Ferromagnetic soldering iron control

-May 15, 2014

 

Presenting one of the runners-up in the TI LDC1000 inductive sensor design contest.

The LDC1000 inductive sensor can be used to make a fixed temperature soldering iron. The temperature will be determined by the properties of a ferromagnetic sensor.

Ferromagnetic materials have high magnetic permeability (>>1), but at high temperature this value goes down. The temperature at which a ferromagnetic body becomes nonmagnetic is called the Curie temperature (Curie point). It is possible to choose various proportions of nickel, iron, and carbon to make tip materials whose Curie point lies within the commonly used temperatures for soldering.

A small rod (2 to 4 inches) of ferromagnetic alloy should be chosen first. Part of the rod is wound using bifilar winding (non-inductive) of nichrome wire. Current is passed through the nichrome wire to heat up the rod. After the nichrome winding, part of the rod should be used for copper wire winding of a few turns. This winding will serve as a sensing coil for the LDC1000. The rest of the rod can be shaped as a soldering iron tip with a ceramic layer.

The LDC1000 is connected to a microcontroller  programmed to give voltage output proportional to the inductance of the sense coil. This voltage is used to control a triac for the heating coil. When the iron is cold, the triac delivers full power to the heater, and the inductance of the sense coil is high. As the temperature increases, the permeability of the ferromagnetic rod goes down slowly, as does the inductance of the sense coil. At the Curie temperature, the permeability reaches 1 and the inductance reaches the minimum. The microcontroller can be programmed to turn down the triac current once this Curie point is reached. Full details of the design are in this PDF.

Note: As it is not easy to find a soldering tip with a magnetic material whose curie temperature is within the melting point of solder, a little change was made. I used an ordinary tip, and used a ferrite cylinder (as used for EMI filters) instead, sliding the iron tip through the hole. These ferrite cylinders have Curie temperatures in the range of 120ºC-250ºC. I used a ferrite cylinder whose inner diameter is a little more than the tip diameter. However, its Curie temperature (175ºC) (Laird Signal Integrity Products: 28b0375-300) is little lower than the melting point of the commonly used lead solder (~185ºC). But, it shows that this technique can be utilized if suitable material can be deposited on part of the iron-tip.

 
Sensing Design Challenge 2013 winners:



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