltage across leads is 10VI'm not able to see any changes in any of these readings based on heating/cooling the sensors. Anyone have an idea of what they are and how to use them?ThanksKevin

author: BennoG
author_rank: Frequent Contributor
timestamp: September 20, 2025, 06:44:59 pm
content: probably a DS18B20 in 2 wire mode (parasitic power mode)Benno

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2025, 08:01:30 pm
content: Depending on how sensitive the current measurement was, it could be a AD592 or similar sensor. These give 1 µA/K with resonable voltage applied.

author: kworm
author_rank: Newbie
timestamp: September 20, 2025, 10:45:59 pm
content: Hi Benno,Thanks for the reply. I should have mentioned the build date of this machine was 1997 and the design circa late 80s. From what I could find the DS18B20 was released in 2001 so based on date I don't think that would be it unless it were some type of previous generation.

author: kworm
author_rank: Newbie
timestamp: September 20, 2025, 10:56:48 pm
content: Hi Kleinstein,Thanks for the reply. So if I understand correctly that sensor would give .298 mA at 298K (about the temp in the room at the time). My power supply only show current draw to .01 A so that would explain me not being able to see any change. I have a fluke 87 multimeter so I'm guessing I should try to measure the current with it in-line with the power supply?

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 20, 2025, 11:56:44 pm
content: But in such a case it would be hard to get a PLC with input suitable for that type of output.

author: kworm
author_rank: Newbie
timestamp: September 21, 2025, 12:04:35 am
content: Yes, I may be better off replacing them with something new.

author: onesystem
author_rank: Regular Contributor
timestamp: September 21, 2025, 12:48:19 am
content: Have you tried supplying constant voltage and measuring fluctuations of current?

author: amyk
author_rank: Super Contributor
timestamp: September 21, 2025, 09:02:28 am
content: Something like this?https://www.sheret.com/account/catalog/product/2076275Likely to be a 1M (at room temperature) thermistor.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2025, 09:12:53 am
content: If the sensor is a semiconductor type one may have to be careful with an external voltage. Some may not like more than some 5 V.Measurung the current with some +-5 V applied would make absolute sense.A high resistance thermistor is a possibility. Another possibilty could be just a thermal switch - so open when below a set temperature, like 50 C. A relatively high resistance NTC could work similar - essentially open when cold and able to read the temperature when relatively hot.

author: kosine
author_rank: Regular Contributor
timestamp: September 21, 2025, 09:50:46 am
content: A lot of industrial equipment from the 80s used screw-in thermocouples, usually J or K type. Often read by an AD594 or 595, which provide 10mV per degree C output for an ADC.

author: amyk
author_rank: Super Contributor
timestamp: September 21, 2025, 07:50:30 pm
content: A thermocouple would measure as a very low resistance.

author: kosine
author_rank: Regular Contributor
timestamp: September 22, 2025, 10:12:25 am
content: That's assuming they're not broken. Thermocouples leads are often solid core and don't like getting bent too much. If they're nearly 30 years old they could well be faulty by now.

author: mikerj
author_rank: Super Contributor
timestamp: September 22, 2025, 02:40:26 pm
content: Are you sure they are not over-temperature switches rather than an analog sensor?

author: kworm
author_rank: Newbie
timestamp: September 22, 2025, 04:12:23 pm
content: Hi,I'm fairly sure they are sensors and not switches because the manual says they are used to turn on the chiller when there is a 1 degree difference between them. I will play around with them a little more but I will probably end up just getting new ones and adapting to the threads.Thanks for all the help.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 22, 2025, 04:14:58 pm
content: Quote from: kworm on September 22, 2025, 04:12:23 pmAre you replacing the controller? If so then I think new sensor is best if one can be mounted fairly easily.

author: kworm
author_rank: Newbie
timestamp: September 22, 2025, 04:31:29 pm
content: Yes, I'm replacing the controller with Beckhoff components.I did some more testing and it does appear to be a 1M thermistor. It starts at 1.051M and goes down as I cool it. Without getting additional cards for the PLC I might still be better off getting new sensors since I think I would have to make some type of circuit to use this with 0-10V analog and since I don't know the model I would have to figure out the curve or at least what 1 degree difference was.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 22, 2025, 04:40:42 pm
content: Yes I think it's best just to replace it with a thermocouple or RTD and get a Beckhoff input module for that.