Reading soldering iron temp

i have taken a 24vdc soldering iron apart and found only 4 wires. 2 for the thermistor "thats the symbol they used" and 2 for the heating element.

when i read the thermistor at room temp i read about 4 ohm. as it heats up the resistance increases and decreases with temp. i have confirmed this on my multi meter between 4-100 ohm.

i built a arduino multimeter using a 200ohm resistor as the r1 in this project

i can successfully read a 200ohm resistor and shorting reads about 0.15ohm. however when i try to read the thermistor of the iron on or off if just read 1.5 -1.9 ohm. however at the same time i can watch the resistance increases on my multimeter but the arduino is not reading it. im using 5v as ref.

Why cant i read the thermistor?

I think it’s time for a circuit diagram and some code .

Be aware that thermistors if this type are often in series with the heating element to automatically control its temperature and 24v on your Arduino analog in will kill it .

Low impedance thermistors are difficult to read as significant current is needed to generate a voltage you can read and then the current can give problems with volt drops across the wiring causing false readings .

Ohms law - 1v across 1ohm implies 1A current , Arduino cannot supply that.

Check out equations for voltage dividers and ohms law .
I think I would look to adding a second , say “10k” thermistor , or thermocouple , just for temperature measurement

Could you help me identify what im working with, what i can tell you is my multimeter reads low '2-4" ohm at room temp and around 100ohm at high temp. I still have not determined if this is a type k or type s thermocouple or a thermistor, its a very generic 24v cheap soldering iron from ebay.

its seems strange my multimeter reads that ohm range but my arduino multimeter wont read it, it will however read other actual resistors.

As far as circuit goes, im just using the example code in the link i posted. its exact except im using 200ohm for R1 na unknown temp sensing device for R2

Re read my replies ! It’s likely the thermistor is in series with the heater element , so is passing current anyway , which affects your circuit .
That current isn’t constant either - don’t think it’s going to work !

But as said need a wiring diagram - be aware you are likely to destroy your Arduino here with the 24v

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I checked for resistance between temp circuit and heating element and there was open circuit. i also check for voltage on the temp sensing wires while the element was powered on and my 10 dollar meter didn't show any voltage on the temp sensing circuit.

Still need a circuit diagram to know what is going on !

Ok sounds like the thermistor maybe part of a separate circuit ???? But … it will have a voltage across it from the existing circuit affecting what you are trying to do , without a circuit diagram who knows , it’s just guessing from our part as to what is going on .

im sorry i dont have a circuit diagram of the soldering iron. after taking apart the handle i see 4 wires, two solder to the spot with symbol of coil, the other two solder to spot with symbol of thermistor. it dont seem to matter if i power the heater or not, if i just heat it with a torch the resistance goes up, my arduino circuit and code is in the link i provided at the top.

im not worried about getting the right temperature value, i just need to map out the resistance values to a percentage, problem is i cant read the resistance with the arduino ohm meter project, i can only read it with my meter.

is it a possibility im not dealing with a thermistor but a thermocouple. from what i've gathered thermistors don't really exist in the range im getting. it seems more likely it is a thermocouple also because we are dealing with pretty high temp. How can i find out for sure without damaging the heater?

I'm pretty sure is thermocouple and not thermistor. my oscilloscope shows a positive voltage increase/decrease with temperature around 10mv-30mv 'ish. i didnt let it get all the way to temp but about half way.

I have merged the two convergent programming and electronics topics as requested.

Regards, Per

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A thermocouple has a very very low resistance, and you measure its output in mV.

It is very hard to workout what these resistive devices are.
PTC or thermistor unless you can make a function curve of resistance vs temperature.
PT100 is 100R at 0DegC.
PT1000 is 1000R at 0DegC.

Tom... :grinning: :+1: :coffee: :australia:
PS, Hard to write a post when the moderator is combining/shifting threads.
The number of times I have been told my edit page is being edited in another window. :grinning: :grinning: :grinning:

Sorry about that Tom. I think everything is stabilized now.

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Measure the voltage produced by the thermocouple at ambient temperature - should be 0mV pretty much.

Now heat the element up (boiling water perhaps?) You should see a millivolt or two change in the output voltage.

Thermocouples only produce very low voltages of a few millivolts, you absolutely need a thermocouple amplifier module to work with them. To find out which type of thermocouple you would need to plot its voltage at a set of different temperatures, which could be quite an involved procedure. But you don't really need to do this if you want to use it in as a thermostat - calibration can be done afterwards using the melting point of solder?


@notsolowki does not have a thermocouple.
Thermocouples do not read 70 Ohms.
It is a PTC of some sort.

Can you please post images of the soldering iron and its control board,
What chip part numbers are on the PCB?

Tom... :grinning: :+1: :coffee: :australia: