RSUs are available from a number of manufacturers, such as Exactoscale (around $350 plus freight) and P-B-L (about $770 plus freight). Until recently, I had not seen them for sale locally, but I note from the April 1999 Journal that Von Strapp Forging Co-op now lists them ($400). I cannot comment on this latter unit, as I have not had the opportunity to examine one. In any case, it is easy and not too expensive to make an RSU, and this article describes a proven device that works safely and well for a cost of around $150. There are undoubtedly other viable possibilities, but do beware the exponent of recycling bits of old tellies unless they (a) can demonstrate it actually works, and (b) have the skills to do so safely.
Before launching into a description of how to build an RSU, perhaps a few words on why one would want to. Resistance soldering has become popular in the UK with the advent and success of etched brass kits. Etched kits are creeping onto the New Zealand market too, but in any case RSUs are equally handy for scratchbuilding in brass sheet.
Resistance soldering works by generating heat with a low voltage and high current. As in arc welding, the work forms part of the circuit. Heating occurs either within, or on the surface of, the work rather than within the iron as is usual. Some readers may be familiar with �Scope� soldering irons that generate heat within the body of the iron in a similar fashion as an RSU.
The RSU generates a lot of heat but it is localised in the right place. The heat is generated rapidly and briefly, which means less burnt fingers. Another major advantage is that because the probe cools rapidly it can be used to hold the work. This fast heat delivery to just the right spot is one side of the success equation. The other requirement is efficient solder delivery. This is commonly done using a solder paint (such as Carr�s 188), but pre-tinning parts with a conventional iron and solder also works well.
An RSU will enable you to make rapid, reliable, clean soldered joints in sheet metal. It is possible to achieve similar results by other means, but usually more difficult.
I�ve been using the technique for a couple of years now with the home-made unit described here. The cost of commercial units and the idea of freighting a heavy transformer from overseas prompted the Wednesday Wafflers to look for a local solution. The result was an unqualified success and three of the units are in use, with a few more on the way.
In its simplest form a resistance soldering set-up might comprise the workpiece earthed to one side of an appropriate power source, with the circuit completed by a carbon probe connected to the other side. The probe is positioned at the spot where the heat is required. It really is that simple. Some form of control is needed, however, and some practical aspects are also worthy of mention. The following components are essential: a suitable transformer as a power source, a means of switching the power on and off, a means of varying the power, a foot switch, a good earthing set up and an appropriate probe.
Transformer
The usual way of generating the low voltage, high current supply we need is to use a mains transformer. Doubtless there are solid state methods, but I have not encountered an RSU that uses them. For our project we had transformers custom wound to our specifications by Electro Products and Services, 11 Darroch St, Belfast, Christchurch. Earlier this year the cost was $110. Not cheap, but only about a third of the cost of a commercial RSU and the only major cost. Ours were rated 150VA with 0, 1, 1.5, 2, 3 and 4.5V tappings. No doubt a less muscular unit would be cheaper.
To achieve the power rating the secondary is wound from solid 1/8-inch copper! In hindsight 150VA was overkill as it allows 50A at 3V or over 30A at 4.5V, continuous. The latter will not only blow a hole through sheet brass, but also generates a carbon arc that will damage unprotected eyes! Initially this aspect is rather fun to play with, but it is rather destructive, and I have yet to find a use for it in model construction.
An RSU requires only intermittent power and 20% duty time ought to be adequate. Thus, a 30VA transformer should be quite sufficient, though thermal protection might be advisable. In practice I rarely need even the 3V setting, with 2V or 2.5V (obtained by connecting across the 2V and 4.5V windings) being sufficient for most uses.
Control
There are two common means of controlling the power output of an RSU. The first is to control the voltage on the primary or mains side of the transformer. I am told this restricts the power output, and does not seem to be favoured in information I have read, though it is certainly possible. If you go this way, do not use a cheap solid state light dimmer. These are for resistive loads only and are liable to fail catastrophically in this inductive situation. Only use a controller designed for this type of application.
The second method of control, and the one we chose, is to have a transformer with tappings for various voltages as described above. This is simple, but will probably need to be custom made. With this second method of control you need to alter the connections whenever the voltage setting is changed, and you have stepped rather than continuous voltage control. In practice this is not a problem. Settings do not need to be changed often and the wing-nut arrangement of terminals shown is convenient enough.
Foot switch
This is essential as it leaves the hands free for more important things. I made mine from a cast-off sewing machine foot pedal (Free from your local sewing machine repair person) and an appropriately rated micro-switch. I connected this to a PDL Cat 40A plug (available from electrical wholesalers.) These look like a standard tap-on plug but have a red or orange base. Inside there�s an extra phase-side terminal that allows interruption of the tap-on circuit. Connecting the foot switch to these terminals allows it to control whatever is plugged in, and means that my foot switch can be used to control any of my power tools � a useful bonus. Foot switches are, of course, available commercially too.
Probe
he probe needs to be a piece of carbon rod. I have used the rods from inside dry cell batteries, but welders� gouging rods are very cheap (50 cents will last a long time) and work well. The probe needs to be shaped to a point and I tend to do mine batch-wise in the lathe. Filing will do though, and I suspect a pencil sharpener might also serve. A holder is needed, and I have adapted a cheap aluminium modelling knife (less than $10) by drilling the clutch arrangement to grip the carbon rod. I wired the probe and earth strap using hi-fi speaker cable. This has a sizeable cross section to carry the power without overheating and is quite flexible. Many other cables of adequate size are inconveniently rigid. The cables are not permanently wired to the probe, but are terminated in a brass stub. This fits in a bore and can be secured by a set screw. The advantage of this is that different probes etc. can be fitted to a single set of leads.
Earthing
t is important that the work is well earthed to make sure that there is good current flow where needed. I have a brass clamp for this purpose, but I also sometimes wire up a vice. Another approach is to earth a steel work surface. This allows the work to be held in place by magnets, but more on this later. One thing I do not suggest is an alligator clip. Arcing can occur where the teeth contact the work and this is undesirable.
Assembly
bolted my transformer into an aluminium box from Dick Smith Electronics (about $12 on sale). The wires are terminated on hefty 6mm brass bolts mounted on a substantial piece of Tufnol fixed to the front of the box. The leads attach with wing nuts. I took care to make sure that all current carrying components had an adequate cross section to make sure only the desired portions (the job!) heated up. Lightweight terminals or connections should be avoided.
Conclusions
n hindsight, it is evident that the whole unit described here is somewhat over-engineered. At the design stage, however, we had little information, and I am not unhappy that we erred on the side of caution. The pictures show the general idea.
An RSU is not essential. There again, neither is a power screwdriver, but both make life a little easier. Resistance soldering works brilliantly on sheet metal because heat transfer to the joint is very rapid. As so much heat is generated in such a small area, I would keep the RSU well clear of whitemetal. Large brass castings are also not that amenable to resistance soldering as so much heat is soaked up by the casting that the advantages of the resistance technique are lost. Joining small parts to large parts works very well though.
Using the RSU
Before building or buying an RSU, you should probably ask yourself how and why you might use it. Hopefully the accompanying article has suggested why, so now a few words on how�step by step.
� As with all soldering operations, cleaning the parts is recommended. This seems to be less important with RSUs though. Heat transfer is not usually a limiting factor!
� Next, sparingly apply solder paint to the parts (or tin with a conventional iron and apply liquid flux).
� Earth the workpiece to the unit by some convenient means. Usually I just wire up the vice, or whatever clamp I�m using to stop the work scooting around the workbench.
� Position the parts and gently press down on them with the probe.
� When you are happy that everything is just how you want it, press the foot switch. There is a hum from the transformer, the probe may glow a little and the flux sizzles. A second or two is usually more than adequate.
� Release the foot switch and let everything cool for a moment.
� Finally, remove the probe and admire the result.
With practice, joints need virtually no cleaning up. This saves a great deal of time and generally results in a somewhat crisper result than traditional soldering. If the procedure seems easier than the usual tussle between reluctant brass and temperamental tin, then I have conveyed the desired impression.
If you feel the need to further simplify matters, Exactoscale�s Universal Assembly Jig is particularly useful because the RSU can be earthed to it, avoiding the need to make earth connections for each new job. It uses magnets to hold the work, and while Exactoscale sell high strength ones (which are excellent), something more ordinary will suffice. The jig is basically a steel plate, but you can add accessories, such as perpendicular guides that aid rapid, repeatable and perfectly square joints. Because the RSU�s heat is so localised, heat loss to the jig is not a problem.
2. (bottom left) The foot switch Lawrence adapted from a sewing machine speed control. The rectangular block right of centre is the micro-switch he fitted in place of the original variable resistance. Note the earth connection to the box�very important! Despite the two other wires being different colours they are both on the phase side. They run back to the PDL Cat 40A plug, which has extra terminals that allow power to the phase tap-on terminal to be interrupted.
3. (bottom right) The probe, earth clamp and spare carbon tips. Note the set screw holding the cable end into the clamp. This allows for easy swapping to some other earthing attachment. That�s hi-fi cable, heavy but nice and flexible.
4. (top right) The unit is very simple inside, just a medium-sized transformer with v-e-r-y heavy secondary windings. Care has to be taken to make safe mains power connections to the transformer primary and to reliably earth the metal case. The 3mm copper secondary windings are soldered into holes drilled into heavy turned-brass washers. Heavy brass screws mounted in a Tufnol panel with the necessary washers and lock nuts act as terminals.
Journal and posted here with Mr. Boul's conscent.
supply,and that at time of writing
NZ$1 = US 40 cents.