Mixing resistive heating with inductive heating?

[rwgast_lowlevellogicdesin]

This may be a super silly question for someone more knowledgeable than myself when it comes to maxwells equations and thermodynamics, but after googling I haven't found an answer!
10o Ive been building a custom solder station and looking in to eventually add support for some type of High Frequency induction handle (i.e hakko fx100, metcal, jbc, etc). These guys are pretty expensive especially if Intend to use pid, instead of cury point.

So I started thinking basically induction heating just works by shooting an AC signal through a coil at HF frequency's. Not a big deal, maybe you could just convert a 907 handle to induction by throwing a coil in place of the ceramic heater,  along with welding a longer rod in the 900m tip (where you would usually stick the ceramic heater in) that way metal attached to the tip passes through the induction coil, next add a thermocouple in the tip and you have a way to pid control the iron.

Then I started thinking what if you were to make your induction coil high resistance also (as long as your R and L don't form a lowpass below your AC frequency). I have plenty of kanthal wire around I use for vaping, if I were to build an induction coil out of that would there be any benefit out of that? Im wondering if somehow the resistance could weaken the magnetic feild around the inductor, I just dont know.

As I said cant find an answer to the question online and I assume thats because i am brilliant and the first one to think of it, or it is just really stupid and impractical. Im gonna guess its the later but I would like to know why!

[unitedatoms]

What I heard about induction heaters inside the soldering tips, is that there core is made of magnetic material which looses certain magnetic property sharp at set temperature point. That makes coil simultaneously a heater and accurate instant temperature sensor. Making resistive heater on top of inductive will break the sharpness of reaction. The accuracy will be worse, but may be will still work.

Anyway, for the oscillator the load in original design should look like a lossy inductor, which suddenly becomes lossless. Some sensing of reflected power or so will drop the outgoing power and backwards restore the power when load becomes lossy again.

[rwgast_lowlevellogicdesin]

Cool thanks for the reply, I will have to experiment with this when I have some time.

The tips your thinking of are called curie point, and im not after that at all. Basically the shape of the tip determines when the magnetic core has lost all magnetism a.k.a its curie point, at that point either a hall effect sensor, or magnetic read switch detects the magnetic loss and cuts the HF to the handle. Once the core starts cooling off the atoms reform in to a magnetic material closing the reed switch and allowing current to flow.

Im more after using a pid feedback loop and temp sensor to to regulate the current and predict power needed to hold thermal mass in the tip. The curie point is just a fancy thermostat control which works well because the tip is the sensor, I think a pid loop system with a sensor embedded in the tip could beat using curie tip for bang bang control. Not just that but who wants to have to buy all kinds of different temps for different temps etc... PITA. It would be nice to have the speed of induction in a system that uses common tips and can adjust temperature on the fly.

[David Hess]

Then I started thinking what if you were to make your induction coil high resistance also (as long as your R and L don't form a lowpass below your AC frequency). I have plenty of kanthal wire around I use for vaping, if I were to build an induction coil out of that would there be any benefit out of that? Im wondering if somehow the resistance could weaken the magnetic feild around the inductor, I just dont know.

If the coil was high resistance than RF will heat it just as a lower frequency or DC will defeating the purpose of inductive heating which generates the heat directly inside the tip.

[Berni]

And how do you know its not restive heating? Is it a superconducting coil?

Induction heating typically uses a LC tank circuit to recycle energy back and forth, this way any energy that has not escaped(Resistive losses, or went into the heating target) is fed back trough the coil on the next cycle. Because of this even a few watts of input power can cause surprisingly large currents (tens of amps) in the coil once the oscillation builds up in amplitude.

So the coils of induction heaters do end up heating up quite a bit because it takes only milioms of resistance to cause noticeable heating at such high currents.

Larger induction heating applications where you see industry using it to harden and temper metal parts have coils made out of copper pipe rather than wire. This is so that they can pass water trough it for cooling otherwise the coil would get too hot even if that 10mm diameter copper pipe was solid copper all trough. The same water cooling loop is also used to cool the capacitors on the other side of the LC circuit as even they get hot from such massive currents.

[rwgast_lowlevellogicdesin]

Im not saying induction heating is not caused by the resistance in the coil, maybe a better title should have been conducion heating mixed with convection? As far as raising the resistance of the coil using kanthal/nichrome etc... im not sure why that would't make more heat, the coil itself shows resistance when it is tanked and loaded with AC current, but adding a true resitive element such as nichrome should produce more heat faster I would think. Non inductive, i.e resitive only  elements, will dump there heat in the same manner regardless of dc, ac, hf ac, microwaves etc.. I cant figure out where I read it, so I dont know if I misunderstood but isnt the coils power calculated by something like current^2 times resistance, which means raising the overall resistance not just the impeadence should produce more power?

Sadly even if I am correct about this im sure the idea is not very practical, as stated above keeping the coil cool so it doesn't deform and short causing a melt down is sometime the bigger task than the heating, using thinly gauged heating wire would probably blow a hole through itself fairly quickly, but maybe if the two were used at a lower power they could get to temperature quicker as long as the temperature isn't to hot. Maybe even better the work piece could be wrapped in fairly light gauge quick heating resistive wire, and an induction coil could be used not only to heat the work but pump extra heat in to the already powered nichrome/kanthal?

Like I said i am no expert on this stuff at all, and these ideas could be just plain BAD, not just in a practical way but the theory also. Im sure if they would good ideas someone would have done it and written a paper about it by now, i mean induction heating has been used since the 1940's...  Im just hoping someone can explain to me why its not done....

[Berni]

Well if you want more heating power you simply push more power into the inductive heating coil and the heating target will heat up faster.

The heating caused by the resistance dissipating power just the same as inducing a heating current in the heating target. They both sap away power from the LC cirucit, its just that where the power is turning into heat was moved from the target inside the coil into the coil itself. No extra heat is generated, just the place where the heat is generated is moved from the inductive heating target to the coil. But heat generated in the coil needs to still travel to the target somehow (And some of it will travel in the wrong direction towards the outside and be lost to the enviorment) while the heat generated in the target is already where it is supposed to be.

So ideally to make it most efficient and fastest to heat up you would want a superconducting coil so that all power that is being input ends up dissipating in the target where you want it and none on the coil. That way there is no chance for heat to get lost to the environment (apart from the targets own loss) and you have less mass to heat up since the coil does not need to get up to temperature, just the target.

There is no real limit to how much power can be input with inductive heating. With a resistive heating element you are limited to how much temperature the element can get up to before it melts. But with inductive heating you can theoretically input 100kW of power as long as the coil is kept cool enough (By being so conductive it doesn't get hot, or actively cooling it with water) and get the heating target glowing red hot within a second. This is the reason for using inductive heating in the first place, it magically causes heat to appear inside the heating target rather than making the environment around the target hot and waiting for the heat to propagate into your target (And this is very limiting due to how slowly heat propagates).

So if you want your inductive soldering iron to heat up faster you simply need to hook it up to a bigger RF power supply and be careful not to pump so much power into it that the copper coil melts (But hey you do get the maximum amount of resistive heating that you wanted since the coil is glowing red hot).

[T3sl4co1l]

Does anyone actually make an iron that's induction heated without using Curie temp stabilization?

The advantage would be that you can deposit heat in the tip without incurring the temperature drop and time constant that convection/radiation from a heater gets.

Downside is, you need active cooling on the coil to keep it coil...y, otherwise it's going to get at least as hot as the tip, and then you still get the temp drop and time constant, just at a lower order (a pole-zero response I suppose).

Needless to say, coil efficiency drops severely with temperature, because copper has a huge tempco.  You might not be able to use copper at all, in which case the efficiency is that much lower to begin with (though the tempco can be lower, so at least the efficiency is flat).  And then you're that much closer to the original resistive-heat case, and there's some Q factor where you need to ask, why bother?

(Q factor roughly being in terms of, say, unloaded coil Q, which if it's 1 or 2, isn't much magnetic field, regardless of what you put around it.  If it's 10 or 100, it's probably going to be a reasonable fraction due to induction.  Presumably your coil coupling will be excellent, so that even a low Q could potentially have quite reasonable efficiency.  But if it's 1 or less, even that won't help you.)


As for the power supply, I would assume a variable frequency drive is fine in this case, and that you have a shielded or balanced cable supplying power to the iron.  For low Qs, an off-the-shelf resonant controller will do, probably with a transformer to adapt to the coil impedance.  (The resonant cap can be onboard, or possibly in the iron if there's room.)  For higher Q (more than maybe 5 or so), a slower controller may be desirable, like a PLL+VCO.  This may not be as feasible using OTS controllers (that tend to have intentional quirks, which improve regulation or efficiency in traditional topologies like LLC), but a CD4046 or the like, can be used with a little glue logic, to good effect.

If constant frequency is desirable, that's probably fine too, and can even be direct drive (nonresonant) up to modest Q.  (The downside to nonresonant is, if you're delivering say 100W at a Q or 10, that's 1kVA the cable, inverter and bypass caps need to handle.  They don't know any better, they think you're making 1kW.  So they need to be that much higher efficiency, and costs go up very quickly.  We're already talking boutique quantity and pricing, so I assume there's a little budget for that, if it should prove useful.)

Tim

[rwgast_lowlevellogicdesin]

Awesome thank you Berni, this is exactly what I had wanted to know

Im not even sure if I will ever attempt to really convert a 907 handle to induction but I got the idea from this blog post when searching around

https://hackaday.com/2017/06/01/diy-induction-soldering-iron/

I also know there are a few inductive based stations with temperature control that do no use curie point. One is made by quick and can be bought in US 120vac format rebranded and sold by some online tool vendor handytool or something, I would have to do some history searches to get the correct vendor site. Anyways this is the 220Vac version on Amazon

https://www.amazon.com/Quick-Induction-Lead-Free-Soldering-Station/dp/B00YUQ4MZE

I am acually considering buying this station, it was $140 on the site I mentioned, but they only sell four tips and im just not sure its worth it when you can get a decent cartridge station for a little more.

The other one is an atten AT306DH which I just can not find a seller for, all the ali express links are broken. There is a picture on this page if anyone is interested



It is listed as a 90W HF powered station, atten is crap anyways and that is why im just building a station.

My idea if I ever were to do it is to just take a regular old 900m style tip and stick a rod made of ferrous metal in the tips hole along with thermocouple, you know the hole where the ceramic heater would usually go, then use an OA torch to braze the rod in to the hole, that joint and filler is good to 1800F much higher than solder temps. Lastly wrap the rod in kapton like in the hackaday video and slide the coil around it as far up the rod as close to the tip as possible and hope it all fit with in the original hakko 907 sleeve . Not saying this will give a JBC a run for its money but this should be far superior to a ceramic heater iron with the thermo couple so close to the tip, and the speed/accuracy of induction. Hell why not just use small copper piping and a little medical DC pump to water cool the thing.. lol probably not.

 I just thought of something, if you dont own a gas welder and dont want to spend $400 on one, you could do this on the cheap without the Oxy Actelyne even, just have to ebay a cheap induction coil kit that gets hot enough to melt the brazing rod!

[Jwillis]

There is a  review on you tube about the Quick 202D, but by looking at the video I can tell the person couldn't operate a BIC lighter without issues. Try reading a manual .The problems pointed out are frivolous and didn't even put the thing to work to see how it performed.
The Quick products are pretty good . I have the QUICK861DW hot air rework station and QUICK TS 1200 Soldering station. Both  heat up  really fast and have excellent temperature stability. I did look at the  202D but settled for higher watts on the  TS 1200 although its not a HF soldering station. The only problem I had was with the tips. The TSS02 tips are more expensive and the T12 tips won't fit the handle.
Other that that I'm very pleased with the Quick products.