Tuesday, 20 October 2015

Weller soldering iron add-on.

Now I'm a great fan of the Weller TCP "Magnastat" iron; robust, simple, great parts availability. Yes, there are better irons out there, but these are usually available secondhand for peanuts.

They are available in various wattages, and I have two, both 50W, one with a nice fat screwdriver type tip, for valve stuff and tagstrip etc, and a slender one, for more delicate work.

The tip has a temperature rating stamped on it's base. This contains a special magnet, which loses it's magnetism at a certain temperature (known as Curie temperature). This magnetism is used to operate a magnetic switch (like a reed switch, but a little more robust) to switch the element on and off, which heats the tip.

As bench space is limited, I only use one power supply, and got sick and tired of pulling one iron plug out of the power supply, and plugging the other in when I wanted to switch between irons. What was needed was a switch box....

... then I got a bit geeky (well, there's a surprise!). Let's have an led on the box, to show when the iron is heating. Useful to know if the iron is up to temperature, or if a fault has occurred (occasionally the switch or element fail , see here ) , so I devised a simple circuit to put an LED on when the iron is drawing current.

Simple. When the iron is drawing current, there is a small voltage dropped across R1, which causes the op-amp IC1A to attempt to drive it's output to rail. Now the current flowing in R1 is obviously AC, so the output is a 50Hz (or 60Hz, depending on your mains) waveform, but it's enough to light a red LED. The power supply for the op-amp is a crude supply derived from the 24VAC from the weller PSU. It's half-wave rectified by D1, and D3 and R2 Clamp the voltage peaks to 5.1V. It's smoothed by C1.

It's crude, horrible but works!


  1. I suppose you could consider one of those AC only LEDs now available? Probably just two LEDs back to back and more expensive than we have lying around in our workshops... Just a thought.

  2. Could do, but it's not turn AC, it's just half wave-rectified 0-5VDC, not swinging either side, as there's no other side to swing to!