Probably The Simplest Electronic Temperature Controlled Soldering Iron

We’re all used to temperature controlled soldering irons, and most of us will have one in some form or other as our soldering tool of choice. In many cases our irons will be microprocessor controlled, with thermocouples, LCD displays, and other technological magic to make the perfect soldering tool.

All this technology is very impressive, but how simply can a temperature controlled iron be made? If you’re of an older generation you might point to irons with bimetallic or magnetic temperature regulation of course, so let’s rephrase the question. How simply can an electronic temperature controlled soldering iron be made? [Bestonic lab] might just have the answer, because he’s posted a YouTube video showing an extremely simple temperature controlled iron. It’s not the most elegant of solutions, but it does the job demanded of it, and all for a very low parts count.

He’s taken a ceramic housing from a redundant fuse holder, and mounted it on a metal frame to make a basic soldering iron holder into which the tip of his unregulated iron fits. To the ceramic he’s fitted a thermistor, which sits in the gate bias circuit of a MOSFET. The MOSFET in turn operates a relay which supplies mains power to the iron.

Temperature regulation comes as the iron heats the ceramic to the point at which the thermistor changes the MOSFET and relay state, at which point (with the iron power cut) it cools until the MOSFET flips again and restarts the process. You may have spotted a flaw in that it requires the iron to be in the holder to work, though we suspect in practice the thermal inertia of the ceramic will be enough for regulation to be reasonably maintained so long as the iron is returned to its holder between joints. Nobody is claiming that this temperature controlled iron is on a par with its expensive commercial cousins, instead it represents a very neat hack to conjure a useful tool from very few components. And we like that. Take a look at the full video below the break.

We’ve dealt with many temperature controlled soldering iron projects before here at Hackaday, including this inexpensive way to use Weller RT tips without paying for the full soldering station, an iron controlled by an industrial PID controller, and this iron powered by a PIC. This iron however takes temperature-controlled simplicity to a new level.

29 thoughts on “Probably The Simplest Electronic Temperature Controlled Soldering Iron

    1. I’ve been using Pt100 temperature probes a lot lately. Low end ones are not expensive and some are good up to 500 degrees C. It’s basically a PTC thermistor with better linearity in the temperature range of soldering irons viz. temperatures over 180 degrees C.

      I would totally build this, but here in Asia I can get a decent temperature controlled soldering iron with hot air rework station, variable DC power supply output, and RF meter (for phone repair shops I guess) for about 40 US dollars. Building this would cost me about 22 USD for a single unit (about 5$ in bulk though).

      Still this is a cool idea and I’m going to keep in in the back of my head! If I ever have to outfit a whole classroom this would be better than the firestarters that most students get around here and I could probably justify the cost. Students would of course build it themselves as part of the curriculum.

      All that being said I think the circuit needs some hysteresis though to avoid a bunch of annoying switching noises and improve relay life. Could probably do it with a Schmitt trigger or opamp or whatever. Even a transistor flip-flop would work with a little cleverness. I’d also add a fuse and 2 LEDs (to indicate power status and when it’s heating up).

      Thanks for sharing!

  1. I like the idea and the initiative – simple to execute, and probably gets you a lot of the benefit of a commercial solution.

    What I really need is a way to keep from accidentally leaving the iron on. I thought of timers, but having to refresh them while I’m working seems like a nuisance. I ended up plugging the iron into the switched outlet I have for the shop lights, which gets me most of the way there (I rarely solder in the dark). But, I do occasionally get pulled away from a project for something and leave the lights on, thinking I’ll be right back, and of course, don’t end up back in the lab for hours (or more).

    1. Heh, I had the same problem. Recently I made a plug-in electrical box for my 1970s vintage non-temp regulated iron. Box has a switch and a 40 W lightbulb. So now I always know if the iron is on. I never leave it on if I have to leave.

    2. Some irons have a timeout, using sensors to detect tilt, motion, or whether or not the iron is in the holder. The TS-100 iron that showed up last year on the Chinese sites has an accelerometer to tell if it’s being held upright or if it’s been moved.

    3. Rip the digital timer from a coffee pot and permanently wire it in circuit as an on switch. They usually stay on for two hours max. You could also leave a colored bulb in a lamp over in the corner of the room on your bench circuit. If you see Red, the power to that rail is still on. Finally, if you’re working on more critical things that need you to be *right there*, one of those child/parent trackers that let you set the distance would chirp at you when you left the burn-in/chemistry experiment/souffle’…Just thots from an absent minded man…

        1. This was @ Ted, about leaving his iron on. Although I doubt that any of my ‘done in under ten minutes’ kludges would be considered “Fully Automated” by the talented Hackaday community at large, I will accept your comment as a compliment ???? .

    4. Soldering Irons draw more current when soldering [citation needed]…
      Reset your timer if the current draw is more than the idle current draw threshold.

      If you have one of the more fancy soldering Irons (the ones that have a blinky light), you can use the frequency as the threshold.

      1. I’ve always thought this is how the Metcal 500s work. They won’t time out if you’re soldering, but they time out after some time when you’re not. However, the timeout doesn’t seem to matter much as the temperature control of a Metcal generally prevents them from burning up tips. I have never had to replace a Metcal tip because it was left on.

      1. I bet if you aim the PIR detector at your workbench, just the act of waving the hot iron around will keep the detector from shutting off. Otherwise, because you’re probably pretty still while soldering, you might end up having to do the “PIR dance” periodically to turn the lights back on.

    5. I think my Weller includes some kind of circuitry that measures the temperature of the iron, and if the temperature doesn’t fluctuate for a while (from using it, wiping it on sponge, waving it around), it shuts itself off. I’m guessing the typical acts of using the iron change the temperature significantly enough that it can be measured and used as a timeout-resetter?

  2. “so long as the iron is returned to its holder between joints.”

    Waaaat? Don’t tell me anyone places one lead at a time, this might be acceptable for harnesses or something, but boards, I place all the components and go until I’m done…. unless I need to swap to a 75W or something to get enough heat into a large shielded enclosure, or big ground tab.

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