What type of thermocouple is used in JBC irons ? (Maybe type N) - Page 1
Forum section: electronics / projects

author: whatsthegeek
author_rank: Contributor
timestamp: December 21, 2017, 04:43:11 pm
content: Hi, I'm trying to design my own controller for a JCB series 245 iron (I know there's a project on dangerous prototypes that does it but I's quit complex and I won't learn anything by just using it ^^). I want to use an all-in-one chip but I need to know what type of thermocouple is used :/Some people says it's a type K and others say it's not, so it's quit confusing...Maybe if one of you guys have one and have a bit of free time, you could heat up the tip to 200° and measure the voltage drop between the metal body and the last contact,It could tell me exactly what type of thermocouple is used.Thanks in advance !

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 04:54:36 pm
content: If you have a cartridge, you can easily measure it yourself. Type K is a most common thermocouple type and it is well suitable for such task. Say put it into boiling water compare measured voltage with a chart. Or if you have multimeter with thermocouple function, you could connect it and see if measured value is matching.

author: whatsthegeek
author_rank: Contributor
timestamp: December 21, 2017, 05:28:27 pm
content: I don't have the cartridge yet :/ I want to be sure I can easily make a circuit to control it before buying it, It's super expensive...That's why I'm asking here, otherwise, I would be already done

author: AndyC_772
author_rank: Super Contributor
timestamp: December 21, 2017, 05:53:21 pm
content: OK, I've tried a very simple and not very scientific experiment:- fit a cartridge into the handpiece and set to 200C- apply a blob of solder, which does just about melt, to aid thermal conductivity- place the tip of a second cartridge (C245-907, for what it's worth) against the tip of the first cartridge- connect DVM probes between the body of the second cartridge and the contact right at the end- wait a couple of minutes and watch the voltage increaseFinal result was about 4.14mV. It takes much longer to reach anything like a steady state than I would have expected.

author: whatsthegeek
author_rank: Contributor
timestamp: December 21, 2017, 06:01:17 pm
content: Ok, thank you sooo much ! Comparing it to the chart, the closest thermocouple type is a type NOn a type N, 4.14mV corresponds to 144°C. Since the conctack between the cartridges wasn't perfect, it looks like it matches pretty well !I don't know why they would use a type N instead of a type K though ? Maybe because it has a longer life span ?

author: AndyC_772
author_rank: Super Contributor
timestamp: December 21, 2017, 06:18:33 pm
content: Another data point for you: at 350C, I get 7.12mV. Does that correlate?

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 06:27:06 pm
content: This is a completely lousy measurement, you cannot just touch the tips and expect another side to heat up to the right temperature when there is basically a huge heatsink on the other end.

author: pigrew
author_rank: Frequent Contributor
timestamp: December 21, 2017, 06:32:19 pm
content: Quote from: whatsthegeek on December 21, 2017, 06:01:17 pmSeems that type N has better stability.The two have very similar temperature curves, so it'd be hard to differentiate them. Note that without the "cold-junction" temperature, you can't tell which thermocouple you have (from a single data-point). You either need two known temperature-voltage pairs, or a (T_thermocouple,T_coldjunction,voltage) tuple.

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 06:34:25 pm
content: Quote from: whatsthegeek on December 21, 2017, 06:01:17 pmYou made assumption based on incorrectly done measurement.

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 06:35:48 pm
content: Quote from: pigrew on December 21, 2017, 06:32:19 pmYou can estimate temperature on the cold end and get a precise enough result. There is quiet high difference between K and N to not be able distinguishing them.

author: whatsthegeek
author_rank: Contributor
timestamp: December 21, 2017, 06:43:49 pm
content: QuoteI get 236.03, very far, but still the closes of all.QuoteThe only other way would be to power the connect a wire inside the handler and measure the temp while on which is hard to do and dangerous.QuoteHow could we get the could junction temperature ? (I don't know a lot about thermocouple)QuoteStill better than nothing ^^Thanks again

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 06:48:04 pm
content: Quote from: whatsthegeek on December 21, 2017, 06:43:49 pmNo it's worse.EDIT: and I can say why. There are a lot of people who screwed up calibration of their soldering iron thinking they are adjusting it to display correct temperature. Put regular thermocouple on the tip and think they make correct measurement while actually it may be like 50oC off best case. Error is much worse in this case and you already made an assumption which, most likely, is wrong.QuoteAs I said, put the end of cartridge into boiling water.

author: whatsthegeek
author_rank: Contributor
timestamp: December 21, 2017, 06:51:36 pm
content: QuoteMaybe AndyC_772 could try this ? But, isn't this bad for the tip ?And also, at 100°C I don't think the difference will be easily visible on a common multimeter :/

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 06:58:07 pm
content: Quote from: whatsthegeek on December 21, 2017, 06:51:36 pmIf we assume cold end is at around 25oC temperature, there will be around 3mV vs 2mV. Not so tiny difference I can say.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: December 21, 2017, 10:57:29 pm
content: I have made many measurements on jbc cartridges in the past for my diy jbc station. righ now I am not near my lab, but I can take a look after christmas. I for myself came to the conclusion that it is pretty much an N type thermocouple, not an K type.

author: kubeek
author_rank: Contributor
timestamp: December 21, 2017, 11:53:47 pm
content: I guess you could also do it the other way, and put a know voltage into the soldering base and see what temperature it thinkgs the tip is at. Should be a bit more precise and provide a definitive result.

author: wraper
author_rank: Unknown
timestamp: December 22, 2017, 01:20:54 am
content: Quote from: kubeek on December 21, 2017, 11:53:47 pmI don't think it will be that easy to do. Most likely there is also a current sensing, so it will just show an error if no load present. And if there is a load it probably could show an error because it does not respond to heating.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 22, 2017, 08:12:01 am
content: Quote from: wraper on December 21, 2017, 06:27:06 pmThe thermocouple junction has to be right at the tip, by design, and the two tips are thermally connected by liquid solder which is an excellent conductor of heat. The two junctions will be much closer in temperature than you're assuming.What's not known is the temperature of the junction between the cartridge body and the meter probe; it's different between the two leads, and does change over time as the cartridge warms up, which means the measurement shown on the meter isn't a true representation of the thermal EMF produced by the thermocouple.I wonder how JBC deals with that issue.

author: alanb
author_rank: Frequent Contributor
timestamp: December 22, 2017, 12:37:41 pm
content: Do you need to know whether it's N or K type at this stage? You can get an IC that caters for both types of thermocouple (MAX31855 for example). I assume the different type of thermocouple is selected in the firmware.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 22, 2017, 01:10:59 pm
content: The thermocouple can be considered as a linear device for soldering temperatures, so all you need to determine is how many microvolts per degree it has.Dip the unpowered soldering tip into water with ice (0°C), then into boiling water (100°C), and measure the generated voltage.Any fine tuning corrections for the uV/°C constant of the thermocouple will be made later, at calibration time.When calculating the temperature at soldering time, don't forget to add the cold junction compensation offset (the room temperature).

author: wraper
author_rank: Unknown
timestamp: December 22, 2017, 01:15:22 pm
content: Quote from: alanb on December 22, 2017, 12:37:41 pmMAX31855 has separate version for each thermocouple type.

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2017, 01:22:32 pm
content: Cold junction compensation can be one of a few methods, using either a diode or thermistor to read ambient at the controller and adding it to the measured voltage, or the more common thing done in these and simply assuming ambient is 25C and having a fixed offset. The fixed offset is quite common, as it does not worry too much if the ambient varies 10C either way, the variation in tip temperature in any case varies more just from orientation, or the amount of solder on the tip, along with it being in contact with the objects being soldered.If you want true cold junction compensation you need either a simulated cold junction using a proper thermocouple amplifier and conditioner, or just add the scaled offset. Otherwise you need a cold junction with the melting ice bath and all to have the cold junction referenced to.

author: wraper
author_rank: Unknown
timestamp: December 22, 2017, 01:24:15 pm
content: Quote from: AndyC_772 on December 22, 2017, 08:12:01 amFirst of all there is some distance. Don't expect it being closer than 5-10mm in each of cartridges. Secondly, on the other side of it, there is much bulkier part which acts as a heatsink. And your measurements do not even remotely match nor N, nor K type. You apparently did not take into account what I wrote about big measurement error even when using usual thermocouple with exposed junction which does not have such thermal mass and a heatsink on the other end.EDIT: I just made some tests with thermocouple as on the pic below. When just inserting junction into a solder blob (like most people do), measurement is around 30oC lower than actual tip temperature. Only if inserting it into solder blob together with around 4-5 mm of wire, measurement is correct, because those wires no longer can act as a heatsink. In your case, situation is an order of magnitude worse.

author: whatsthegeek
author_rank: Contributor
timestamp: December 22, 2017, 08:01:32 pm
content: Well, I think I can safely assume it's a type N. it's the only curve that is close to K, goes higher than 400°C and gives lower voltage for the same tamperature. Since I can't buy any MAX ic that handles type N on LCSC, I'll just convert the temperature result from a MAX6675 all-in-one type K ic with this code:if (t < 412){return in * 1.256;}else {return in + 126.65;}it's precise to +-2°C which, I think, is enoguh for a soldering iron

author: wraper
author_rank: Unknown
timestamp: December 22, 2017, 11:14:14 pm
content: Quote from: whatsthegeek on December 22, 2017, 08:01:32 pmThe only thing safe to assume is that measurement was done incorrectly. Yes, reading is way lower than soldering station set temperature, however how much lower, you can only guess. 200oC (170+30) at thermocouple junction, assuming given conditions, 350oC set temperature and around 30oC at cold end seems completely possible to me.QuoteAnd get cold junction compensation working incorrectly. Therefore reading dependent on ambient temperature.

author: tboy32
author_rank: Unknown
timestamp: December 24, 2017, 09:52:48 am
content: Just did my own experiment.Tip: C245-903Ice water: -0.600mVBoiling water: +0.970mVNotes: The boiling water experiment was tough. I think bubbles hitting the tip caused fluctuations. I tried keeping the tip+heating section under water but not touching the bottom of the pan. Steam condensed on the top and the alligator clips holding onto the contacts. This was tap water so I wouldn't be surprised if impurities caused increased conduction between terminals and thus a smaller output voltage than it should have been, but I have no idea how to estimate how much of a difference it was. I tried shutting off the heat source (eliminating bubbles) and immediately taking a measurement but it looks like (according tohttps://us.flukecal.com/Thermocouple-Temperature-Calculator) +0.970mV on an N type thermocouple with a 25C reference temp (guess) is only 60C. I did see measurements above 1mV (like +1.025mV IIRC) but was unable to reproduce it. Also: shorting clips together lead to a measurement of about -0.003mV.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 24, 2017, 12:02:42 pm
content: Quote from: tboy32 on December 24, 2017, 09:52:48 amThat would be about 15.7uV/°C, which is not a sensitivity for standard types of thermocouple in this table:http://www.analog.com/en/analog-dialogue/articles/measuring-temp-using-thermocouples.htmlMaybe the soldering tip does not have a standard type of thermocouple.Since almost any 2 different metals can be used as a thermocouple, they might probably used as a thermocouple whatever 2 different metals were there anyway for the mechanical build of the tip.

author: whatsthegeek
author_rank: Contributor
timestamp: December 24, 2017, 12:10:35 pm
content: Quote from: RoGeorge on December 24, 2017, 12:02:42 pmThat would be pretty annoying :/Quote from: tboy32 on December 24, 2017, 09:52:48 amThese are pretty weird results, I would have thought the voltage drop would have been higher at 100°C :/

author: Bgdn
author_rank: Contributor
timestamp: January 08, 2023, 11:30:06 am
content: Hello,i did today some measurements in a electric oven.Placed a type K termocouple connected to a multimeter and osciloscope channel 1Placed a jbc tip C245-931 connected to osciloscope channel 2.I tried to place the thermocouple as close as posible to the proximity of the jbc tip.I used the averaging function of the scope, because the raw signal is quite noisy from both thermocouples.Bellow are the measured values and the temperatures wish small aproximation maybe.type K JBC5.24 mV - 0.61 mV - 132 degrees5 mV - 0.54 mV - 128 degrees4.84 mV - 0.5 mV - 125 degrees4.6 mV - 0.42 mV - 120 degrees4 mV - 0.36 mV - 105 degrees3 mV - 0.26 - 80 degree2.04 - 0.2 - 50 degreeWhat do you think?

author: wraper
author_rank: Unknown
timestamp: January 08, 2023, 12:58:54 pm
content: Quote from: Bgdn on January 08, 2023, 11:30:06 amI think you have no idea how thermocouples work and placed a whole cartridge connected by usual copper wires into the oven. The problem is that thermocouple measures temperature difference across its length, not absolute temperature.

author: Bgdn
author_rank: Contributor
timestamp: January 08, 2023, 02:33:34 pm
content: The type K thermocouple wires which comes with my multimeter its not from copper, i think its something "silverish".I dont have a reference junction.Maybe just flood both into temperature controled oil? without exposing the wires to temperatures?

author: wraper
author_rank: Unknown
timestamp: January 08, 2023, 02:55:43 pm
content: Quote from: Bgdn on January 08, 2023, 02:33:34 pmThat's why you had a correct reading with thermocouple. In most cases their wires are just a continuation of thermocouple.QuoteWell, you have them, just don't understand it. Reference junction is where thermocouple is connected to normal wires. It's just that temperature where thermocouple ends must be known to calculate a correct result. On the picture it's in water at 0oC. Multimeters and thermometers usually have a resistive temperature sensor in them.QuoteYou can do that. Just avoid cartridge heating up as a whole. Terminals must remain cold or special thermocouple extension wire must be used.

author: Bgdn
author_rank: Contributor
timestamp: January 08, 2023, 09:56:35 pm
content: I googled a bit about cold junction compensation...Now i am wondering, since i connect the osciloscope probe directly to the C245 cartrige.Based on the picture above, the point of conection between the probe and the cartrige shall be at 0 degrees, right?But if its at room temperature? Shall i consider my reading to be the heat temperature + rom temperature?Its a bit puzzling, i see alot of discussions on the internet about these jbc tips.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2023, 10:38:54 pm
content: A termocouple is made out of two different metals, in this case two wires of different metals.It does not measure the absolute temperature, like a thermistor or a mercury thermometer.The two different wires are tied together at one end. At the other end you connect a voltmeter. The points where you connect the voltmeter is often called the "cold" junction, but in fact the cold junction can be at any temperature. Can be even at a higher temperature than the tip of the thermocouple.The voltmeter will indicate a voltage proportional with the temperaturedifferencebetween the so called "cold" junction and the tip where the two wires are twisted together.For a multimeter to measure the temperature with a termocouple, it needs to know the temperature of the "cold" junction (usually the cold junction is at room temperature). The very cheap DMMs will asume the room temperature is always 25*C or so, but most of the DMMs will have a thermistor inside, so the DMM can measure the room temperature. You can check this on a DMM with temperature by unplugging the thermocouple. When you unplug it, the DMM should indicate the cold junction temperature (in this case - probe unplugged - will indicate the room temperature).When you put the tip of the thermocouple inside the oven and the other end at the room temperature, the voltage on the oscilloscope will tell (Toven- Troom).The cold junction is considered the place where the two type of metals (the two wires of the thermocouple) are connected to a 3rd type of wires of the same type (here the oscilloscope probe is the 3rd type of metal).For example, if you have the oscilloscope probe connected to two copper wires (to extend the oscilloscope probe's lenght), then those two copper wires are connected to the thermocouple wires inside the oven, the "cold" junction will be also inside the oven, so you will measure very close to zero volts.Even more, if you let the cold junction inside the oven and put the tip of the thermocouple outside, you'll measure a negative voltage.For your K thermocouple, the cold junction is where you connected the oscilloscope (or a voltmeter). For the C245 cartridge, the cold junction is somewhere inside the cartridge, on the opposite side of the soldering tip. To measure correctly, you will need to keep inside the oven only the soldering tip, and the other end (where the cold junction is) must be kept outside of the oven. If you do so, the voltage will be proportional with the difference in temperature between the two ends of the thermocouple (Toven- Troom).To recap, if you have 2 wires of metals type A and B, the cold junction is where you connect them to a 3rd type of metal, C. Any two metals put in contact will form a thermocouple, weather you want it or not, just that usually the voltage is very small and hard to measure. Thermocouples like K type or J type have the wires made out of metals that gives higher voltage with temperature, so to be easier to measure.Even more, the effect is is reversible!If you put a difference of temperature you'll read a voltage. If you put a voltage you'll force a temperature difference in the thermocouple. That's how the Peltier elements are working.

author: Miti
author_rank: Super Contributor
timestamp: January 09, 2023, 02:14:26 am
content: Quote from: RoGeorge on January 08, 2023, 10:38:54 pmNope, you forgot to short the TC inputs.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 09, 2023, 07:35:45 am
content: Have you checked? Mine still sees 0V (and thus will display the room temperature) with the thermocouple unplugged, so no need to short the inputs.

author: Bgdn
author_rank: Contributor
timestamp: January 09, 2023, 09:51:14 am
content: Thanks George for the usefull explanation.So i wil try to do the following experiment, hopefully i will get usefull results.On a copper plate 1-0.5mm thick, i will try to solder on the tip of the C245 together with a k thermocouple, and try to externally heat only the copper plate and see what results i will get.What do you think?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 09, 2023, 12:15:18 pm
content: Don't solder the tip on a copper plate. Any force between the copper plate and the soldering tip might peel the tip and ruin your C245. If you want to keep the soldering tip and the thermocouple tip at the same temperature, cover them together in a small blob of Aluminium wrapping foil, or wrap them together with some copper wires and heat the wrapping with something else.Keep the other end at room temperature (for both the thermocouple and the C245), where you connect the oscilloscope. Make a table with all the measurements and post the results. Then, you can check the measurements against the expected voltage for K type and for C245.

author: Miti
author_rank: Super Contributor
timestamp: January 09, 2023, 02:52:27 pm
content: Quote from: RoGeorge on January 09, 2023, 07:35:45 amCheck this out.

author: Bgdn
author_rank: Contributor
timestamp: January 09, 2023, 07:38:19 pm
content: Hello,i gaved up on the copper plate experiment.But fixed the termocouple tip very tight to the c245 tip, and submerged both of them in aprox 5mm of melted solder ( in a small metalic copper box heated with hot air station from underneath).The reading was stable enough to do some readings on the scope connected to the c245 cartrige.What do you think?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 09, 2023, 08:51:05 pm
content: Very well!I think you should be precise with the questions, ask what you want to find out.Now, you can put those numbers in a spreadsheet and plot the voltage versus temperature. It is expected to show a tilted line, ideally a straight tilted line. Inherent measuring errors should still show the tilting trend, though the line will look crested instead of being straight.Then, you can compare the results with others, for example the results shown in this other page:https://www.eevblog.com/forum/beginners/jbc-t245c245-thermocouple-thermal-coefficient/

author: RoGeorge
author_rank: Super Contributor
timestamp: January 09, 2023, 09:51:47 pm
content: Quote from: Miti on January 09, 2023, 02:14:26 amQuote from: Miti on January 09, 2023, 02:52:27 pmGood advice to short the inputs before reading the room temperature with a DMM, thank you.For the cheap clone of Mastech MY64 the reading shows the same temperature, shorted or open.With an expensive Metrix MTX3283, the room temperature read is incorrect with open inputs.First pic is with open inputs instead of a thermocouple, second pic is with a short circuit (a piece of a paperclip for MY64, and the red wire in the back for mtx3283) instead of the thermocouple. The room temperature was about 26*C.

author: Bgdn
author_rank: Contributor
timestamp: January 10, 2023, 05:26:30 am
content: Quote from: RoGeorge on January 09, 2023, 08:51:05 pmi wonder why i missed that thread, there is a very usefull characterization of the c245 tip.After seeing this, it makes sense to consider this to be a very likely type k inside the c245 tip.But at least i have now all the info needed, i will proceed to design the analog front end for this jbc tip.Thanks everyone!

author: RoGeorge
author_rank: Super Contributor
timestamp: January 10, 2023, 07:14:35 am
content: If you just want a controller for that C245, it would be simpler and cheaper to order a clone soldering station, or to use an existing design.If you want to design your own controller, that would be a nice project and a lot to learn from it, go ahead. The main characteristic of a thermocouple is how many uV/*C it produces. From your last measurements, it doesn't look to be a type K, it looks more like type C.Image source:https://www.thermocoupleinfo.com/thermocouple-types.htm

author: Bgdn
author_rank: Contributor
timestamp: March 22, 2023, 01:10:37 am
content: Hello,today i connected a thermocouple to the circuit shown, and trying to read the values over the 24bit adc, running at 8192 samples.In paralel with the tip i have connected a keysight u1272a which can go down under 10uv measure scale.I have around 19-20 degree room temperature, and i measure across this C245 tip:- 24uV from ADC- 27uV measured by the multimeter.Tomorrow i will do another hot solder pot test, to see if the values match the last time i did this...Not sure if i need larger biasing resistorsLatter eddit... Since ad8221 needs under 1nA input bias current, i tryied biasing the tip with 1.5Mega resistors, the same result... surprisingly after letting the tip to sit for a while in the setup at room temperature... the voltage drops quite low.. arround 3uV on both instruments.I have also tried to remove the 1M5 resistor from GND, the value this time stays arround 50uV.Not sure if this behaves liniar at such low temperatures, because if touch the tip with my finger, immediately jumps to 100uv and stays there some time, then it drops back to ~60uV quite slow

author: Bgdn
author_rank: Contributor
timestamp: March 26, 2023, 08:04:48 am
content: Hello,sorry for the short delayi did the hot solder pot test, where both thermocouple & tip share the same melted solder "lake", heated externally.These are the values read on the c245 tipAlso i tried to apply power to the tip, and read the voltage across internal thermocouple ( 150hz pwm wave provided with 10-20% dutycycle), and the signal across the thermocouple is read during the "pwm off" period.I tryied not to heat to much the tip, because both type k tk & heater tip share a solidified solder ballI left some time to reach a constant temperature, then recorded the voltage acros jbc tk.These are the values read aswell.Somehow the thermal inertia seems to cause diferences between hot pot test & direct voltage aplied to the tip. But 17uV/degree is not so far away from 21uv/degreeWhat do you think?

author: wraper
author_rank: Unknown
timestamp: March 26, 2023, 08:13:59 am
content: QuoteThat your circuit is not suited for thermocouples. And has several issues beyond that. Not to say why do you use in-amp to begin with? PWM to tip temperature is even more meaningless exercise.

author: Bgdn
author_rank: Contributor
timestamp: March 26, 2023, 08:22:54 am
content: Quote from: wraper on March 26, 2023, 08:13:59 amwhy not? The GND from the measurement circuitry is isolated from the heater power circuit

author: wraper
author_rank: Unknown
timestamp: March 26, 2023, 08:38:30 am
content: Because you pass quite a bit of current through the thermocouple (R1, R6) which you certainly shouldn't do as it creates significant voltage drop across it. If thermocouple has say 10 Ohm resistance typical to thin longer ones, it's already 1.6mV of voltage on it. JBC thermocouple should have lower resistance but it will still have a significant impact. Also this circuit has horrible CMRR, it would be better with no C1 and C2 at all than like this, proper input filter circuit attached. There is no need for in-amp and you only overcomplicate the thing this way. No cold junction compensation, but let's assume it's okaish if you know ambient temperature.

author: wraper
author_rank: Unknown
timestamp: March 26, 2023, 08:44:59 am
content: R2, C12 also is not how you should do it. I won't look into in-amp specs but it certainly can cause problems with some in-amps or op-amps. Not to say it's a really piss-poor way how to do a low pass filter.

author: Bgdn
author_rank: Contributor
timestamp: March 26, 2023, 08:50:59 am
content: Quote from: wraper on March 26, 2023, 08:38:30 amFor the current passing trough the thermocouple, i am still wondering about that. I saw alot of this type of circuit using resistors on both legs for providing bias current to the in-ampRegarding usage of such circuit just to measure temperature, maybe its a bit overengineered, but had a bunch of them in my "monkey box", said to try to build something.Regarding coldjunction, this will be made with a temp sensor placed somewhere near the connector...You are right about C1 & C2 from my schematic, initially i started with 100nf, but had horrible recovery between the pulses, the voltage swing on the in-amp output was slow. This probably will suffer more modificationsI have changed them down to 10nf, but this was not in focus when i initially started the measurement... sorry from my side here.This is how the signal looks at the entrance & output of the in-ampThe measurements where performed with no tip handle. I tryied to measure with some crocodile wires directly on the tip

author: Bgdn
author_rank: Contributor
timestamp: March 26, 2023, 10:41:06 am
content: Just for my curiosity i removed both bias resistors.Connected the tip handler, the resistance for the wires in the handler is roughly 0.15 ohm/wire.I used a tip which has like a cup shape in front. Placed the external TC right in that cup space ( see photo)The multimeter shows 266 degrees ( room temperature ~23degrees)Reading on the jbc tc shows 5.85 mV, 5.85/266, it gives me a seedback of 21.9 uV/degree.Here i have a question, does the seedback coeficient when its callculated, it must include the cold junction temperature?

author: wraper
author_rank: Unknown
timestamp: March 26, 2023, 01:08:15 pm
content: You still need some voltage bias. Just not like this. Put high resistance voltage divider to one of the inputs in case of single voltage supply. Or single resistor to GND in case of dual supply. So input voltage is within acceptable range for in-amp. Also supply voltage is not shown on the circuit but it must be more than 3.3 V, 4.6V is minimum allowed.

author: Bgdn
author_rank: Contributor
timestamp: March 26, 2023, 07:27:20 pm
content: HelloI tried to modify the circuit, and provide 1.65V via the 1M5 resistors to the non-inverting input. Like in the schematic attached.No differences where observed, on the readout.For example i measure about 245degrees ( CJ 23deg included) - 5.0mV . The multimeter shows 239.5 degrees.260deg @ 5.3mV - multimeter shows 252 degI am beginning to be convinced that this is far far away from a type K, i tend to think its somewhere in the region of 19-22uV, its hard to characterize this due to other contact elements ( holder + connectorto the pcb.. etc ).The closest type would be C, but even that is 17uV/degre, and even so this yelds 17 *245 = 4.165mV or 17*260 = ~ 4.420.Al my measured values today are giving me arround 19-22uv region.Tried to measure 3 C245 tips, the values are mostly in the same region.

author: wraper
author_rank: Unknown
timestamp: March 26, 2023, 08:22:07 pm
content: Your circuit is still wrong. C2 capacitance should be at least 10 times larger than C3, C4. Not the other way around. You did not say from what voltage in-amp is powered.QuoteThis is pretty much spot on FWIF. Different thermocouple type would be way off. Not to say it's hard to measure tip temperature by external thermocouple, if coupling in not perfect, you will get somewhat lower measurement. Also without actual cold junction compensation, you can easily get some temperature measurement offset.

author: Bgdn
author_rank: Contributor
timestamp: March 27, 2023, 06:16:27 am
content: the in-amp is powered from -2V and +3.3v.the 1.25v reference is supplied from a MAX6070AAUT12.I will try today to get a 2nd type k tc, and see how this ciruit behaves with a known seedbeck coefficient.

author: Bgdn
author_rank: Contributor
timestamp: March 27, 2023, 06:05:45 pm
content: So i got my second type k thermocouple.I have tied both thermocouples closer one to each other.Placed them in the kitchen oven ( thanks god my wife wasn't home) )Since my multimeter shows a 23 degree ambiant, i used the same temperature as compensation for my hardware.Waited a bit for the temperature to settle in the oven.On the multimeter the temperature was stable arround 138-139 degree, while on my circuit it was 137-138(compensated with 23degrees), and raw reading was 115 degrees and 4.61 milivolts.4.61/115 = 0.04008 uV/degree like expected.Ok maybe, the pcb where the connections are made with the tc there was maybe +2/3 degrees due to self heating of the circuitryBut both thermocouples follow each other at different speed, but when the temperature is quite stable, both temperature readings stay close to each other +/- 1-2degree.Seeing this i am quite confident that the circuitry itself works well.So i really wonder how JBC deals with the self heating of the contacts inside the holder, maybe the even use a lookup table depending of the temperature range?

author: wraper
author_rank: Unknown
timestamp: March 27, 2023, 06:11:10 pm
content: Quote from: Bgdn on March 27, 2023, 06:05:45 pmNormally special wires are used. Either from the same materials as thermocouple or something else that resembles particular type of thermocouple within smaller temperature range.

author: Bgdn
author_rank: Contributor
timestamp: March 27, 2023, 06:29:54 pm
content: An ideea comes in my mind right now, just for the sake of trying to compensate the heating at the contacts, i would try to improvise a "pcb type holder",and place there like a small smd NTC, or another standard tc, to take into account that temperature as wellMaybe that way there is a chance to find a more accurate sebeck coeficient.What do you think?

author: Bgdn
author_rank: Contributor
timestamp: March 27, 2023, 07:59:48 pm
content: I took a look over a schematic from the jbc CD-2BC , the measurement circuit.I see they can switch between all 3 pins to measure.TC & COM - i think this is what most of us, use for measuring the temperatureWould be interesting to find out what info you can get by measuring e TC & Heat, or Heat & COM.Not sure how SW1 & SW2 control signals are managed from the MCU, all at once or independent

author: thm_w
author_rank: Super Contributor
timestamp: March 28, 2023, 01:14:34 am
content: CD-2BC only supports T245 right?Some of the other handle types have TC/Heat swapped, so it might make sense to have the ability to measure both ways. This could be a generic circuit for most of their stations.Then there is also the detection for sleep when in the stand.

author: Bgdn
author_rank: Contributor
timestamp: March 28, 2023, 04:07:06 am
content: In the mean time searching over the internet, i found here some informationhttps://www.eevblog.com/forum/testgear/jbc-soldering-station-cd-2bc-complete-schematic-analysis/.Concerning the handle, mine is quite old, i have it like for 10-12 years, its a black type handle. It has some writing barely visible on it "2245 / 4700 advanced jbc". What i can tell you.. its a original piece.Not sure if the newer variants with ( greenish type color on the handle have the same wiring as mine)Concerning the wires function/order there are alot of variants on the internet.I tryied to stick to my own measurements.Therefore i took my 4 wire multimeter and did the following measurementsC1 (outer sleeve) - C2 (middle) measured 2.20 ohmC2-C3 measured 2.227 miliohmC1-C3 measured 50 miliOhmGiven these values, C1 i consider to be common for TC & heater, C2 is the heater, and C3 is the TC end pin.Afterward is have placed the c245 tip in the "black old holder" and measured resistance between the wiresJust to be sure which wire is the connected to the tip's outer sleeve i started measuring each wire against the tip's C1.In my case, red wire seems to be connected to C1, since i measure arround 0.139 ohm.Therefore i continued to measure the resistance between each wires and i have the followings:red & green - 1.03 ohm - i consider this to be the TC & commonred & blue - 2.88 ohm - common & heater+green & blue - 3.65 ohm - heater & TCSomeone herehttp://adgd.ru/2021/01/04/jbc-soldering-cartridges-pinouts/had similar results