Any suggestions for a home made Resistance Soldering Unit?

I want to diversify in my etched kit railway building, and resistance soldering seems like the ideal technique.

There are limited commercial offerings, however I'm tempted to build one myself (being fairly competent with electronics). However I'm struggling to find a reliable reference for the hobbyist grade RSU.

Any recommendations appreciated.

Bump.

I saw one demonstrated at a show once and the machine was just a box, some clamps on a steel plate and an electrode with a handle. Aren't they just a transformer giving about 50A at a couple of volts AC - a hundred watts or so? Might be as simple as replacing the high voltage windings of a microwave oven transformer with a few turns of thick wire. Long time ago - if they explained I've forgotten.

I don't think there is much electronics involved in an RSU. It's basically a mains transformer with a tapped, low voltage, high amperage secondary providing the variable power settings by selecting from the tappings. The one I own has a 4.5v secondary with two intermediate tappings which gives six permutations of voltages up to 4.5V AC (measured using my meter so not peak to peak). I believe the power switching is done on the primary side of the transformer using a relay. I think the problem will be sourcing a transformer with a suitable secondary winding, or else wind one yourself.

I believe that some people have used power sources like a transformer from a six volt battery charger to do the job but with no tappings you then lose the ability to reduce the power for finer work.

Jim.

I wonder if one could be made frome a battery and a pwm controller ?

I thought that too and got as far as these MOSFETs from Farnell which are beefy for a few quid.

But then I remembered PWM is DC and can strike an arc. Very hot, capable of melting steel while giving your retinas ultra-violet sunburn. AC is more suitable for this because it automatically extinguishes any arc that forms 50 times a second.

Last time I took a microwave apart the transformer came apart easily. The hard part is finding a few metres of hefty magnet wire to make a new low voltage secondary. ebay only goes up to 2mm (30Amps) when 3 or 3.5mm would be more like it for a cool 50/60A. I suppose you could wind a pair of 2mm secondaries and parallel them up for 60A. Ebay have motor insulating varnish so you could make your own from bare copper. Brake pipe perhaps?

ebay sell carbon electrodes too, so they're easy enough to get. Carbon rods out of dry batteries probably not good for this because they contain copper to lower the resistance. Possibly solid carbon fibre rod might work, though I can imagine it disintegrating or worse!

Dave

In a "Shop Mechanics Handbook" that I have from about 1958 (published by Science and Mechanics magazine) there is a construction article for a soldering transformer (RSU). It uses a home wound transformer using the laminations from a scrap transformer. It has an output of 100 amps at 5 volts with a taped primary to give two heat levels. The laminations are standard "E" type 5 3/16" by 3 1/2" stacked 2" thick. The primary winding consist of 250 turns of #16 wire tapped at 85 turns and the secondary is 8 turns of either 4 #8 or 6 #10 wires in parallel. Note the primary of this one is wound for an input of 115 VAC and is fused at 6A. The electrode is a 1/2" carbon rod held in a suitably insulated handle. Obviously it could be rewound for 240 volts and any number of taps could be added to the primary for multiple heat levels using various sizes for carbon rods. The heat range can also be regulated by changing the length of the carbon rod extending from the holder ( only if they aren't the copper coated type) . This may not be exactly what you are looking for but should give some idea of the basic construction of one.

John

Edited By John Purdy on 04/06/2018 19:00:46

I don't think my RSU works at anywhere near 100 amps. Certainly the cables and plugs and sockets used would never carry anything like that. I just did a check and the resistance of the lead with the carbon rod is about 0.4 ohms so with a voltage of 4.5V, that would give a maximum current of about 11A. I never normally work at that voltage because there would be a danger of burning holes in thin gauge metal. I normally work using 2.5 - 3V on metal gauges of 0.010" and upwards. For any metal thinner than this, I would drop to 1.5 to 2V.

Jim.

Edited By Jim Guthrie on 04/06/2018 21:02:39

Can't say I'd heard of this technique, but a quick search reveals that it's simply a low voltage DC source at modest currents - in the tens of amps. It seems to rely on the highest resistance in the circuit being the carbon rod to work, and hence that's the bit that gets hot.

I can't see where the mention of arcs by SoD comes in, the output voltages aren't high enough to generate an arc. Essentially it's a DC process, so you could modulate the heat by PWM. But a footswitch is probably just as effective. Offhand I can't see why the technique wouldn't work with low voltage AC, and it'd save quite a number of components.

The main problem will be getting the transformer. No doubt it'd have to be a homer. One used to be able to buy transformer kits with a primary and space for a custom secondary from RS, but I think they've been discontinued.

Andrew

Edit: RS still sell 2V Cyclon lead acid batteries. That might be a good place to start. Even small lead acid batteries will have no problem delivering tens of amps over the timescales we're talking about.

Edited By Andrew Johnston on 04/06/2018 21:49:50

I remember back when i worked in a plating shop , we used dc power at many hundreds and sometines thousands of amps but never had an arcing problem because all this power was running at less than 15 volts - had to be careful of sparking though as the hydrogen released as part of the plating process liked to go bang in a big way !

Obtain a micro wave transformer, carry out and insulation test on it using a megger (500 volts DC) and use the high voltage winding as the input from the mains. If you carefully remove the heater winding, that is the thickest and use it treble or even four times you may find it thick enough for a high current secondary when you rewind it back from where it came, might only need a couple of turns. When I made my spot welder I obtained a metre of PYRO cable with plastic on the outside and used that as the secondary,not the plastic but the cable outer with two lugs crimped on. So see your friendly electrician and see if he has a bit of scrap PYRO or maybe the scrap yards itsself. Oh, and by the way its not volts that cause the sparking its the amps. I have a foot operated switch which causes a an old car starter solenoid to close, putting on the power. a relay isn't man enough. John

Long time ago I made one of these to solder closed a small metal case for a filter. Power supply was an old, large C-core transformer that had separate mains primary and LV secondary bobbins. I removed the secondary completely (easy in this case) and replaced it with one "turn" of brass bar, about 1/2 x 1 inch IIRC, which was connected to a couple of brass pads bolted to a tufnol plate with a small gap between them. Item to be soldered was clamped down on the plates with a tooling clamp and the primary supply switched on for a few seconds. C core transformer used because they can easily be dismantled and reassembled to get the bobbins off.

John_f1..

I presume the 'high voltage' winding your refer to is the 220v mains in winding..Not the 1 or 2KV magnetron supply winding..

I guess you could use the high voltage winding as input, but it leaves little space on the core to fit any other windings. Normally folk remove the high voltage winding and use some welding flex-cable - 2 or 3 turns - as the secondary - 2 to 5 volts should do the trick.

Not sure I agree as to the amps being the cause of the spark - think of sparks from your finger to the door knob when rubbing your leather shoes on the carpet in dry WX..a spark plug, Piezo gas oven lighter, etc...

But then I remembered PWM is DC and can strike an arc. Very hot, capable of melting steel while giving your retinas ultra-violet sunburn. AC is more suitable for this because it automatically extinguishes any arc that forms 50 times a second. - SoD

Dave, Arc's do not care where the electrons come from - AC or DC..your AC buzz-box welder generates arcs as good as one with a DC rectifier on the output. Ali welding is done with AC as well, so Arc's are Arc's...And arc's extinguishing 50 time a second will dissolve your eyes just as well!

Joe

I could be wrong, but I do think it is possible to strike an arc at low voltages with this type of machine. Perhaps an experiment is in order!

I suggest this isn't like a welder where a high voltage is needed to deliberately start an arc, or a fixed spark gap where nothing happens until the volts rise sufficient to ionize the air, say 3000V per millimetre, or like the switching in a plating shop.

Arcing occurs when current flows heating air sufficient to ionise it. Normally that doesn't happen. One way to get current flowing is to first ionise the air with a spark. The spark drops the electrical resistance of the air so that current can flow but after that current alone heats the air enough to keep it ionised. An arc depends on heat not voltage. If air is already hot and at low-pressure, its much easier to start an arc. A spark (volts) isn't an arc (amps).

DC current is far more likely to arc than AC. The reason is that, once started, the current in a DC arc flows continually keeping the air ionised. Conversely the current in an AC arc drops to zero every cycle and gives the air time to cool. AC in quantity can and does arc, but it's much harder to strike and maintain a small AC arc than a small DC arc.

I think arcing is not unlikely in a DC soldering unit even at low voltage. It would work like this.

1. The operator applies the electrode to the work and melts solder as intended at 250C. (So far so good.)
2. Without disconnecting power, the shaky operator (me) wobbles the electrode off the work causing a tiny spark.
3. The tiny spark in hot air is enough to allow a small arc to form.
4. The small arc heats the surrounding air and the arc grows.
5. Slowly moving the electrode away lets the arc build to full size. Within the arc the temperature jumps to 3000C, more than enough to boil steel. (Inside the arc; we haven't converted our workshop into a blast furnace, the total power is only 200W, but it's concentrated in a small space.)

The operator of a DC machine could prevent arcing by only applying or removing power when the electrode is firmly in contact. Not difficult. The operator of an AC machine wouldn't need to worry about arcing at all, no matter how shaky his hands! And an AC machine is easier to make.

Be interesting to build a pair of machines and compare them. The heating effect would be identical. The question is, of AC and DC, which machine is easier to make, which is more robust, and which has fewer side-effects. Perhaps there's no significant difference in practice. The key advantage of DC is that it avoids the need for a hefty transformer. On the downside the electronics are more complicated, but not particularly so.

Dave

Sorry to join the discussion belatedly

... I have been away for a few days at the National Health Service's pleasure.

I think you will find this earlier thread informative: **LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=103182

MichaelG.

Tried some unsuccessful arc striking experiments.

• The core of a soft black art pencil turned out to have a resistance of 800Ω per mm, far too resistive to use as an electrode.
• Sawing a 1.5V alkaline battery open made a hellish mess and revealed an electrode suspiciously like a small nail; not carbon at all.

Nothing proved without a proper carbon rod...

Dave

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Alkaline

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Hint:

https://en.m.wikipedia.org/wiki/Zinc–carbon_battery

MichaelG.

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Edit: for no obvious reason that link is going to the 'Zinc' entry, not the 'Zinc-carbon battery' entry.

Edited By Michael Gilligan on 08/06/2018 20:48:53

I still don't understand what's wrong with the link I posted last night, but hopefully the 'desktop' page will work better: **LINK**

https://en.wikipedia.org/wiki/Zinc–carbon_battery

MichaelG.

.

Edit: Nope 

Edited By Michael Gilligan on 09/06/2018 09:41:52