Smart Soldering Iron Adaptor

Every electronics dabbler must have a Soldering Iron. If you wanted a Smart Soldering Iron, you could pick one up from the components vendor near to you, but where the hell is the fun in that? Now it is very easy to build your own by modifying an ordinary one into a smart one with the help of the little adaptor presented here!

The commonly used 60/40 solder wire melts at about 200 degree Centigrade, but in practice the soldering iron tip should be above 370 degree centigrade to make a good solder joint within a stipulated time. At this temperature level, the tip oxidises fastly, requiring regular cleaning. Here is a simple circuit solution to this annoying problem as the circuit helps to control the tip heating level by controlling the heating element through a phase-controlled triac. This not only reduces the oxidisation, but also increases the life time of the soldering iron.

Soldering iron adapter

The circuit operates on AC230V supply, and is optimized for 230VAC Soldering Irons. Usually, the temperature at which a soldering iron normally operates is too high for the purpose, a simple internal modification is employed by the maker to halve the power consumption and thus lower the temperature. The modification consists of fitting an ordinary diode (1N4007) in series with the heaing coil. It is mandatory to bypass this diode to connect the soldering iron with our controller circuit.

As stated above, the controller is exclusively designed for soldering irons working on 230VAC mains supply. Here a BT139 Triac (T1) is used for phase control of the ‘resistive’ heating coil load. The 1M preset pot (P1) is an adjustable trimming potentiometer to tweak the circuit operation. The Red LED (LED1) and associated components builds a flashing activity indicator. After construction and ‘pre-flight’ testing, the whole circuit should be enclosed in a suitable non-metallic box fitted with an suitable female power output connector. Note that, the current configuration of this ‘Soldering Iron Presever’ circuit is not suited for control down the heating power to zero.

T1:BT139, T2:BC547, D1:DB3 DIAC, D2&D3: 1N4007, C1:47nF/400V, C2:220uF/25V, R1&R3:470K, R2: 2K6, R4:100R, P1:2M2, LED:5mm Red (All resistors are ¼W)

Soldering iron enclosure


  • Jumper point JP1, reserved for future, is a zero ohm jumper by default. This allows you to connect a thermistor in its place to limit the inrush current to the heater load (NTC type thermistor), or to build a self-regulating heater load (PTC type thermistor), etc
  • Although the power dissipation of the triac (T1) is not high, a small heat sink should be provided. The heat sink should not be allowed to come in contact with any external metallic objects
  • Prototype tested with a 25W soldering iron. The ceramic heater have a heating coil resistance of about 1K Ohm
  • If you want to access P1 from external world, replace Preset POT P1 with a 2M2 plastic shaft potentiometer, and fit it on the front panel of the enclosure

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