What type of thermocouple is used in JBC irons ? (Maybe type N)

[Bgdn]

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.

For 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-amp 

Regarding 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 modifications

I 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-amp



The measurements where performed with no tip handle. I tryied to measure with some crocodile wires directly on the tip

[Bgdn]

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?

[wraper]

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.

[Bgdn]

Hello
I 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 deg

I 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 + connector
to 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.

[wraper]

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.

For example i measure about 245degrees  ( CJ 23deg included) - 5.0mV . The multimeter shows 239.5 degrees.
260deg @ 5.3mV - multimeter shows 252 deg

This 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.

[Bgdn]

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.

[Bgdn]

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 circuitry

But 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?

[wraper]

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?

Normally special wires are used. Either from the same materials as thermocouple or something else that resembles particular type of thermocouple within smaller temperature range.

[Bgdn]

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 well

Maybe that way there is a chance to find a more accurate  sebeck coeficient.

What do you think?

[Bgdn]

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 temperature

Would 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 

[thm_w]

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.

[Bgdn]

In the mean time searching over the internet, i found here some information https://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 measurements

• C1 (outer sleeve) - C2 (middle)  measured 2.20 ohm
• C2-C3 measured 2.227 miliohm
• C1-C3 measured 50 miliOhm

Given 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 wires

Just 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 & common
• red & blue - 2.88 ohm - common & heater+
• green & blue - 3.65 ohm - heater & TC

Someone here http://adgd.ru/2021/01/04/jbc-soldering-cartridges-pinouts/ had similar results