It is common these days to have a soldering iron where you can set the temperature using some sort of digital control. But how accurate is it? Probably pretty accurate, but [TheHWCave] picked up a vintage instrument on eBay that was made to read soldering iron temperature. You can see the video below, which includes an underwhelming teardown.
The device is a J thermocouple and a decidedly vintage analog meter. What’s inside? Nearly nothing. So why did the meter not read correctly? And where is the cold junction compensation?
The probe seemed okay when used with a modern meter. However, driving the meter directly showed a problem. It seemed like something was wrong with the meter’s movement.
Pulling the meter out revealed a handwritten label identifying the meter as having a full-scale deflection of just over 27 millivolts and using a 0.5 ohm external resistor. Unsurprisingly, the old meter had some issues with the thermocouple.
Pulling the meter showed that the compensation was actually inside the meter casing. The meter’s problem was due to these extra components: an NTC thermistor and a custom wire-wound resistor. Changing the resistor fixed the box. This time, he put the two components outside the meter housing in the vast space of the nearly empty case.
Today, we take measuring temperature for granted. Only the cheapest meters lack temperture measurement and you can use other techniques like IR sensors, too. But this was an interesting look at how it was done “back in the day” and an interesting repair, too.
Thermocouples are an old standby for measuring high temperatures. With modern tech, it is pretty simple to compensate them.
I’ve got a meter very similar to this one. The box is grey, and the probe is just sticking out the front of the box. I think it was unbranded or at least another brand. Grabbed it along with a nice Tektronix scope and some other stuff when they cleared out a storage room at work. I have not found any use for it, but someday I should at least check its accuracy.
If you want to verify the temperature of your soldering station, then just add a blob of solder to the tip and then adjust it to a temperature where it barely melts. And then compare the temperature on the display with the datasheet of the type of solder you use. Some solders have an eutectic point and go solid at a defined temperature, while other solder types have a trajectory in which the solder is a bit mushy.
And as this lets you calibrate your iron right at the temperature where it matters, any further gadgets are just a waste of money.
“It is common these days to have a soldering iron where you can set the temperature using some sort of digital control.”
Not so on my venerable Weller TCP-24! 😁
Haha yeah! Using “6” tips for standard leaded solder and a “7” or “8” when I need some oomph. Love that iron, I have two of them on my workbench and a spare in my drawer. They work extremely well on 20V tool batteries as well. Just not all the time because of contact switch burning by DC…
SP-200 here – the temperature is in the tip design, and I trust them more than I trust a random meter TBH.
Time to upgrade to a Pinecil. We got Weller stations at work but I like to keep my Pinecil in my backpack for serious souldering needs. With their silicone (flexible) USB-C cables it’s so nice
You don’t need cold-junction comp for any soldering equipment. It’s always done around room temp which is 25°C +/-5°C so a few degrees error at worst and insignificant.
I just bought a soldering iron temperature checker off AliExpress years ago. We tested it and it’s accurate.
It’s marked Hakko, but it clearly isn’t by them.
It was only a few euro at the time and I’ve found it useful on several occasions.