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…

And so, to use 2 secondary turns will be better than just to use 1, and in some cases heavy wire is used – particularly if braided – instead of a solid piece of metal, because even braided wire has weaker eddies than a bar of metal would.

Because roughly 200x the amount of current flows through the tip of a soldering gun, as flows through its primary winding, it is also important to tighten the physical screws well, that hold the actual tip, because a poorly-tightened screw will introduce some small amount of series resistance, and we did not want for the screws to heat up, we wanted for the tip to heat up the most.

In fact, if the amount of current is very high, and the voltage low, it is difficult for a piece of metal actually to form a short-circuit in the classical sense. Whatever minute resistivity the metal has, will act as a resistor, and will still limit the current, thus preventing a ‘real’ short-circuit from forming. Yet obviously, the amount of current that will flow, is still high enough to be dangerous, as is just demonstrated by the fact that it will melt solder and more…

And, the fact that such a high current flows through the tip, is also the reason for which the tip itself will generate a powerful magnetic field. The magnetic field the single, or double primary loop forms in the core, still defines the counter-EMF of the primary winding, as long as we have an efficient transformer, and so its magnetic field will also be as strong as that, of the 200-400 turns of the primary winding.

And this is why we do not approach sensitive circuit-boards with the tip, of a soldering gun. When soldering sensitive electronic circuits, we always use a soldering iron, or soldering pencil.

Soldering guns are practical however, if we have house-wiring or car-wiring to complete, because they heat up so fast, and because they deliver a high amount of heat very conveniently.

Caution: This type of transformer is not wound for continuous duty. This means, that the device will only tolerate being powered, for a certain fraction of the time it takes to complete a project. If we were to keep the trigger pulled continuously, eventually both the secondary and primary windings would overheat. This is completely different from how a soldering iron or soldering pencil works, which we plug in 1/2 hour in advance of doing our soldering, which we unplug only after the project is complete, and which we can leave in its stand to cool down overnight, before putting it away properly the next day.

Also, I owned my first soldering gun when I was a child. My father gave it to me for my birthday in the 1970s, and taught me how to use it. But it took until the 1980s before I was old enough, to be allowed to open it up and to peer inside.

My parents worked on the assumption, that no part of my own space could be cut off from them. It was okay if Dirk wanted to solder something, but also understood that Dad would be watching him do it…

If we were to look inside a soldering gun, we might find it revealing, as to how low-quality the device might be, which simply has a nice, plastic, molded shape on the outside.

When it came to the soldering gun I had in the 1970s, I found two apparent defects in its design:

  1. Even though the bars into which we screw the tip were thick where they could be seen from the outside, inside, they only connected to some cheap copper wire, that actually made 2 turns to form the secondary winding. Thus, that copper wire would also become hot inside the soldering gun, when the tip became hot in the front. I am not really sure if this did harm, because even if the enamel on this internal wire did char, the voltages were so low, that there was little to fear from this wire shorting against other metallic components, such as the ferritic core… Comparatively little current would flow there.
  2. In the 1970s, the primary winding presumably had the correct number of turns, but only consisted of enameled wire which was loosely held together into a bundle. Even if I just peer in through the cooling slots of my present soldering gun – which I do not plan to open up – I can see that its primary windings are much more neatly-wound. This gives hope that my present soldering gun is also less likely to overheat, than the one in the 1970s was.

The way transformers generally work, if their secondary winding experiences an open circuit, i.e. a no-load condition, the counter-EMF of the primary winding as it its maximum, and an efficient transformer will draw a minimum of current through its secondary winding. (If certain parasitic phenomena can be discounted, such as resistivity and capacitance within the windings, then this current will be phase-shifted 90° with respect to the applied voltage, and form a constant component of the current flowing through the primary winding.)

As soon as current is allowed to flow through the secondary winding, i.e. a load connected, its magnetic field tends to become opposite that of the primary winding, so that the primary will produce less counter-EMF, (if its current does not change,) and will thus end up drawing more current, according to unchanged, alternating applied EMF.

If the secondary winding of the transformer is fully shorted in the classical sense, the primary winding will also no longer generate counter-EMF, and then the series resistance of the wire, of the primary winding, will act to limit its current. This is generally a dangerous condition to put a transformer under.

But, as I did write above, because of the way this transformer is wound, it would be difficult for the secondary loop actually to short it out fully, because current flowing through its secondary winding is still limited by the series resistance, of the secondary winding. Even if that just consists of thick chunks of metal.

Dirk

 

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