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Mixing resistive heating with inductive heating?

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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:

--- Quote from: rwgast_lowlevellogicdesin on January 09, 2020, 12:43:09 am ---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.
--- End quote ---

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.

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