Answer 3—There are at least two different soldering systems based on Curie point temperature control, the Weller WTCP series of irons and the Metcal system. In each case, the tip itself controls the temperature.

In the Weller system, the back part of the tip is made of a ferrite material that has a Curie point of approximately 600°, 700°, or 800°F as indicated by a "6," "7," or "8," respectively, stamped on the end.

A mechanical switch that has a magnet as an actuator controls power to the heater. When the tip is cold, the tip attracts the magnet and the switch is on. When the tip gets to its Curie point, it no longer attracts the magnet, which is released, and the switch turns off. When the tip cools below its Curie point again, the cycle repeats. This system has the advantages of being simple, inexpensive, and rugged, but the opening and closing of the switch tends to induce all kinds of RFI in nearby equipment.

The Metcal system uses Curie point in a different way. The tip is primarily copper but with a nickel-iron alloy plated on the surface. A high-power RF oscillator is magnetically coupled to the tip. When the plating is below it's Curie point, it absorbs the energy and, being relatively resistive, it heats up. When the plating reaches its Curie point, the induced current flows through the copper core instead and, because its resistivity is much lower, the heating is much less.

This system provides tighter control of the temperature and less RFI (except at the oscillator frequency), but it is more complex, and both the base and tips are more expensive.

Contributor: David Tweed

Published November 2003