Hacking Amazing Soldering Features Into The Already Great Weller WMRP

Weller, the German soldering tools manufacturer, has a nice range of micro soldering irons (pencils) designated as the WMRP series. These are 12V, 40 W or 55W units with a 3 second heat up time, and allow quick tip exchange without needing any tools. [FlyGlas] built a neat soldering station / controller for the WMRP series based around an ATMega microcontroller running Arduino.

It’s packed with most of the features you see in a professional rig.

  • low offset op amp for soldering tip temperature measurement with type c thermocouple
  • cold junction compensation using the PTC (KTY82-210) included in the WMRP soldering pencil
  • input voltage measurement
  • soldering pencil current measurement
  • recognizing if the soldering pencil rests in the stand (–> standby)
  • 3 buttons to save and recall temperature values
  • rotary encoder to set soldering temperature
  • illuminated 16×2 character LCD module
  • USB for debugging and firmware update
  • 4mm safety socket for +12V power input and a protective earth socket for connection to ESD protection

WMRP_controller_02A PWM signal from the microcontroller controls the load current using a MOSFET. Load current is measured using a Hall Effect-Based Linear Current Sensor – ACS712. The corresponding linear output voltage is buffered and slightly amplified using AD8552 zero drift, single supply, RRIO Dual Op Amp before being sent to the microcontroller ADC input. To ensure ADC measurements are accurate and stable, a low noise precision voltage reference – ADR392 is used. Another precision resistive voltage divider allows input voltage measurement. The supply input has over-current and reverse voltage protection. A set of buttons and a rotary encoder are connected to the microcontroller to allow settings and adjustments. An analog section measures the thermocouple voltage from the soldering pencil as well as the stand-by switch status. The handle has an embedded reed switch that is activated by a magnet in the support stand which puts it into stand-by mode. Another analog section performs cold junction compensation using the PTC sensor within the soldering pencil.

The Git repo contains the initial Arduino code which is still a work in progress. While the hardware source files are not available, the repo does have the pdf’s, gerbers and BOM list, if you want to take a shot at building it. Check a demo video after the break. Thanks [Martin] for sending in the tip.

35 thoughts on “Hacking Amazing Soldering Features Into The Already Great Weller WMRP

    1. We’re talking about Weller, not Lexmark. No guarantee of course, but I find it hard to believe they have any desire to alienating themselves from their customeres base.

  1. This is a very cool project, and seems to be very nicely made.
    There are quite a few controller boards on eBay for various Hakko soldering iron handles (but not for the higher-end ones, and since Hakko sells the handles for a fairly high price, you might as well pay the difference and get the original high-quality).

  2. The pencil alone costs more used than my Weller WES51 package new. The quick change tips are nice but I can buy a $5 silicone oven glove if I want to change mine. lol. Will defiantly make one if I come across a pencil with chisel tip for $120 or less.

    1. after I discovered metcal, there’s no going back ;) I used to be a weller fan and they do make good stuff, but metcal – if you can afford it (used on ebay is not bad if in good condition) – is hard to beat. the tip geometry (curved tips) are surprisingly good and the heat-up is as fast as weller’s best. tons of specialty tips on ebay, too.

      1. + ^ This ^

        Metcal – No stinking control other than off and on – It heats up instantly and it’s smart enough to know how much heat your solder joint requires. And if you have to follow a procedure that requires a calibrated temperature, the Metcal can do that too.

        Sorry, but this is just another overkill project of shit you really don’t need to know. Measurebation at it’s finest.

  3. Just curious – similar low voltage (12/24V) soldering irons, the supply voltage is usually given as AC voltage, just like for WRMP http://media-weller.de/weller/data/OI/OI_WMRP_WMRT/OI_WMRP_WMRT_DE_EN_ES.pdf though those days of power MOSFETs and DC/DC convertors it would be simpler to feed the heater with DC voltage. Why do they use AC voltage?

    I vaguely recall somebody used Hakko soldering pencil – rated to 24VAC – with 24VDC suppply and it failed after a short time, so he switched to AC and everything was OK. I also recall something around electromigration, but I’m unsure about this.

    Does anybody have practical experience with this?

    1. As far as I know it’s just because the tech is ancient, and at the time (I guess still), it’s cheaper / lower parts count to use a triac and AC. I can’t see DC hurting it though… just it would cost somewhat more to implement.

    2. I might be mistaken but isn’t it because 24VAC would be closer to 24 * 0.707 = 17VDC? also controlling it from actually overheating under full voltage is of course a possible factor.

      1. Like j says below, that’d be true if it were 24V amplitude sine wave. However when we use the form ###VAC we are always talking about RMS voltage. The nice thing about RMS voltage is that it’s the equivalent sine wave amplitude that would dissipate as much energy as a the same value of DC voltage. Hence, 24VAC = 24Vrms = 24VDC dissipation.

    3. the weller uses ac and switched full sine cycles instead of PWM. the measuremetn is done by holding up a power cycle, such that the wire resistance does not mess up the measurement. the disadvantage is that the temperature is a smapled mesurement and that the iron is quite sensitive to RF fields, I complained about that at weller as I am developing wirelss charging circuits at 6.78Mhz and 13.56Mhz. they updated the softwhere to alleviate the problem, but never really solved it due to the chosen concept. the DC methos should also measure only when power is temporarily off, otherwise there temp errors can eb considerable. did not yet check the code if it is done that way.

  4. Hi jaromirs,
    i know the problem with AC versus DC. But when you take a look on the PCB of the original WSM 1 (http://blog.spitzenpfeil.org/wordpress/2012/01/01/weller-wsm-1-teardown/) you can’t see any hints for AC. The power supply is just the usual laptop-brick with DC output and there is no visible H bridge or an other DC/AC converter. So i think DC is ok and the erosion on the tip is faster than the electron migration in the heating element.

  5. German made? That’s cool!
    How nice that Danaher hasn’t bought them and outsourced all manufacturing to China, resulting in a huge decrease quality and a brand that no longer means anything.

    1. danaher is – umh – ‘a piece of work’ (to put it mildly). they destroyed tektronix, fluke and while I didn’t know they own weller too, that might explain the downturn in weller quality.

      really too bad the T&M companies all had to get bought out by a corporate raider ;(

    2. Weller is now part of the Apex Tool Group, which is a joint venture between Cooper Tools and Danaher.

      My soldering station is a WD1M, and I absolutely love the micro-pencils. They heat up quick and can handle anything even though being “only” 40W. The “tips” cost so much because it also contains the heater element, a big part why it works so well.

        1. I really like those, the tips aren’t too expensive and come in a reasonable amount of variants. The problem is that the temperature levels are very coarse and of course changing the temperature requires changing the tip…

  6. Really cool hack to make your own controller, but in USD the handle alone is $128 and the tips are $35! I’d probably just buy a Hakko 888D if my current setup failed me.

    1. I have the Nano after many years of using a home made thermostat with a soldering iron from a garden variety 48W station(pensol). I appreciated them for the good quality tips(i only replaced a sharp one that i broke). The changes are huge, but it’s not anything you cannot live without.
      I would say that nowadays you can find many decent quality temperature controlled irons at low cost, it’s not worth building your own.

  7. I already own a Weller temperature controlled iron. Soldering iron is not going to make much of a difference for the type of SMT parts I am working with lately. I think I’ll look for a cheap hot air station instead.

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