How a Soldering Gun Works

Many people should already be familiar with the concept of a transformer, which could be a step-down transformer, that has a certain number of turns – anywhere from 200 to maybe 400 – as its primary winding, and which has a secondary winding with fewer turns.

The output voltage follows as the AC input voltage, times the ratio of secondary turns to primary turns – or, divided by the ratio of the number of primary turns to the secondary turns. The output current gets stepped up, by whatever factor the output voltage was stepped down.

What some people may not have the imagination to realize, is that it is fully valid for the secondary winding, to consist of only 1 turn, or of 2 turns. If that is the case, then the output voltage will also be ~1/200 the input voltage, and the output current will be ~200 times the input current, as long as a well-designed transformer, with a suitable core material, preserves the energy well.

If that amount of current flows through a solid bar or chunk of metal, even a solid chunk of metal will get hot. And this is the basis of a soldering gun. Its secondary winding is just a solid bar of metal, that loops through the same transformer core, which the primary winding looped through.

There are some caveats to this design. One is, that if the secondary winding really does just consist of a solid bar of metal, we will have problems with eddy currents, stealing away some of our energy. After all, the ferritic core material itself is commonly laminated, to avoid the possibility of eddy currents there; the same consideration could be given to this secondary loop…

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First Stage of my Project Reached

Often it can happen that before we open up some technical work that was performed by somebody else, we have one idea about what to find, but that after we do so, we find something quite different. Today, while working on my project, this was the case.

When Electricians connect two leads to a single-pole, single-throw switch, what they are supposed to do is remove the insulation a certain distance from the free end of each lead, loop the bare wires around each terminal of the switch, and tighten the screws of the terminals.

When the switch is ‘open’, no current can flow, and when the switch is ‘closed’, current flows with minimum series resistance.

If one wire is black and the other red, which are both codes for phase-wire, past experience tells me that they have never been connected to supply, 180° out of phase with each other, because the first time the circuit was closed, there would have been fireworks. In this case, the supply-wire was black, and the load-wire to be fed was red.

But this is how the previous design was screwed up: Instead of having a free end of supply (black) wire, the previous Electrician had de-insulated a segment about 1.5cm long along this wire, leaving the rest of the wire insulated. Hence, the black wire came from out of the wall, looped through one of the switch-terminals where it was de-insulated, and went back into the wall in one continuous path, to feed circuits elsewhere, which that switch was not supposed to be able to turn off.

But, the load (red) wire was passed in as one free end, and lodged in so tightly that even after I had completely removed the screw of the terminal, I could not remove this red wire. My only solution was actually to cut the red wire, and to leave some length of it in the terminal, rendering the rest of the switch useless, even if it was to be reassembled – ever.

What this meant, was that the plastic screw-on caps, with the little springs inside, that are normally excellent for this sort of thing, were useless to me, because nowhere did I obtain 3 free leads facing in one direction, to twist together mechanically. All I could do, was to entwine the red (load) wire with the bare section of the black (supply) wire a little bit, and then rely on my soldering job 100% to hold everything together.

Then, I had to apply several layers of professional-grade electrical tape, to prevent this blob of metal from shorting against the inside switch housing.

When we apply any soldering job, a detail which most inexperienced people may not know, is that we never really apply the solder to the tip of the soldering iron or gun. We always use the soldering tip, to heat up the copper wires or terminal, until the copper itself becomes hot enough to melt the solder. If we neglect to do that, liquid solder will fail to bond to the colder parts of the copper, will bead instead, and will yield a faulty connection which we will hear bad news from…

It comes as a secondary fact, that we actually cannot avoid the liquid solder additionally touching the tip of the soldering iron or gun.

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