Resistance Soldering: Electrodes

With transformer, circuitry, and firmware in hand, the final step is to get juice to the workpiece. Resistance soldering depends on passing a high current through a relatively low resistance: the power varies as the square of the current, so more current is much better.

The catch is that the transformer produces a relatively low voltage, so the initial circuit resistance must be exceedingly low. With 5 Vrms from the transformer secondary, a mere 0.5 Ω in the secondary circuit limits the maximum current to 10 A. Even with most of that in the joint, it’s not gonna work as you expect.

I scrounged some very flexible 6-conductor signal cable with several shields that, with everything conductive crimped-and-soldered together on both ends, worked out to 1 mΩ/foot. A pair of 7-foot leads with copper lugs swaged-and-soldered on each end, bolted together to the transformer secondary, produced 280 A at 4.1 V: a bit over a kilowatt. The secondary winding and lugs evidently contribute 4 mΩ of resistance to the total.

CAUTION – That much current makes the cables twitch in their own magnetic field. If you wear finger rings, bracelets, or metallic body jewelry, remove it. A metallic ring looks like a single shorted secondary winding that can couple magnetic flux from the cables and get surprisingly hot surprisingly quickly.

Don’t put the rings in your pocket, either. Your pants are not a good magnetic shield.

With that in mind, some electrodes:


The black block is a slab of machinable graphite clamped to a brass plate that probably served as a wall bracket in a previous life. It serves as a nice base for most operations: conductive, non-sticky for solder, doesn’t produce nasty arc scorches. A cable from the secondary bolts firmly to the brass plate and the vast surface area provides a low-resistance contact. The cheesy plastic clamps work fine: the block doesn’t get too hot.

The huge electrode comes from a carbon-arc spotlight. It’s actually too long, with too much resistance, and doesn’t work well at all.

The tiny electrode is a steel welding rod (for gas welding). It works well for very small setups, but has essentially no resistance and requires low duty cycles.

The Goldilocks electrode in the middle is a length of 5/32-inch carbon gouging rod (for arc welding). It has a copper coating that tends to burn off near the tip, but the overall resistance remains low enough that the joint heats well. The middle glows yellow-hot if you overdo things, hence the discolored section.

To date, I’ve used a few inches off the end of one 12-inch rod. There are 49 more rods in the package. If you build one of these things and don’t want to pass a similar box along to your heirs, drop me a note and I’ll send you a rod.

The scrap box emitted a sturdy cardboard tube that slipped over the cable so well that I simply gave up thinking about making an actual handle with a contact switch and all that stuff.

Resistance soldering electrode handle
Resistance soldering electrode handle

All the electrodes terminate in homebrew clamps made from copper lugs that bolt to the transformer’s secondary terminals with 10-32 machine screws. The gouging rod has steel rings (forged from husky wire) holding the lug closed around the rod; they’re a pain to (re)move, but ensure very solid contact. The cable termination is swaged-and-soldered.

AA Cell Clamping Pliers
AA Cell Clamping Pliers
Center Electrode - Front Detail
Center Electrode - Front Detail

I have not yet conjured up a pair of tweezers, as almost everything I’ve done has been suitable for pressing against the carbon plate.

One exception: a pair of snap-ring pliers became a clamp for AA cell positive terminals and worked well in that capacity, along with a repurposed oil burner tungsten electrode that even provided its own ceramic handle.

If I ever get around to building tweezers, I’d probably use chunks of that tungsten rod. It’d be easy to put a contact switch in there, too.

Right after I got it working, I grabbed some copper junk and tried it out:

Resistance soldering test pieces
Resistance soldering test pieces

Notice that there’s not enough heat in the surrounding metal to discolor it. I’m not always that lucky good, but it’s possible.

The copper wire was instructive: even though copper is a great conductor, the joint is the lowest-resistance part of the circuit and gets all the heat. If you think of it as a parallel circuit, the ring has a relatively high resistance and sees much less current.

Cleanliness and good joint preparation are vital, because any nontrivial resistance will reduce the heat to zilch. The tip of the carbon electrode sometimes acquires an insulating flux coating; a swipe on a file solves that problem

Solder foil works well, because the current passes through the solder and starts the melting process in the middle of the joint. That’s easier than fiddling with solder wire, although your mileage may vary depending on what the joint looks like.

Projects done with the equipment you’ve seen…

It’s a great tool; I wonder how I got along without it.

Now, I really must put that widowmaker breadboard inside an enclosure. There’s a dead dehumidifier near the bench (slated to contribute its compressor as a vacuum pump for a hold-down chuck on the mill) with a case that just might be the right size…

3 thoughts on “Resistance Soldering: Electrodes

  1. I have a fairly large number — a couple hundred — of tungsten arc lamp electrodes, that are as I recall about 1/8 inch in diameter by 2 inches long, with rounded ends. They’ve been sitting in an unopened bag for 20 years. If you’d like some to experiment with them I’d send you a bunch. But, tungsten electrodes for TIG welders are dirt cheap and they’re available pure or with interesting alloys.

    1. Sounds like those are pure tungsten, which is probably exactly right for a non-stick electrode. If I can get that working, then I’ll try thoriated TIG electrodes.

      A few percent of your lifetime supply should serve my needs forever… thanks!

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