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

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

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

There was one commercial? unit demonstrated at an SMEE stand a couple of years ago – the bit that gets hot is the small amount of solder paste between the two bits of brass being joined – the short pulsed operation of a footswitch to the mains power to the transformer and the large thermal mass of the rest of the brass soon sees the solder joint made – the low voltage high current is applied between a metal base plate and a small handheld carbon rod which holds the two bits of brass and solder paste whilst the footswitch is briefly depressed – nothing complicated + seems to work very well on small items assuming that the light pressure is held whilst the joint is made – the relatively short length of cabling on the secondary side is thick enough to carry a short high current pulse without overheating but flexible enough for easy operation – the carbon rod can be handheld

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.

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

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

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

I'm posting from an iPad, Frances

You may be onto something …thanks for the thought.

MichaelG.

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I think you will find that they are still around, Dave

… It's just that 'Carbon' is such a dirty word these days, that they try to avoid mentioning it.

**LINK**

http://www.poundland.co.uk/leisure-and-entertainment/batteries

The 'Zinc' ones are almost certainly 'Zinc Carbon'

MichaelG.