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I got a used Metcal station from ebay for my home-lab and everything I read about soldering with it is right, it is a completely different feeling.

I understood that stuff about controling the temperature using the Curie-effect in the soldering tip.

But I do not understand why they use 13MHz HF power to heat the tip. Why not DC, switched DC or 50HZ or 60Hz AC?

wollud1969
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1 Answers1

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The effect of the Curie point is to change the skin effect. Skin effect is an AC effect (the depth of penetration of an AC current) and is approximated by:

\$\delta=\sqrt{{2\rho }\over{\omega\mu_r\mu_0}}\$

where ρ = resistivity of the conductor

ω = angular frequency of current = 2π × frequency

\$\mu_r \$ = relative magnetic permeability of the conductor

\$\mu_0 \$ = the permeability of free space

As you can see, the skin effect is inversely proportional to the square root of the relative magnetic permeability of the conductor. So, if the permeability drops due to increasing temperature near the Curie point, the penetration depth increases.

As shown in this source, the Metcal irons have a construction with an outer layer of metal on a highly conductive core as so:

enter image description here

When they are energized with a constant-current RF source, the current is shunted by the conductive core when the permeability is low (temperature too high) and thus the heating decreases. At low temperatures, the permeability is high and the current flows mostly through the resistive outer layer, causing a large amount of heating. Thus, the temperature is maintained near the Curie point.

The actual physical configuration, as shown in the patent US 4,877,944, is apparently more like this:

enter image description here

The transformer configuration is used to match the heater impedance to the impedance of the coaxial cable feeding the hand piece.

It's not impossible to make such a device work at 60Hz, but few would want a soldering iron with a 150mm (6") diameter tip, it would make it difficult to solder SMT parts.

Spehro Pefhany
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  • Excuse my naive question but what is the point of this fancy induction sensing? Why can't an IR sensor measure temp and control the output of the induction coil? – user391339 Nov 03 '18 at 13:16
  • I can’t see how one would use an IR sensor that way and still make a useful soldering iron but maybe you can. – Spehro Pefhany Nov 03 '18 at 14:00
  • Or a contact sensor for that matter, such as a thermocouple. There must be a significant bottleneck somewhere in closing the temperature loop using other more traditional strategies. I guess the above sensors are not physically close enough to the tip to accurately measure the temperature, where as here the tip itself is the sensor and feedback loop. – user391339 Nov 03 '18 at 16:30
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    A simple thermocouple is a traditional way, as is the magnestat method use by Weller WTCP. There are subtleties, both in the heating and in the sensing. You'd prefer all the sensing and all the heating to fit in about a very narrow space as close to the working end of the tip as possible, which is not easy. – Spehro Pefhany Nov 03 '18 at 16:53
  • The "magic" of curie point irons is that they are using physics to servo each square millimeter of the surface of the tip to a particular temperature, instead of the average temperature of the tip as a whole. Why heat up the whole tip when you can just put heat into the parts that get cold? – Cameron Tacklind Jul 18 '20 at 20:07