More induction heating

[Alberto]

Hello. I saw some threads about this topic but since my question is different I don´t want to change the topic of the other threads.

I want to start making vacuum triodes and to heat the titanium getter I need a induction heater. I have some research but all of them have a lot of turns in the coil and to heat parts acurately I think I only neeed 3 or 4 turns.

Also I think it would be around 300 watts. For the power supply I have thought in a modified MOT to get 12 or 24V wit a FUUUUULL bridge rectifier. I don´t know if I would need a big capacitor for filter. Since a power suppky of this power if a little expensive

I saw some schematics of ZVS for induction heaters, so probably I onlye need to change the values of some components to get that amount of power and for 3-4 tusn coil, but as you can see, I have no idea.

I will appreciate any advice.

best regards

[romy]

on mine a 80mm od, 70mm long, 6x1mm, 81/2 turns coil produces around 50 khz.

these formulas should give a very rough approximation (i would look them up i a more intuitive presentation than i can do here):

(1) L = µ0 x rel. premeab. x area X (turns)^2 / length

(2) F res = 1/(2pi) X sqrt(1/(L x C)) = 1/(2pi X sqrt(l x c))

so if you add turns and lenght they tend to cancel out, but turns prevail (lower F).

(somebody correct me if im wrong.)


how large are your objects and what temp are you aiming at? i understand these devices will work up to about 100 khz and tend to let out the smoke above that.

[Alberto]

Thanks for your answer!

It is for heating getters and other parts on homemade vacuum triodes. I think the maximun diameter of the triode will be arround 20mm

So the parameters of the coil change the frequency, but it has something to to with the power of the heater? I mean if I use a scoil with to many or too few turns, can I burn the circuit?

[romy]

yes, thats what i was getting at. if you play around with coils and decrease inductance too much (e.g. 2-turns, much smaller diameter) the cirquit cannot handle the switching speed any more and burns.

(mine burned for another reasons, but thats another story.)

[Alberto]

yes, thats what i was getting at. if you play around with coils and decrease inductance too much (e.g. 2-turns, much smaller diameter) the cirquit cannot handle the switching speed any more and burns.

(mine burned for another reasons, but thats another story.)

Thank you!

My idea is making a induction heater with only 2 or 3 turns  and 200 or 300watts to heat getters. But I have no idea how to make it because all the examples on the internet are for big coils with a lot of turns

[Anders Mikkelsen]

The standard Mazzilli ZVS driver does not work well with low values of work coil inductance, for various reasons. If used with small values of resonant capacitance, the operating frequency is too high, and if used with large values of resonant capacitance, the tank circuit impedance is too low for the circuit to work well. You could potentially use an output transformer to step down the output voltage of the ZVS to a level where it can drive a coil of low impedance, but the resonant capacitor would likely need to be on the primary side of the transformer to avoid introducing too much stray inductance between the MOSFETs and resonant caps, and the transformer would consequently have to be sized to handle the full tank VARs, i.e. many times the actual processed power.

A multiturn coil with a copper flux concentrator would be worth a try, but like the transformer solution this is not something that has been tried and documented, so you would have to experiment on your own.

I've experimented with induction heaters for getter firing and neon electrode bakeout, and I found that transformer coupled series resonant or LCLR can work very well. These circuits can drive any practical work coil inductance by correct sizing of the matching transformer/inductor, but they are a bit harder to design. If you only want a few hundred watts, then a fixed frequency design can be worthwhile, with a pot to adjust the frequency. Just make sure it doesn't drop below the resonant frequency, as loss of ZVS will massively increase MOSFET losses.

[petespaco]

Alberto-
   You mention wanting to use a work coil with only 3-4 turns.  As others have said, there is a good chance that the resulting low inductance will damage the drive circuit and I agree.  The issue, as I see it, is that the high frequencies (much over about 120 kHz) force the  Mosfets in the ZVS circuits spend too much time in linear mode causing excessive heating and subsequent failure.
  So, I ask myself:  "what's the big deal about wanting only 3-4 turns for a work coil?"
Then I answer myself: "I guess it's because he wants to concentrate the power in a narrow area".
If that is true, then consider a multi-layer, multi-turn coil design.
You can see how one works here:

/>This video was  the culmination of some experiments I did a few years ago to maximize power transfer for annealing brass rifle cartridge casings. 

[Alberto]

Thnak you both for your answers!

...

So I could use a mosfet conected to a 555 oscillator and choose the frequency of the coil right?

...

Yess. I didn't realise that I could make a lot of turns in a small volume.

[romy]

i didnt know what getter heating is untill i looked it up, but imho the "internet zvs" devices provide around 50khz and thats way too low to heat up such a small object fast. also i see commercial equipment starting a 3kw. why does it have to be heated that fast, btw?

anders, could you expand a bit an the "copper flux concentrator"? it wont be out of copper, right? do you need a custom made powdered metal piece or is there a simpler way? how is it to be placed on a helical coil? iv seen them as plates together with pancake coils.

[Alberto]

i didnt know what getter heating is untill i looked it up, but imho the "internet zvs" devices provide around 50khz and thats way too low to heat up such a small object fast. also i see commercial equipment starting a 3kw. why does it have to be heated that fast, btw?

anders, could you expand a bit an the "copper flux concentrator"? it wont be out of copper, right? do you need a custom made powdered metal piece or is there a simpler way? how is it to be placed on a helical coil? iv seen them as plates together with pancake coils.

You can see it in action in 1:04:37. It only has 200 watts

[romy]

so what is in the wooden box?

(i have not the slightest idea about the stuff you are doing, but im impressed.)

[Alberto]

No no I wish that video was mine!!!

I dont´t have idea what is in the box, this is what I want to know. I need a induction heater like this one to make my own triode valves.

[petespaco]

romy- as regards "What's in the box?"---
Here is the link to that video:

/>I see that, as recently as about a month ago, someone was still replying to comments.  And new comments were still being made as recently as one day ago.
Why not go to the video ask the person?
 If you do that, you should also ask what they mean by "to get a higher frequency". 
Actually you need to know a bit more than just "what's in the box", since they mention that they have a separate 12 volt gate drive power supply.  We MAY even be seeing heat sinks behind that huge bunch of capacitors.  Are the power supply rectifiers there or are they for Mosfets?

(My use of "they" attempts to avoid trying to guess gender of the technician from the voice)

[Alberto]

Yess I was one of these guys in the coments, but I asked for the diagram of the induction heater and the guy don´t answer.

I bought a induction heater like this one



I made a coil with the same inductance as the original one but smaller. It works at 12V and with a screw or a blade of a cutter it drains 3 or 4 amps. The blade get red hot in seconds, and all ferromagnetics material get hot, but the titanium wire no and I want the induction heater for heat the titanium wire to act as a getter

[romy]

the guy in the vid mentions higher frequency, right? what does you contraption run at? fortunately titanim has hight resistivity.

[Alberto]

Ok, finally he replied!

The circuit is that

https://markobakula.wordpress.com/power-electronics/500w-royer-induction-heater/

but the capacitor bank is 3,15microfad and it runs at 350khz

[RoamingD]

Hello, I have a suggestion for the power supply, by far the cheapest power supply that will output 200-300w at 12V with no problem are old computer ATX power supplies. You can get one at a second hand market for very cheap (most don't cost more than 5-10 dollars) or even for free 
A traditional full bridge rectifier power supply will require huge capacitors that come at a big price if you don't have them already, run a simulation on https://www.circuitlab.com/ and it will plot out for you the voltage and ripple.

[romy]

check my math: 350khz/3.15µf → 6.4µh. 

[klugesmith]

Math check: what are you trying to figure? To resonate at 350 kHz with 3.15 uF, L would be less than 0.1 uH.

[petespaco]

Re: "I made a coil with the same inductance as the original one but smaller. It works at 12V and with a screw or a blade of a cutter it drains 3 or 4 amps. The blade get red hot in seconds, and all ferromagnetics material get hot, but the titanium wire no and I want the induction heater for heat the titanium wire to act as a getter"

Well, 12 volts at 4 amps is only 48 watts gross  Probably only half of that for net power to the work.  Is that induction heater rated for higher voltages?  If so, go to the maximum voltage that the thing can handle and try again.  Many of us started out experimenting with induction heaters by using car batteries in series to get higher voltages.  Even garden tractor 12 volt batteries can output lots of current for a short period of time.
  One other thought:  Maybe you could flatten the titanium wire to make it more like a thin piece of sheet metal.  I seem to remember seeing getters shaped like that about 200 years ago when I was young. <G>