A Tale Of Three Soldering Iron Controllers

[ZL2PD] needed to replace an old Weller soldering station and decided not to go with one of the cheap soldering stations you can find all over the Internet. He has a long story about why he had to design his own controller, but you never have to explain that to us. He kept detailed notes of his journey and in the end, he built three different controllers before settling on one.

He started with a Hakko hand piece that uses a thermistor for temperature measurements. The first iteration of the controller had analog controls. He wasn’t happy with the number of parts in the design and the simple LED display. That led him to replace the controller with an ATTiny CPU and a use a serial LCD.

The third and final version still uses the CPU. [ZL2PD] realized the LCD display wasn’t very useful, so he went back to the LEDs and created a PCB. Finally satisfied, he created a 3D printed enclosure, an artistic dial scale, and scavenged a tool holder. The results look pretty good and there has to be something satisfying about soldering with a station you designed yourself.

Seems to us all he has left is to hack a fume hood. Or perhaps he’ll build a desoldering companion. If you want to duplicate his results, be aware that some irons use different types of temperature sensors. Or you might just want to blow $16 on a cheap station (see the video below) and work on other projects. We won’t tell.

33 thoughts on “A Tale Of Three Soldering Iron Controllers

  1. I own a Daxis TO0601 (~40 euros) which I bought 4 years ago and looks like a knock-off of the 936. The PCB inside is actually good quality, components well soldered and the ground cable is correctly crimped. The tip lasted for a long time, now I have a genuine hakko tip. For the hot air stuff I have an Atten 858D+ (~40 euros) also bought around 4 years ago.
    Both tools serve well, I never needed to pay for more expensive stuff.

      1. I had this issue with my first nicer iron – tossed all my firestarters, but ended up needing to solder in a new ceramic heating element after a few months… Ended up getting a Hakko instead, and that other one is just sitting idle…

      2. My first soldering iron was a Heathkit, with three power levels (way before temperature control was available). Honest to god, what they recommended if you didn’t have another soldering iron, was to crimp all of the connections with needle-nosed pliers, turn it on high and let it get real hot, then unplug it and solder as many connections as you could before it cooled down too much. Repeat as necessary. This actually worked.

    1. I wondered the same thing, but “However, the simple resistor divider arrangement used in the previous design required too much current from the 5V rail.”

      Wouldn’t this be a good use for a wheatstone bridge?

      1. Nope, neither a resistor divider nor a wheatstone bridge would work here. Limitation is the 78L05 dissipation which sets a max current drain on the 5V rail, as the website says. That LM317 is a cunning solution.

        1. Ah! I wasn’t even looking at the ‘L’ in 78L05 lol. Odd though to use a medium power LM317 Linear Voltage Regulator because the main Linear Voltage Regulator is too small (low power) lol. He could have used a 78M05 (normal sized REG). The dissipation in the series resistor (for PTC) was not too much under 250mW so I thought he might have wanted to have some more room (leeway) to use 250mW resistors. Perhaps it came down to what was in the spare parts box.

          All the same it’s interesting to see a Linear Voltage Reg being used as a sort of high current output comparator (Op Amp) sense amplifier. Makes me wonder if he also used the 1.2 Volt offset to shift the sense range to better suite the micro-controllers ADC (and improve Resolution).

          +10 for an interesting circuit technique!

        2. No, the only reason why the LM317 is better is because it is drawing the current directly from the 24V line, not from the 5V regulator. He could have used a lower current in the divider(lower output voltage range like in the current source) and get the same thing. I would prefer a small heatsink on the regulator than add more parts.
          Plus, using a divider, the precision of the measurement is strictly related only to the resistor in the divider, everything else is not important, so you need a 1% resistor and done. Now, the precision of the measurement depends on the 5V regulator used as ADC reference, the LM317, the precision of R2 and R3, so, worse solution.

          1. Not sure that’s right. Seems to me your approach would use more parts. And 1% parts too? A heatsink like you suggest is a large (heat generating) thing inside a tiny PLA (low temperature melty-plastic) shell. This design seems to avoid all of that.

            Besides, precision is not a requirement of a soldering iron like this, is it? Near enough is good enough, right?

          2. @Jim…not sure if trolling or not…really, a single 1% resistor is a problem for you?
            Calculate the code value of the ADC in this case and it will be a function of all the things I mention before. 2 5% resistors, a 5% 5V reg, a 5% LM317 and you are at 20% already.

          3. I haven’t done the graphs for this specific circuit but I have done the graphs for a straight resistive divider and a clone Hakko 903 Iron PTC.

            A straight divider presents some problems.

            1) Firstly for the range of temperatures you would expect for a soldering iron, change in voltage is greater at the cold end of the graph and very minimal at the hot end of the graph where you need the greater accuracy. While you don’t need great accuracy for one particular iron, poor resolution can make the difference great between the PTC’s of different irons.
            2) The cold temperature resistance of a PTC is very low which means you have to use a fair amount of current at the cold temperature so that you have enough resolution at higher temperatures.

            So to summarise a straight divider gives high resolution at the cold end and poor resolution where you need it at the hot end and this problem is made even worse as you reduce the current through the divider.

            The Vcc to the divider also makes a difference. A higher voltage means a higher series resistor (for the same current) and results in a greater change in sense value at the hot end of the scale.

            The thermal dissipation is directly related to the current through the PTC irrespective if the voltage it is supplied from as it still is derived from the 24Volts.

            So ideally you don’t **just** want a constant current supply (a high Vcc to the divider is more like a constant current supply) you would also want a sense amp that performs the function reciprocal.

          4. It’s is indeed flat when you measure resistance against temperature but a micro-controllers analog input measures voltage – not resistance.

            To get the same flat response you need a constant current supply and that was my original point – ie the chosen design is ‘more’ like constant current than it is ‘like’ a resistive divider.

            For constant current Vsense = Rptc * x

            For divider Vsence = x * Rptc / (Rbias + Rptc)

            In both cases Rptc is closely (but not exactly) proportional to temperature.

            Totally different.

            Thanks for the link. I am in the process of making a solder station with Hakko 936 clones so the link is very helpful.

  2. I can’t even get a google cache of the page!

    Maybe HAD should give website owners a heads up about a new inbound link that’s going to suck the very last binary ‘0’ out of their site! The ‘site down’ doesn’t help readers, it doesn’t help HAD and it doesn’t help the site owner. Even a separate cache that HAD maintains would be good.

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