Adding PID Control To A Non-Adjustable Iron

Do remember your first soldering iron? We do. It plugged into the wall, and had no way to adjust the temperature. Most people call these kind of irons “fire starters.” Not only are they potentially unsafe (mainly because of the inadequate stand they come with) they can be hard to use, slow to heat up, and you never know what temperature you are soldering at.

[Mike Doughty] wondered if you could hack a cheap iron to be temperature controlled. He began by taking apart an iron, and adding a K-type thermocouple to the mica heating element with the help of a fiberglass sleeve. After a few tries at fitting and finding the right placement for the thermocouple, he then reassembled the iron, and attached everything to an off-the-shelf industrial PID controller.

Not one to trust that everything was working, [Mike] began to test the iron. He used a Hakko FG-100 soldering iron tip thermometer to measure the “real” temperature of tip, and compared it to the value the K-type thermocouple was reporting it to be. The results were fairly impressive (as seen in the video after the break). Only about 10 degrees out. Not too shabby.

He concluded that although it did work, it wasn’t a replacement for a high quality soldering station. We suspect the real problem with this idea is that the mica heating element is way to slow to respond to any thermal load that the tip is given (but then neither did the unmodified iron.) If you’re interested in hacking together your own soldering station, you might be interested in the open source soldering iron driver.

[via Dangerousprototypes]

10 thoughts on “Adding PID Control To A Non-Adjustable Iron

  1. The problem with the speed is actually the important point here. It’s pretty cool to stabilize the iron with PID to turn it from dangerous crap into safe suboptimal solution. But nonetheless, these irons heat up super slow, because they can’t naturally rely on any regulation,so that it will be very difficult to get any useful regulating performance at higher temperatures. Plus, the tip is so poorly shaped that it won’t follow any sort of load. The price of the PID controller is better used for some cheap regulated iron. They start at around 30 bucks and are already pretty decent.

  2. The Hakko FX-888D offers full temperature control for less than $100. That Hakko tip temperature is more expensive ($230.00). Unless you have a lot of time and money to waste – go buy the new FX-888D unit!

    1. Unless you’re in a 230V region, then the FX-888 is double the price.

      However, any iron designed with temperature control will be better than this hack – and they start at about the same price as that PID controller.

  3. If you want something cheap, just get yourself one of these.

    110V version:
    220V version:
    240V version:

    There was a teardown / review of it on EEVBlog a while back. I have one because I just started out building up a ‘workshop’ and therefore had to buy way too many things at the same time with little money and I think it is pretty good for the price.

    1. I’ve got one of those (not from HobbyKing as the shipping was a bit excessive – plenty on eBay).

      I’ve also got a few real Hakko 936 irons. The knockoff works fine. As expected the build quality, while ok for a cheap iron, is a lot less than the Hakko. eg the Hakko lead is silicone rubber, while the other is just normal PVC.

      I’d take it over a pencil type iron any day. Spare tips are only $1 on eBay as well.

  4. Not that is matters much for this, but don’t connect your temperature probe to the PID with copper leads like that.

    Screws up the reading a bit.

    Take the plug off and wire it straight in (or make the leads as short as possible.)

    1. The length of the leads does not matter.

      A thermocouple does not measure the absolute temperature, a thermocouple measure the _difference_ between the two ends of the thermocouple wire. So, if someone have only a thermocouple, without a thermometer or another object with a _known_ temperature, then the absolute temperature of the K probe head can not be calculated.

      What actually matters is the temperature of the cold junction. The cold junction is the place where the K type wires end, usually the yellow socket pins. Good (precise) temperature controllers have cold junction compensations, which means they have a temperature sensor that measure the temperature of the junction between the K type wires and the socket (or the normal wires).

      That is why we should not use extra leads, no matter their length, because any extra leads will put the cold junction far from the cold junction compensation sensor. If the leads temperature is kept the same (by any magical process) on both ends of the leads, then we can have leads as long as we like, without affecting the controller precision.

      Also, if the temperature controller is a cheap one, without extra temperature sensor for the cold junction, then the leads does not matter at all, because controllers without cold junction compensation assume that the junction temperature is always constant (the room temperature, approximative 24 *C).

      I know, tl;dr, and everybody already know all this.
      Sorry, I promise I won’t do it again.

  5. Haha, I did almost same hack in 2012! :)) with same REX-C100, but used laptop psu and ‘proper’ 24V 48W iron. Problem was REX-C100 has minimum 1 second cycle time, abandoned it quickly for a proper soldering station.

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