Why does the soldering gun only get hot on the tip end?

[gary350]

I have been making different size and shape soldering gun tips out of wire wires. The smallest radius gets the hottest. A 4" long piece of #10 solid copper wire bent with a 1/8" radius heats up very fast and gets very hot. The same wire bent with a 1/4" radius heats up very slow and barely gets hot enough to melt solder.

Do the same experement with a 5" long wire the small 1/8" radius still gets the hottest but not as hot as the shorter 4" long wire.

Why does the smaller radius get the hottest?

Why does only the tip get hot?

 

[Reloadron]

What happens to the resistance at that point?

Ron

 

[Joe G]

if i run the quarter mile track on the inside lane, wouldn't i go a shorter distance then the person on the outside lane? Why are the start marks not in line, but staggard?




oops, thought of the same length wire, i'm gonna learn to comprihand and spell english

 

[BobW]

Bending the wire stresses it and increases the resistivity of the copper in that region. When the radius is smaller the stress is higher and the resistance is higher.

Eventually, with heating and cooling, the copper will anneal, and the stress will disappear, however the act of bending it in the first place will have reduced the wire diameter at that point, and so the bend will continue to be the spot where is gets hot.

 

[gary350]

Bending the wire stresses it and increases the resistivity of the copper in that region. When the radius is smaller the stress is higher and the resistance is higher.

Eventually, with heating and cooling, the copper will anneal, and the stress will disappear, however the act of bending it in the first place will have reduced the wire diameter at that point, and so the bend will continue to be the spot where is gets hot.

That is so logical it makes perfect sense. But what about factory made parts they are larger on the tip end where it gets hot.

 

[kinarfi]

As you bend the wire tightly to make a point, the heat is concentrated where the wires are closest to each other because there is less area for cooling, If you're making tips that you plan to use for soldering, they will last a lot longer if you bend them tight so the wires touch each other and then braze them

 

[gary350]

I am repairing my 39 test leads. I notice if the soldering gun tip gets too hot it self destructs pretty quick. I did some experements to determine how long the wire should be. The 2" long tip made from #10 copper wire gets hot enough to unsolder the solder joints of the metal chassis of a micro wave oven but it only lasts long enough to solder 7 alligator clips to 7 test leads.

A 4" long tip made from #10 copper wires heats up slow and does not get as hot but it lasts a very long time.

A 5" long tip made from #12 copper wire is about perfect for soldering it heats up slow and lasts a long time.

A 6" long tip made from #14 wire heats up slow and lasts a long time.

If I put 1 coil or 5 coils of wire on the tip it still heats up in about the same time and gets hot enough to solder. I have learned the wire length needs to be made long enough to have a long life for simple soldering like circuit boards and wire solder joints. For an extremely hot short life tip the wire tip needs to be shorter. The smaller the radius on the tip the hotter the tip gets all the heat seems to be in a smaller area.

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[user_88]

The physical principle behind resistance heating is called Joule Heat.
It is described mathematically as I[SUP]2[/SUP]R, with units of watts.
The process of mechanically bending wire induces stresses within the metal, which increases resistance at that particular location.

You might try taking the #10 copper wire and bending it several times back and forth, without actually breaking it. This should increase the resistance at the bend location. The larger diameter of the #10 wire should increase the current flow, compared to smaller diameter wire.
Another possibility would be to mechanically work the wire at the location of the bend by hammering on it, shaping it into a useful sharp point.

As long as the copper wire is not heated to the cherry red color, it will not be annealed and lose its mechanically induced resistivity.

 

[kubeek]

Why don´t you use a solder pen instead of trying to guess the right wire? I always asumed you need to do "manual PWM" with soldering guns to get anywhere.

This was one of the first real curcuits I built when I started with more advanced electronics, since I had a 24V 100W transformer already laying around I just bought a $10 solomon replacement soldering pen and hacked together a temperature regulator. If you want, I can try to find the original schematic, it was a simple circuit with just a 7909, LM741, U2008, and a triac with a few other passives.

 

[Reloadron]

Gary

The question begs to be asked. A typical Weller Soldering Gun tip (or similar) retails for about $2.49 as can be seen here so why screw around trying to fabricate one? Additionally, using a gun for the repair work pictures in your link is just a little overkill. A simple 15/30 watt soldering pencil is more than adequate. Considering the cost of gun tips or a simple soldering pencil plus a few tips I can't see the merit of making a tip for a gun.

Just My Take
Ron

 

[gary350]

Gary

The question begs to be asked. A typical Weller Soldering Gun tip (or similar) retails for about $2.49 as can be seen here so why screw around trying to fabricate one? Additionally, using a gun for the repair work pictures in your link is just a little overkill. A simple 15/30 watt soldering pencil is more than adequate. Considering the cost of gun tips or a simple soldering pencil plus a few tips I can't see the merit of making a tip for a gun.
Just My Take
Ron

I have a solding pencil it is too slow it takes a while for the part to heat up so solder will stick. I have bought factory soldering gun tips they are very slow to heat up. It took me 2 hours to replace and repair the wires with several packages of new alligator clips and new wires. If I had used a slow factory tip it would have taken three times longer. Those factory tips take about 20 seconds to get hot enough to solder, my homemade tip is hot in 4 seconds. I have a 75 watt and 100 watt soldering iron I could have used one of those but they are too big for this job.

 

[Reloadron]

Gary, I have been using Weller and similar soldering guns just like these for well over 40 years. When I pull the trigger I get heat and I get it well within 5 seconds well in excess of the 500 or so degrees needed to melt average diameter 60/40 solder. Most such guns have a two stage trigger as can be seen in the link and even at stage one heat quickly. What diameter solder are you using and what is the solder composition?

As to bending AWG 10 or AWG 12 wire the actual bend forming the tip will be larger than the marketed tips for any standard gun sans the large guns using a heating pad type tip.

As to using a pencil. Yes, a pencil takes a few min to heat but a 30 watt pencil Like the one attached in the link when using the correct solder diameter of about .031" the solder melts and flows quickly for each job as in maybe 5 to 10 seconds per joint.

Aside from the El' Cheapo pencils there are nice controlled heat stations out there which have become relatively inexpensive and they attain temperatures in excess of 500 degrees F in less than a min.

Also attached is an image of a 50 watt resistor terminal soldered to a bare copper AWG 12 wire that was part of a load bank I built. Using the attached 30 / 15 watt pencil I soldered dozens of those junctions in a few hours tops averaging 10 seconds per solder joint.

So I guess I don't get it as solder must melt and flow different in OH. than TN.

Ron

 

[polychrome]

With apologies for reviving an old thread, it looked like gary350 had almost worked out the correct answer before he was given well intended, logical, but incorrect explanations that he apparently accepted at face value.

The reason the tip of the soldering gun heats up rather than the rest of the wire isn't because the copper wire is cracked or stressed at the tight bend. Its because the magnetic field created by electrons traveling in a conductor causes them to repel each other, pushing the current to travel at the surface, this is called the skin effect. The skin effect is always present, but its more pronounced for AC than DC, and especially at high frequencies or very high current. The tight bend at the tip puts currents flowing in opposite direction in close proximity causing an intense skin effect there, the traveling electrons use just the surface of the copper, only a few atoms deep. The internal bulk of the copper isn't carrying current, as if it were hollow. This is why that area presents a high resistance to the current and heats up far more than the identical wire nearby.

As the tip ages it may (will) also develop stress cracks as the differential heating near the tip causes mechanical stress. Even a small surface defect like a teeny corner crack will present disproportionately higher resistance causing locally more intense differential heating, promoting rapid crack growth. This is the most common operating failure, while being mashed in the toolbox appears to be common non-operating failure.

The skin effect is a well worn topic for power transmission systems and RF equipment among others. Engineers are usually more interested in calculating it to avoid it than to exploit it.

 

[gary350]

With apologies for reviving an old thread, it looked like gary350 had almost worked out the correct answer before he was given well intended, logical, but incorrect explanations that he apparently accepted at face value.

The reason the tip of the soldering gun heats up rather than the rest of the wire isn't because the copper wire is cracked or stressed at the tight bend. Its because the magnetic field created by electrons traveling in a conductor causes them to repel each other, pushing the current to travel at the surface, this is called the skin effect. The skin effect is always present, but its more pronounced for AC than DC, and especially at high frequencies or very high current. The tight bend at the tip puts currents flowing in opposite direction in close proximity causing an intense skin effect there, the traveling electrons use just the surface of the copper, only a few atoms deep. The internal bulk of the copper isn't carrying current, as if it were hollow. This is why that area presents a high resistance to the current and heats up far more than the identical wire nearby.

As the tip ages it may (will) also develop stress cracks as the differential heating near the tip causes mechanical stress. Even a small surface defect like a teeny corner crack will present disproportionately higher resistance causing locally more intense differential heating, promoting rapid crack growth. This is the most common operating failure, while being mashed in the toolbox appears to be common non-operating failure.

The skin effect is a well worn topic for power transmission systems and RF equipment among others. Engineers are usually more interested in calculating it to avoid it than to exploit it.

 

[KeepItSimpleStupid]

Even more fundamental is that R=pL/A where p (Rho) is resistivity, a material property and L is the length and A is the cross-sectional area.

In a bend, the resistance of the copper changes because of L/A. To properly do it, you would have to integrate p*L/A from L=0 to the end.

 

[gary350]

I knew stress in the metal was wrong and so is metal gets smaller at the bend because factory made tips have a large blob of copper on the tip 3 times larger than the wire and it gets hot. The magnetic field idea makes good sense. This is like winding 20 turns of wire on a transformer then stop and wire 20 turns of wire in the opposite direction. That is like having two 20 turn coils connected out of phase it makes a short circuit and gets hot. Solder gun is 1 turn out and 1 turn back like the transformer.

 

[Vizier87]

Gary, cheers.

Sorry if I sound ignorant, but are you actually building your own soldering gun using resistive wires? That sounds real badass.