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I have a soldering station equipped with a 24V/60W soldering iron. Its heating element (pins 1-2) resistance measures around 50 ohms. The resistance of the thermocouple (pins 4-5) measures around 2 ohms.

Soldering Iron Pinout

PIN WIRE    PURPOSE
1   Blue    Heating element  ' ~50 ohms
2   Green   Heating element  '
3   Yellow  Case ground
4   Red     Thermocouple     ' ~2 ohms
5   Black   Thermocouple     '

I also have a desoldering gun from a desoldering station with the same power requirements and ratings 24V/80W. The thermocouple resistance measures the same as my soldering iron around ~2 ohms (pins 1-2). The resistance of the heating element (pins 3-4), however, measures only 4 ohms.

Desoldering Gun Pinout

PIN WIRE    PURPOSE
1   Black   Thermocouple      ' ~2 ohms
2   Blue    Thermocouple      ' 
3   Red     Heater Element    ' ~2 ohms
4   Red     Heater Element    ' 
5   White   Trigger switch
6   White   Trigger switch
7   CENTER  Vibration sensor

Both instruments are using a ceramic heater. The temperature range for the desoldering iron is 160℃ - 480℃ I don't understand how the heating element's resistance can be so low. Am I missing something here? If I were to connect the heater and thermocouple wires of the gun to my soldering station, wouldn't it overheat? If so, why doesn't it overheat when connected to the desoldering station given the units have very similar power requirements. Is there some other internal circuitry I am not accounting for? Am I misreading something?

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    \$\begingroup\$ Can you link to datasheets or other product descriptions for the soldering iron and desoldering gun? \$\endgroup\$
    – The Photon
    Commented Jan 22, 2019 at 21:53
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    \$\begingroup\$ Heating elements can have a quite high positive temperature coefficient and the resistance increases with temperature, lightbulbs is a common example. \$\endgroup\$
    – RobinSt
    Commented Jan 22, 2019 at 22:57
  • \$\begingroup\$ How can you say that a 60W iron and an 80W iron have the same power requirements? Are you including power used for circuits other than the heating element? \$\endgroup\$
    – Elliot Alderson
    Commented Jan 22, 2019 at 23:39
  • \$\begingroup\$ I should have said similar. I was thinking about the output power of both stations. The desoldering gun says 24V/80V on it, while the soldering iron says it in the manual. \$\endgroup\$
    – user148298
    Commented Jan 22, 2019 at 23:42

3 Answers 3

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Oops! I discovered the source of the issue. It turns out soldering stations and irons use one of two or more temperature sensors. The majority use either PTC sensors, where it's resistance increases with the temperature (positive temperature coefficient), or K-type thermocouples, where it's voltage increases with the temperature (in millivolts). It explains why my readings were way off. Unfortunately, many of the soldering equipment manufacturers don't explicitly state which temperature sensor is used. Since both sensors had the same number of wires and the irons were identical looking, I assumed it was the same.

I've read several tutorials on various hacks for soldering stations. There's quite a few who came up with elaborate schemes for calibrating the temperature after seeing their soldering irons glow to a cherry pink, not realizing the iron uses the wrong temperature sensor.

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24V across 50 ohms equates to 11.52W. I think the iron needs a new element. I would say that the desoldering gun element has a high positive temperature coefficient.

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  • \$\begingroup\$ Interesting you say that because the desoldering gun us brand new. I got the courage to finally wire it and it worked, but of I exceed 300C, the temperature runs away, causing the gun to glow red. I didn't consider the heating element as being faulty and I've only encountered one other blog where someone else tried to do hook it up to a different, but similar soldering station and experienced the same runaway temperature. \$\endgroup\$
    – user148298
    Commented Aug 25, 2020 at 23:38
  • \$\begingroup\$ @user148298 If your soldering equipment glows red, that's not a good sign. I think you got a faulty soldering iron. \$\endgroup\$
    – Hearth
    Commented Dec 12, 2021 at 1:15
  • \$\begingroup\$ @Hearth LOL! The soldering gun uses a PTC sensor and not a thermocouple. See my answer. \$\endgroup\$
    – user148298
    Commented Dec 12, 2021 at 1:38
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The 50 ohms is the sensor. The 3 ohms is the element. I have a similar one and it uses around 10-11V for the heater

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  • \$\begingroup\$ You station uses PTC sensors whereas mine uses a thermocouple, See my answer. \$\endgroup\$
    – user148298
    Commented Dec 12, 2021 at 1:40
  • \$\begingroup\$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. \$\endgroup\$
    – Community Bot
    Commented Dec 12, 2021 at 16:05

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