Resistance Soldering

Resistance soldering is a technique similar to welding where a large electric current is passed through the joint of two metal objects. The electrical resistance of the joint itself causes localised heating – enough to cause normal electrical solder to melt and complete the joint. It can be a very effective way of soldering to heavy metal objects (such as rail) that conduct heat away from the joint too quickly to allow a convential soldering iron to work properly. It can also help minimise damage to nearby plastic rail ties!

Commercial resistance soldering stations are quite expensive – yet the principle behind the technique is very simple. Here are some ideas on how you can go about building your own resistance soldering iron.

Scope Transformer

Close-up of old Scope Transformer rated at 5V, 30A intermittent.

The set-up is extremely simple. You need a “Scope” low voltage transformer capable of delivering ~30A across a short circuit without blowing up. “Scope” soldering irons date back to the glory days of valve radios and amplifiers, and they are not main stream items any more. For an interesting history of their rise and fall, head over to the Australian Valve Amps web site.

You can still buy them new from a few places. I tried a quick search on Google for “scope soldering irons” and came up with quite a few hits: Wiltronics in Victoria look promising listing Scope transformers new for A$94 each. eBay is another good place to try to find a second hand one.

The transformer we are using was manufactured locally and is quite old. One of our members has had it for years. As you can see from the picture it is rated at 5V, 30A intermittent.

Resistance Soldering Iron made from an old broom handle, some copper water pipe, and the core of a 'D' cell battery.

Resistance Soldering Iron made from an old broom handle, some copper water pipe, and the core of a ‘D’ cell battery.

The tip is made from the end of broomstick, a short length of copper water pipe and the carbon core from a standard ‘D’ cell battery (carbon cell of course). Experience has shown that the cheaper the battery the better. Name brands (like Eveready or Duracell) tend to have quite porous, soft carbon rods. The really cheap ones (like you get as “own brand” in a supermarket or from salvage disposal stores) seem to use much harder carbon – which lasts longer in this application. Take care when cutting a dry cell battery open – it can be a messy job. Use some sand paper to file a conical tip onto the carbon rod.

You also need some two core flexible wire – pretty heavy gauge (it has to carry 30A after all!). I recommend some heavy duty speaker wire – basically the thicker the better.

Complete resistance soldering kit.

Complete resistance soldering kit.

Drill a small hole through the middle of the broomstick to accept the wire and a larger hole half way through to accept the copper pipe as a press fit. Thread one wire through the broomstick and solder to the inside of the copper pipe. You want this joint to be very solid – spread it out over us much area of the copper that you can. Too small an area and it might get too hot itself during use and cause the solder to melt.

Cut a slit (with a Dremel or similar) into the other end of the copper pipe. This is so it can be expanded enough to accept the carbon rod – which itself is a simple press fit into the copper pipe. Press fit it all together. Put a large alligator clip on the end of the other wire. Attach the other ends of the two wires to the low voltage output of the transformer.

To use, attach the alligator clip to the rail. Hold the broomstick handle in one hand. Plug in the mains side of the transformer. Using the carbon tip, press the wire dropper you are soldering to the track on to the rail. Using your other hand simultaneously apply the solder – normal resin cored electronic solder is used. It gets hot very fast – there is 150W of heat concentrated right on the tip of the carbon rod as soon as you press it against the rail! Even though this is low voltage, you should wear safety glasses as there is a potential for sparks. It is possible to do it all by yourself but typically at the club it is done as a two person operation – one person holding the iron and the solder, the other person holding the dropper in position. This allows you to remove the iron to allow the solder to cool. Adding a foot switch on to the primary (mains) side of the transformer is a useful addition to allow one person operation.

As with convential soldering, cleanliness is paramount. If the joint is dirty, not only will the solder not stick to it, but it will probably not conduct much electricity and won’t get hot in the first place! I always recommend scraping the joint surfaces with a small file and/or “wet-and-dry” paper. Some club members like applying extra resin (or bees wax) prior to soldering, but I have never found it to be necessary myself.

Experiment and have fun!

For lots more information on resistance soldering techniques, try searching Google for “resistance soldering”.

Since passing this information on to other clubs, we received some useful feedback and suggestions. Instead of using a broom handle you can use an old soldering iron handle to hold the carbon rod. A size ‘C’ carbon cell battery has a small enough carbon rod for this purpose.

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