Thermocouple Amplifier with Cold Junction Compensation Explained

[@qt1qg]

how do soldering irons perform cold junction compensation? from what i have read online there is no thermal sensor in t12? jbc or hako 900 series handles(cold end of t12 tip can get up to 50C), and there can be a sensor on the board or they can use temperature sensor of mcu itself. How do they actually do it without knowing temperature of the handle and using only ambient temperature?

[@STEMprof]

Soldering irons, such as those using T12, JBC, or Hakko 900 series tips, often don't explicitly perform cold junction compensation in the same way thermocouple systems do, because they typically don't use thermocouples for temperature sensing. Instead, they rely on a different approach like the PT100 RTD Temperature Detector with Error Compensation kzbin.info/www/bejne/fKTHXotsbsaqfM0 to manage temperature and ensure accuracy. Here's how they work: 1-- Temperature Control Mechanism: Most modern soldering irons like the T12, JBC, and Hakko 900 series use a sensor (usually a thermistor or RTD) placed at the tip or very close to the tip to measure the temperature directly at the point of soldering. This sensor is crucial because it directly monitors the temperature of the tip in real-time. A related video is Thermometer Circuit Design with Op Amp and BJT transistor kzbin.info/www/bejne/a2a8pKWXe6pjqMk 2-- Cold Junction Compensation (or Equivalent): The handle of the soldering iron, including the cold end of the tip, can indeed reach higher temperatures (around 50°C, as you mentioned), which might cause concerns about temperature readings being influenced by ambient heat. However, soldering irons don't typically use thermocouples for sensing the temperature, so the issue of cold junction compensation (which primarily applies to thermocouples) is less of a concern. Instead of compensating for the cold junction as thermocouples would, the system relies on sensor calibration. The sensor used in the soldering iron (often a thermistor or an RTD) is calibrated to account for the temperature at the tip and its surroundings. Even though the handle may get warm, the system compensates for the temperature through direct measurement at the tip, ensuring it maintains the correct soldering temperature by adjusting power to the heating element. In summary, soldering irons do not need cold junction compensation in the traditional thermocouple sense because they do not use thermocouples to measure the temperature. They rely on direct temperature measurement at the tip using sensors like thermistors or RTDs and compensate for ambient and handle temperatures through sensor calibration and power control mechanisms. The heating system ensures the tip maintains the correct temperature based on real-time feedback from the sensor at the tip itself. Watch this video kzbin.info/www/bejne/a5vQk41vltBpe6s to see more about Thermometer Sensor Circuit Design.

[@qt1qg]

​@@STEMprof Thank you for the response! I recently found out that original JBC tips are TC based and have compensation inside the cartridge, t12 tips are also TC-based and original Hakko handle has RTD inside for software based compensation. 900 series are RTD based(don't know why they did not use RTD in T12 and JBC). Clone cartridges and handles probably don't have any compensation. I know that some soldering stations that use T12 cartridges have thermal sensor on the board or use MCU with built in thermal sensor, is it possible to achieve any kind of accurate regulation by estimating cold junction temperature based on thermocouple readings and ambient temperature with some kind of empirically discovered formula?

[@STEMprof]

@@qt1qg You're welcome! Your insights are spot on about how different temperature sensing technologies used in soldering irons. To clarify the part about achieving accurate regulation by estimating cold junction temperature using thermocouple readings and ambient temperature: yes, it’s possible in theory, but it’s definitely more complex and less reliable than using a direct measurement approach say using thermistors or RTDs (Resistance Temperature Detectors) similar to the kzbin.info/www/bejne/fKTHXotsbsaqfM0 video discussing PT100 RTD Temperature Detector with Error Compensation to manage temperature and ensure accuracy. In soldering stations using thermocouples (like those with T12 or JBC tips as you stated), cold junction compensation is typically achieved using internal compensation circuitry that accounts for the ambient temperature at the cold junction (the connection point between the thermocouple and the board). This allows the system to correctly interpret the temperature difference measured by the thermocouple. However, if you're estimating the cold junction temperature based on ambient temperature and thermocouple readings, it would require a precise mathematical model or empirical formula, which might not be universally accurate for all setups. This model would need to account for factors like thermal resistance, environmental factors, and how the tip and handle are thermally coupled. Even with this, the result could vary between different soldering irons and environments. On the other hand, as you mentioned, the Hakko 900 series and T12 tips use RTDs or thermistors that are typically placed near the tip. These sensors provide much more accurate and stable temperature measurements, since they don’t rely on interpreting a voltage difference like thermocouples. In these cases, ambient temperature and handle heat still need to be considered, but the MCU or onboard temperature sensor can use software-based compensation to account for the temperature variation. To summarize, while estimating cold junction temperature with thermocouple readings and ambient temperature is a theoretical approach that could work with proper calibration, most modern soldering stations with RTDs or thermistors achieve much more accurate and reliable regulation using direct temperature feedback from the tip. This eliminates the need for complex compensation methods, making temperature control easier to manage and maintain. I hope this follow-up explanation is helpful.

[@qt1qg]

If there are multiple junctions of similar metal pairs in the same direction between thermocouple and measuring device, is cold junction only a place when bare thermocouple wires connect to copper?

[@STEMprof]

No, the cold junction is not only the point where the bare thermocouple wires connect to copper. The term "cold junction" refers to the reference junction, typically the location where the temperature is known or controlled. In a thermocouple system, the cold junction is usually at the point where the thermocouple wires are connected to a measuring device or reference material (such as copper or another metal). But if there are multiple junctions formed by different metal pairs in the circuit, each additional junction can also influence the measurement. When thermocouple wires are connected to other metals (like copper in the measuring device), each junction between dissimilar metals creates a potential difference, which can contribute to the overall voltage (e.g. series voltages that add up). The cold junction compensation method is used to account for this, ensuring that the temperature at the connection point is accurately represented in the measurement. Therefore, the cold junction is primarily considered where the temperature is controlled and known, and where compensation for other junctions in the system is applied, not just where the thermocouple connects to copper. I hope this is helpful. Here are a few related circuit videos: Thermometer Sensor Circuit Explained kzbin.info/www/bejne/a5vQk41vltBpe6s PT100 RTD Temperature Detector with Error Compensation kzbin.info/www/bejne/fKTHXotsbsaqfM0 Thermometer Circuit Design with Op Amp and BJT transistor kzbin.info/www/bejne/a2a8pKWXe6pjqMk

[@qt1qg]

@@STEMprofThank you! So while other junctions will influence measurements to some extent, is junction with bare thermocouple wires more likely to introduce the most error? Since it probably is closest to the measured object(larger temperature difference with ambient) and due to the fact that it has different pairs of metals for example chromel to copper and alumel to copper, that have different Seebeck coefficients(other junctions are more likely to be two similar pairs, something like two pairs of copper to bronze that have the same Seebeck coefficients and will compensate each other if their temperature is the same)

[@STEMprof]

@@qt1qg You're welcome! Good points regarding thermocouple measurements and the potential sources of error due to additional junctions. Yes, the junction where the bare thermocouple wires connect to copper is more likely to introduce the most error. This is because this junction is exposed to the largest temperature difference between the measured object (the hot junction) and the ambient environment, which can create a significant thermoelectric potential. Also, as you mentioned, the metals used in the thermocouple (such as chromel and alumel) and the metal the thermocouple connects to (like copper) have different Seebeck coefficients, which means they will generate different voltage contributions depending on their temperature difference. This results in potential measurement errors unless properly compensated. The junction formed by two similar metal pairs (like copper to copper, or brass to brass) would likely have less impact, as the Seebeck coefficients of similar materials are closer, and their temperature-induced voltages can cancel each other out more effectively. This is why the system is usually designed with materials that have well-matched properties for minimizing such errors. In practical thermocouple systems, cold junction compensation is employed to correct for these types of errors, by either measuring the temperature at the cold junction directly (using an RTD or say a thermistor) or using a known reference temperature to adjust the reading accurately. I hope this explanation clarifies things a bit further.

[@qt1qg]

Would current of 25 to 250 microamps(most mosfets that start from IR can have such leakage currents) passing through a thermocouple influence the readings? For example if it's in series with the heater like in t12 tips and mosfet leakage current is passing through it when it's off

[@STEMprof]

Good question. For practical amplifier design, all components including op amps need to be selected carefully. I suggest using zero-drift low-noise CMOS precision Op Amps from Texas Instruments or Analog Devices. When carefully selected, drift currents and voltages of the CMOS op amps are on the order of tens of nano amps or nano volts. For related videos please see: Thermometer Sensor Circuit Explained kzbin.info/www/bejne/a5vQk41vltBpe6s PT100 RTD Temperature Detector with Error Compensation kzbin.info/www/bejne/fKTHXotsbsaqfM0 Temperature-Compensated Programmable Current Source Circuit Design with Zener Diode, BJT Transistors kzbin.info/www/bejne/h4qXaXyHja98iKs Thermometer Circuit Design with Op Amp and BJT transistor kzbin.info/www/bejne/a2a8pKWXe6pjqMk I hope this is helpful.

[@erikev]

Is the zener temp coeff. value accurate at all? The part is specified for 2.4V Zener. There is no binning from production not any uncertainty. Have you built this circuit, and confirmed the accuracy?

[@STEMprof]

Good question, Yes for 2.4v Zener and for the cold (PCB/copper) temperature range of 0 to 50 degree Celsius that matters. Note that Zener Diodes between 1.8v to 3.3v have remarkably stable temperature coefficients assuming proper DC Bias current (say 6-10 mA).

[@docteurlowbat]

Good ! But i didn't get the use of the 1 meg resistor.

[@weinihao3632]

I also wondered about that

[@STEMprof]

Thanks! Good question, we want to avoid leaving the thermocouple terminal float in the circuit.

[@docteurlowbat]

@@STEMprof Thanks ! A kind of pull up résistor ?

[@STEMprof]

Yes, you are welcome.

[@weinihao3632]

@@STEMprof but it is tied to ground through the 100 Ohm resistor

[@STEMprof]

A Thermocouple Amplifier with Cold Junction Compensation Circuit is explained in this video. For more sensor signal conditioning and amplifiers see kzbin.info/aero/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj and the following videos: PT100 RTD Temperature Detector with Error Compensation kzbin.info/www/bejne/fKTHXotsbsaqfM0 Thermometer Sensor Circuit Explained kzbin.info/www/bejne/a5vQk41vltBpe6s Digital Stethoscope Microphone Amplifier Explained kzbin.info/www/bejne/m6uYfKehhcemfsk PhotoDiode Amplifier with Data Compression Explained kzbin.info/www/bejne/nqLVg6toqst0rck Thermometer Circuit Design with Op Amp and BJT transistor kzbin.info/www/bejne/a2a8pKWXe6pjqMk Thermometer Current source kzbin.info/www/bejne/fZjJYax5lrmHn7s Strain Gauge Wheatstone Bridge Instrumentation Amplifier Explained kzbin.info/www/bejne/n6CUqnWZeNiDY7s Temperature-Compensated Programmable Current Source Circuit Design with Zener Diode, BJT Transistors kzbin.info/www/bejne/h4qXaXyHja98iKs Temperature-Independent Current Circuit Design with Op Amp, BJT, Zener, Schottky Diodes kzbin.info/www/bejne/nnfFn52YmcmIrNU Instrumentation Amplifier with Electronic Gain Control kzbin.info/www/bejne/eWXXmJuQYtZpkNU For more analog circuits and signal processing examples see: kzbin.info/aero/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt I hope these sensors circuits videos are interesting and useful.

[@stevenbliss989]

Why not use the 3.9 & 4.3 Zeners in series (with higher drive voltage & higher divider etc., as that would be near 0 temp issues?

[@stevenbliss989]

Never mind, I see they are using it to compensate connection issues.

[@STEMprof]

@@stevenbliss989 Correct, 2.4 volt Zener Diode is intentionally used since its -0.085% per degree C temperature coefficient results in -40.8 micro volt per degree voltage sensitivity which nicely compensates for the unwanted effect of non-zero cold junction temperature Tc as explained around minute 8:00

[@SANJAYSINGH_73]

How to use J-Type Thermocouple in this design?

[@STEMprof]

Good Question, While you can use both Type J and Type K thermocouples in this example, J-Type can operate well at lower temperature and in vacuum. It is not preferred in very high temperature or very corrosive environments where K-type is a better choice. For more sensor signal conditioning and amplifiers see kzbin.info/aero/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj video playlist.

[@Guishan_Lingyou]

Very interesting circuit. I have a couple of thermocouples in a box waiting for me to get time to use them. I wonder how this circuit compares practically against the off-the-shelf modules that can be purchased from Adafruit and other places. I worked on the installation of a new furnace in an aluminum foundry and installed thermocouples and the associated devices, but there was no opportunity for me get any insight into the circuit design. Also, if anyone is interested, there is at least one easy to deal with supplier of trustworthy, USA made thermocouples, if you don't want to take your chances on Amazon or other such places.

[@STEMprof]

Glad that you liked this Thermocouple Amplifier video. Thanks for the comment and sharing your thoughts. Here are a few related sensor amplifier videos: Thermometer Sensor Circuit Explained kzbin.info/www/bejne/a5vQk41vltBpe6s Temperature-Compensated Programmable Current Source kzbin.info/www/bejne/h4qXaXyHja98iKs Strain Gauge Wheatstone Bridge Instrumentation Amplifier Explained youtu.be/io1yBcC Thermometer Circuit Design with Op Amp and BJT transistor kzbin.info/www/bejne/a2a8pKWXe6pjqMk I hope these circuits are interesting as well.

[@heinzergrinder1901]

Good morning. What is a composite amplifier ?

[@STEMprof]

Good morning as well. Composite Amplifier usually means cascade of more than one stage. More examples kzbin.info/aero/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj are posted in the sensor signal conditioning and amplifiers playlist.

[@Chupacabras222]

Very nice and fascinating.

[@STEMprof]

Thank you. Glad that you liked this video. For related sensor signal conditioning and amplifier videos please see kzbin.info/aero/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj which is the collection of Sensors & Instrumentation Amplifiers Videos.