Fixing and Reverse Engineering Cheap Temperature Controlled Soldering Iron

Last year, I bought a temperature controlled soldering iron from local supplier. I bought a hot air rework station soon and I seldom used this iron because new rework station also has a temperature controlled iron. I like the idea of this iron because of its design. It is not like others temp-controlled soldering stations. See the photo.

Here is close up. You can see inside circuitry.

It is just an iron and temp-control circuit is inside it’s handle.  No dedicated controlled station like famous Weller gun. There is a knob at handle to adjust temperature from 100~400 C.  Yes, it is made  in china. The price is very cheap, about 5$ (do you believe it or not). But the quality and performance is quite good.

One day, I used this iron and it was not working. So, I opened fixed it. Fixing is quite simple and I checked the fault within minute.

– Removed the screw and knob first.

– I checked circuit.You can see an IC, components and a Triac. There is no SMD devices or expensive, complex thing. I noticed that there are 4 wires output to heat element tube. Interesting, I first thought there will be no feedback circuit or sensor in such cheap tool. I was wrong. The red-blue pair is a nichrome coil and the white pair is a thermo-couple. It is a closed loop control circuit, unbelievable.

I started to examine the circuit.

The fault was nothing. The soldering joint of heat coil was dry. Soldering directly to a nichrome coil is not a good idea in such circuit. It should be crimped. But this is no room for this and I just do re-soldering the joint with acid paste flux (which can help to solder metal joint like nichrome, tin sheet ect.. Don’t forget to clean the joint after soldering, acid from flux may conduct your circuit lines). Then, the iron worked again.

I started checking the circuit detailed. The IC is simple common Op-Amp, LM358, low power dual operational amplifier. The Triac is MAC97A6, logic level triac. This means that this Triac can be directly switched with logic level, 5V.

The close up,

As usual, I tried to trace the circuit with the help of back light.

This is final circuit.


# Updated 21 July 2015 : Added the value of C and corrected polarity according to Nguyen Ngoc comment, Thanks.

– The principle of circuit operation is simple. The Op-Amps are configured as comparator. The Triac drive directly by Op-Amp2 via C and LED1. The thermo couple is connected to U1, non-inverting input. The preset VR1 is used to adjust the comparator reference voltage. When temperature is lower than preset, the output of U1 will be lowered than U2 non-inverting input, The output of U2 will be high state and switch on Triac to heat up iron. When temperature is reached to set-point and U1 output will changed to high and U2 output will set to low to switch of Triac.

– The C1,ZD1, D2 and R7 are a simple transformeless zener regulator supply.

-Although it is not a linear feedback controlled circuit, it works, at least for this type of iron. Quite a simple and working design. You cannot defeat china 🙂 .

– I haven’t construct this circuit yet. If you want to construct, I cannot grantee it will work. Do it at your own.  If any one have experience on this, pls let me know.

#Updated August, 2015.

“who care”(may be someone 🙂 ) shared this schematic with some useful calibration tips. Thank a lot. If someone has experience in these tips, pls share your experience also.



57 thoughts on “Fixing and Reverse Engineering Cheap Temperature Controlled Soldering Iron

      1. Hi thanx for your reply that helps me alot. One last question, how you put the pcp out of the case. Just pulling out? Thanx alot again

      2. Me again , if I use a thermocouple that range is between -200°C bis 1200°C has then my soldering more temperature?
        thanx again.

      3. Hi,
        Temperature range depends on the rating of your soldering coil (heating element). Not depends on thermocouple . If your soldering coil’s max temp is 400°C, your thermocouple should cover this range. So, check your heating element first. But in short answer, you can use thermocouple (with -200°C bis 1200°C) in most soldering circuits. You need to adjust your feedback circuit to match with heating element range(In my post’s circuit, its R5 and R6).

  1. I do not know much about electronics but I would like to build this circuit for my cheap 120 volt iron.Ineed details on the ratings of the resistors.I saw a50 watt version that uses 2 trimmers but i do not know for which ones ,plus one of the caps has no designation. can u help me?

    1. Hi,
      If you are a hobbyist and not familiar with AC circuit, don’t simply try this. Better should use a cheapo soldering iron. I learned this circuit just for knowing how and never build this circuit. Also cannot grantee my circuit may correct.
      (I don’t notice your comment and my respond is a bit late. )

  2. My interpretation of the circuit is a little different. It seems to be that U1 is acting as a hysteresis comparator and U2 is a normal comparator. Resistors R5 and R6 function only as a voltage divider so that U2 sees a low voltage AC signal. The temperature range would be set by varying R1, R2, R8 and VR1. The amount of hysteresis would be set by varying R3 and R4.

    I am somewhat confused by the section after the output of U2. When U2 goes high, C would see a voltage opposite to its polarity, and LED1 and the triac gate would see fairly large voltages. Am I misinterpreting this part of the circuit?

    1. Hi Eli Gibson,
      Thanks for you attention. Honestly, I never build this circuit and my circuit may be something miss/wrong while I traced this . So, your analysis may also correct.
      But for U2, it’s supply voltage is 22V and the triac gate will see only 22V. For C, I think the polarity is wrong. I will recheck the circuit.

  3. The polarity of C (C2) is wrong, ( i checked on my board). I measured the thermocouple while it hot: 1.8mV , which formula to calculate it’s temp. i’m trying to DIY a temp display for more accuracy.
    thank you for sharing! very useful.

    1. I would also like to know the value of C.
      I’m already a few months testing heating-element A1322 to fazar a thermal tweezers to my soldering stations controlled yx 881D.
      I’ve already tested some resistances, but those that are on the market do not serve. In little correte series, parallel excess current.
      Tonight I thought of another way, combined tweezers, soldering iron + forceps working together, still will have to be with specific resistances, however easiest to work because of their characteristics.
      One of the problems with enfreto is the time to reach these elements, sometimes more than 60 days.
      I think these resistances here can Help, this reverse engineering controllable iron.>
      Wed you guys think ?.
      Thank you.

  4. Forgive my ignorance, but if I want to divide the temperature range of the iron in half (50C – 200C). What changes do I need to make. Thanks

    1. Hi,
      I want to adopt this circuit for temperature range 25-90C and fine hysteresis.
      What to target?
      R1 10k?
      VR1 5K?
      R2 10k?
      Maybe R4 500K (hysteresis width)?
      Please help.

      1. Sorry ilijavk for lately reply. You should use trail and error methods. Honestly, I cannot figure out how hysteresis should be :).

  5. I am in the process of building this circuit. It is a very smart circuit.

    The first opamp is a hysteresis comparator as said. The second opamp inverts the comparators output and in the same time generates triggering pulses to the TRIAC’s gate which are synchronized with the AC line. This is done by feeding a damped (i.e. a scaled-down) version of the AC signal to the input of the second opamp.

    Simulation shows that the potentiometer can provide a reference (compare) voltage from 8mV to 24mV, which is different from the 1.5mV-when-hot measurement mentioned in the replies. I am looking forward to measuring the comparator-set-point-voltage in the physical circuit for the entire range of the potentiometer.

    Note that I intend to use this circuit to drive a Hot-Air reflow soldering station assembly. The thermocouple in this assembly can go from 1mV all the way up to 20mV. Temperatures are easily in excess of 500 centigrade.

    Really interested to test the circuit.

    Where should I share my build? And how do you pronounce your name so that I can acknowledge you properly when done?

    Thank you.

    A maker at the Originbase makerspace, Dubai (UAE)

      1. Xin chào bạn, chúc bạn một ngày tốt lành. ^^ (i’m Vietnamese)

        i wonder why did you think Oak Kar is Vietnamese. ^^ (his nick name and his language didn’t show any thing related to Vietnam). 🙂

        Back to the topic, I measured the thermocouple while it hot: 1.8mV, long time ago, i decided to DIY a temperture display on the iron. I tried to find out the formula that convert that milivone to Celcius.
        Is that feedback voltage linear relation with temperature?
        I have no equiment to test the relation, i multiplied by [a large number] to calculate the Celcius and the result is not accurate. Will you build a temperture display on your soldering station ?

        By the way, his name is Oak Kar (in ABOUT PAGE ^^). Nice guy. Useful topic.

  6. I have a question about this iron’s maximum soldering temperature. You said you were quite happy with your iron’s performance. I have what appears, from the photos, to be an identical model.

    I now have had the opportunity to use it to repair cold joints on a surface mount board. My impression is that the tip fails to get hot enough to make the solder flow properly. I am using Radio Shack solder, of the standard type, intended for electronic applications. But, even the solder already on the board is ‘reluctant’ to melt sufficiently.

    I am going to disassemble it to see what’s wrong. Since you have already ‘been there, done that’ much appreciated if you could give me one or two likely problems to look for?

    Is a construction fault possible? Could the thermocouple be out of position? Any other ideas? Otherwise it’s a nice iron, but unless I can repair it, it won’t be very useful 😦

    1. Hi marlosol,
      I used this iron and It works well. I never set max temp (400C). I set the control knob about 300C and it does the job. I never test temperature accuracy of this iron but most lead soldering temp is about 300~350 C and I assume this iron work at standard temp.
      For your iron, I am not sure what is going on. You can check connections, solder joints first.

      BE SURE to do that with caution. THIS CIRCUIT is a LIVE CIRCUIT (I notice that you have experience in this but take care).

      you can test heat coil by connecting directly with ac power source. So you can check heat element work well or not with max temp.
      Also check the placement of thermocouple. Then, also test thermocouple by using a DMM with temperature reading.
      If both heatelement and thermocouple are working well, check the circuit. It is not a complex one.
      Hopefully your iron will soon work well.

  7. Thank you very much for your troubleshooting directions. You have given me some good ideas from your observation of the iron and its construction. I think I will begin by setting the temperature of 450º and then observing the pilot light to see if the heating element cycles on and off, or stays on constantly. I suspect in a correctly-functioning iron, it should stay on constantly, or nearly so. This may give me a clue as to what to look for, when I begin disassembly.

  8. At the maximum temperature setting, the pilot light is mostly on, with only brief periods of flashing off. The only thing I can think of is poor thermal conduction between the soldering tip and the heating element. Seems any other cause of a low temperature (low line voltage, etc.) would be compensated for by feedback from the thermocouple. I will have to give this some through … any ideas on your end?

    Perhaps I’ll give it a try with a direct 220v connection to the heating element, as you suggested.

    1. seem controller circuit work well. before direct connection test, how about measuring the controlled voltage (at pair output of heat element) with multimeter?

  9. Good idea. I’m getting 230VAC, measuring at the wires that lead directly to the iron’s hearing element. The wall outlet is measuring 232VAC with the same digital multimeter. Do you see any problem with these values?

  10. If these voltages look OK, another possibility has occurred to me. The thermocouple measures heating element, not soldering tip, temperature. When you use your iron, do you have to compensate for this?

  11. I have been checking references on the Internet: the official EU voltage is now 230v (up from the former value of 220v) with the exception of the UK (voltage tolerances were ‘fiddled’ by EU authorities to allow them to remain at 240v).

    So, because I am taking the measurements in Portugal, the voltages I measured do not appear to be the problem.

    I am now looking at the screws holding the tip inside the heating element. If the small screw is tightened first, the tip’s rod is pushed toward the center of the heating element cylinder. If the large screw is tightened first, the tip’s rod is pushed into firm contact with the heating element’s interior wall.

    I just noticed this, but haven’t yet reassembled iron for a soldering-test. Can you confirm this, and have you any other ideas?

    1. At least, on the iron I have, the soldering tip’s rod presents a very-slight gap on one side of the heating element chamber. It would be better if its diameter were a fraction of a millimeter larger. If your tip fits exactly, this would eliminate the sensitivity to positioning by the setscrews. In this case it would be a manufacturing quality-control problem, with different production runs being provided with tips of varying diameters.

      1. My iron also has a little clearance like you said. I don’t think it may be problem. My old fixed temp normal iron also has the same clearance. So, I don’t think it would be the root case. If you still suspect heatelement, should try direct connection and test soldering like normal iron.

  12. If it continues to fail heating sufficiently, I will take your suggestion to try a direct connection of the heating element to the line voltage direct from a socket. According to the measurements you suggested I make, this will give me 232 v on the element, as opposed to what I now have, 230v. This seems a very small increase, but I will try anything to make the iron useful.

    If the element is defective, given the local (non-) availability of a replacement part, the cost of replacing the element vs an entirely new iron, I think I will replace and keep the old one for parts. I’m well beyond the guarantee period when an exchange would be honored. As well, I would be liable for postage costs for its return to China — several times the cost of buying new.

    Right now the iron is reassembled, and waiting for the next soldering job, to test it, in practice. The rod-section of the soldering tip has a fine line of ZP-340 thermal compound along its length, positioned with this compound bead facing the heating element’s wall, and with the large screw opposite to this, tightened as much as I dare without stripping the threads. If this doesn’t work, it’s time to get another order off to DX … 😉

    I never imagined when I ordered this item I could get technical assistance and have a circuit diagram as well. The iron didn’t even come with written directions, at least not in English. Thanks very much, you are performing a wonderful service! 🙂

      1. Feedback on my last repair attempt: yes, it is now working fine, just as you reported that yours does. Completely satisfactory. I am very happy there is nothing wrong with the internal circuit or heating element.
        I have seen a newer model, same price, at Banggood, where a hollow tip slips over the heating element. I expect this will resolve heat transfer problems, between the heating element and the soldering tip, itself.
        Thanks again for the advice and ‘tech support’.

  13. better quality one is this using this model for a long time


  14. I have the same type of soldering iron, and it failed some point. With this excellent post, I managed to fix it. In my case the triac, the zener diode, and the ic failed. Cost around 1$, works as before 🙂

  15. i like your work sir! but can you please explain what the improvements of the updated version is, and how they work? it really confuses me.

  16. This product is no doubt , but i have experienced that the thermocouple wire corrodes after a period of time due to the fact that it runs from the center of the element where the heat id much higher than the tip ,
    i would use a mch ceramiic element with a built in sensor ,
    please comment .

    1. Yes pard, a ceramic element would be better. Another thing is this type of controller uses bang bang control (just ON/OFF). Today, you can get or build soldering iron with better controller with cheaper price.

  17. I have almost the same soldering iron. It worked a while and then stopped heating . led is up but not heating . I don’t know much about electronics, it looks the problem might be the same as you had with one you repaired……. Can you post or send me some more photo what you re-solder on yours .

    re this this :
    I The fault was nothing. The soldering joint of heat coil was dry. Soldering directly to a nichrome coil is not a good idea in such circuit. It should be crimped. But this is no room for this and I just do re-soldering the joint with acid paste flux (which can help to solder metal joint like nichrome, tin sheet ect.. Don’t forget to clean the joint after soldering, acid from flux may conduct your circuit lines). Then, the iron worked again.

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