More induction heating

[Alberto]

Thanks for your answers.

At first I´ll try the new circuit with a car battery. But usually people uses higher voltage than 12V

In the link there is another link to the web of Mads and he uses 35V. If I need more than 12V I´ll use small lead acid bateries in series rated for the amperage needed

But Yes, for a 350khz resonance, the coil should have a very low inductance, so I think I missing something. Can I decrease the value of the capoacitance whitoud damagin the circuit?

[davekni]

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.

Yes, a plain thin non-magnetic wire will be VERY difficult to heat with induction.  Vacuum tube getters are usually in the form of a closed ring to make induction heating reasonable.  Per WikiPedia, induction getter flashing is usually at 27 or 40MHz ISM frequencies.  Getters are usually thin nickel sheet metal U-channels containing barium or other getter material and bent/welded into a closed ring.  Titanium wire might work if formed into a closed ring.  Lower (than 27MHz) frequency might work too with sufficient power.  (Lower frequency might make it more difficult to heat getter without heating other tube structures.)

[Alberto]

Thanks for your answers.

At first I´ll try the new circuit with a car battery. But usually people uses higher voltage than 12V

In the link there is another link to the web of Mads and he uses 35V. If I need more than 12V I´ll use small lead acid bateries in series rated for the amperage needed

But Yes, for a 350khz resonance, the coil should have a very low inductance, so I think I missing something. Can I decrease the value of the capoacitance whitoud damagin the circuit?

Thanks for your answer. 

People used to heat everything's in the tube to degass the metal but, what effect does the frequency has in the type of heating?

[davekni]

People used to heat everything's in the tube to degass the metal but, what effect does the frequency has in the type of heating?

To heat everything inside a tube, lower frequency (well below 27MHz) is probably better.  I was surprised to read that 27 and 40MHz were used.  In general, preferred induction heating frequency is higher for non-magnetic materials and higher for smaller objects.  However, the only possible reason that comes to mind for going all the way to 27MHz is to minimize heating of other tube structures.  At that frequency and with an induction drive coil similar in size to the getter and placed as close as possible to the getter, the getter should block much of the magnetic field from penetrating deeper into the tube.
BTW, this is based mostly on reading and physics theory.  I've only dabbled with induction heating at home and built one tiny unit for work, to heat SS parts just enough to speed curing of epoxy.  I've never built a vacuum tube.  My last tube electronics project was ~50 years ago in high-school.

[romy]

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.

thank you, thats what i was getting at. (i messed up the calculation forgeting to put in nano instead of µ and i was one decimal point off anyway.)

now im getting 65.6 nH*. first i thought such a coil was not possible. then i found a wire loop calculator (using a totally different formula). apparently 1 turn/40mm/3mm wire will produce 67.1 nh.

btw, how do you get 3.15µf with 0.27µf caps?


* L= 4pi x 10^-7 x N^2 x A / length (for empty coil)


edit: not a single "calculator" yields the same result as an other and they seem not to adhere to the formula, e.g. L is in no way inv. proportional to lenght. in literature i see stuff like this, and thats only the beginning for a foil, for a wire it gets twice as long. what is the difference?

[Alberto]

Thanks for your answers.

Davekni, Ok I´ll try differents values to change the frequency, but Can I change the frequency (changing for example the capacitance) without damagin the circuit?

romy, Yes my calcularions is the same value for the inductance, but the guy uses a 4 turn 40 mm diameter (at leats that seems to me) so I´m not sure. I have asked him about that

[romy]

lets practice (40mm/4T/30mm long): 12.56x10^-7x1x16x1.26x10^-3/3x10^-2 = 844 nH

a calculator: 610 nH

https://www.translatorscafe.com/unit-converter/en-US/calculator/coil-inductance/?D=2&Du=cm&l=1&lu=cm&N=10

[klugesmith]

Let's suppose 350 kHz is the frequency of choice.
Designer can trade L for C to facilitate the drive electronics. Here's an example:

First we figured 3.15 uF and 0.0656 uH. Characteristic impedance sqrt(L/C) is 0.144 ohms.
For the getter heating application, if we wanted 2 kVAR that would be 17 volts and 118 amps.

Could get same magnetic field, frequency, and VAR with 3x electrical scaling (51 volts, 39 amps).
C, L, and impedance would be scaled down or up by 9:    0.35 uF, 0.591 uH, 1.30 ohms.
New coil could be 3 turns in same space as the original 1 turn.
Volts per turn and ampere-turns would be same as original.

Note that tank V and I can be much higher than the DC voltage and current going into inverter.

[romy]

this "trading off" where you square the factor is clear. but is there an optimal proportion? if you can chose the power supply characteristics. how does P=I^2xR influence the choise?

 

[Alberto]

Thanks for your answers. I had incomplete information. The circuit could work at 100khz, and the gates of the mosfets work with an independent power supply of 15 volts.

To make that I sould cut in the red cross and put the 15VDC with the + in the point A and the - in the point B right? Maybe it is a silly question but I´m still learning



And the guy uses IRL540N but I have IRFP250N. Wich one is more suitable? It makes a big difference?

I really appreciate all your help. The level of the forum is very high and I have not much idea

[davekni]

To make that I sould cut in the red cross and put the 15VDC with the + in the point A and the - in the point B right?

Yes.  Turn on 15VDC supply before main power supply.  Also, reduce 470ohm gate resistor values to increase frequency.  100ohm resistors would keep power dissipation under 2W/resistor given 50% duty cycle at 15V for voltage across resistor.  Might be OK to 100kHz that way (with 100ohm gate resistors).

And the guy uses IRL540N but I have IRFP250N. Wich one is more suitable? It makes a big difference?

IRFP250N has higher gate capacitance, so requires even lower gate resistance to achieve 100kHz.  Perhaps drop to 60 ohms, which will consume almost 2W each.  IRFP250N is higher voltage, but a bit lower current capability.  Allows supply voltage up to theoretically 60V (if no startup transients).  IRL540N requires supply voltage of 30V or lower.

Have you tried heating a closed loop of titanium wire?  I expect a closed loop will be required for any success at all.  Titanium loop should be parallel to induction coil loops (coaxial to induction coil).

If you are more comfortable with vacuum tube designs, why not build a high-frequency induction heater with perhaps a pair of 6L6GC or whatever you may have easy access to?  Will likely require lower power at higher frequency (1MHz or higher).

[Alberto]

To make that I sould cut in the red cross and put the 15VDC with the + in the point A and the - in the point B right?

Yes.  Turn on 15VDC supply before main power supply.  Also, reduce 470ohm gate resistor values to increase frequency.  100ohm resistors would keep power dissipation under 2W/resistor given 50% duty cycle at 15V for voltage across resistor.  Might be OK to 100kHz that way (with 100ohm gate resistors).

And the guy uses IRL540N but I have IRFP250N. Wich one is more suitable? It makes a big difference?

IRFP250N has higher gate capacitance, so requires even lower gate resistance to achieve 100kHz.  Perhaps drop to 60 ohms, which will consume almost 2W each.  IRFP250N is higher voltage, but a bit lower current capability.  Allows supply voltage up to theoretically 60V (if no startup transients).  IRL540N requires supply voltage of 30V or lower.

Have you tried heating a closed loop of titanium wire?  I expect a closed loop will be required for any success at all.  Titanium loop should be parallel to induction coil loops (coaxial to induction coil).

If you are more comfortable with vacuum tube designs, why not build a high-frequency induction heater with perhaps a pair of 6L6GC or whatever you may have easy access to?  Will likely require lower power at higher frequency (1MHz or higher).

Thanks for your answer.

So, If I use IRFP250N i have to change the 470ohm resistors with 60ohm resistors but if I use IRL540N the resistors should be 100ohms. Right, but excuse me again for my few knowlodge, if I want to supply the main circuit with 36V, wich one would be more suitable, the IRFP or the IRL?

Thank you!

[petespaco]

Please look into IRFP260N (50 amps) instead of IRFP250N (33 amps).
Most, if not ALL of the Chinese 1000 watt to 3000 watt ZVS induction heaters use the IRFP260N, as do I.
The value of this higher current capability means, (to me anyway) that, if you select an operational frequency that is on the high side for the Mosfet, making it turn fully ON a bit slower,  you get less heating of the Mosfet die.  The higher current value comes from the lower RDSon of the IRFP260n (40 mOhms) vs the IRFP250N at 85 mOhms, doesn't it?

[Alberto]

Thanks for your answers. But another question. Is there any problem if for the 35v supply I use a switching power supply? I have heard it could be problems

davekni I forgot to say, yes I was making a loop, What I wasn´t making is putting parallel to the coil. Now gets a lot hotter! No red hoy but maybe 200 or 300 ºC.

petespaco I already have the irfp250N. If IRFP260N maes a bid difference I could buy them They are not expensive, the problem is the shipping.

[davekni]

Right, but excuse me again for my few knowlodge, if I want to supply the main circuit with 36V, wich one would be more suitable, the IRFP or the IRL?

I home my previous answer was clear:

IRL540N requires supply voltage of 30V or lower.

Is there any problem if for the 35v supply I use a switching power supply? I have heard it could be problems

With separate 15V gate supply, should be fine.  Turn on 15V supply first before 35V supply.

The value of this higher current capability means, (to me anyway) that, if you select an operational frequency that is on the high side for the Mosfet, making it turn fully ON a bit slower,  you get less heating of the Mosfet die.

petespaco I already have the irfp250N. If IRFP260N maes a bid difference I could buy them They are not expensive, the problem is the shipping

IRFP260 has ~1.7x gate capacitance of IRFP250, so will turn on that much slower (unless gate resistor value is further reduced).  ZVS circuits generally work best with fast switching.  Switching at "zero" volts avoids the typical fast voltage slew rate issues associated with fast switching of H-bridge circuits.  Partially-on FETs during switching dissipate more power because Vds is higher.  At high enough frequency this extra switching loss will exceed the reduced loss of lower Rds-on.  Would require simulation and/or testing to know precisely which part is better at specific frequencies and currents.
At least for now, I'd suggest staying with IRFP250.

[Alberto]

Right, but excuse me again for my few knowlodge, if I want to supply the main circuit with 36V, wich one would be more suitable, the IRFP or the IRL?

I home my previous answer was clear:

IRL540N requires supply voltage of 30V or lower.

Is there any problem if for the 35v supply I use a switching power supply? I have heard it could be problems

With separate 15V gate supply, should be fine.  Turn on 15V supply first before 35V supply.

The value of this higher current capability means, (to me anyway) that, if you select an operational frequency that is on the high side for the Mosfet, making it turn fully ON a bit slower,  you get less heating of the Mosfet die.

petespaco I already have the irfp250N. If IRFP260N maes a bid difference I could buy them They are not expensive, the problem is the shipping

IRFP260 has ~1.7x gate capacitance of IRFP250, so will turn on that much slower (unless gate resistor value is further reduced).  ZVS circuits generally work best with fast switching.  Switching at "zero" volts avoids the typical fast voltage slew rate issues associated with fast switching of H-bridge circuits.  Partially-on FETs during switching dissipate more power because Vds is higher.  At high enough frequency this extra switching loss will exceed the reduced loss of lower Rds-on.  Would require simulation and/or testing to know precisely which part is better at specific frequencies and currents.
At least for now, I'd suggest staying with IRFP250.

Thank you for your answer and help (and your patience haha)!!

Ok, IRFP250N and 35V volts with 2 separate power supplys 15V in the gates and 35 volts in the main. The 15V probably will be swiching, but for the 35 I think I´ll rewound the secondary of a MOT a full bridge rectifier and a bunch of capacitor in pararlell. Since the power will be higher and the value of the coltage it hasn´t to be exact, I wont need a regulator right?

[davekni]

Since the power will be higher and the value of the coltage it hasn´t to be exact, I wont need a regulator right?

Correct.  15V supply current is low, ~0.3A with 60ohm gate resistors.  A simple cheap wall-wart supply should work fine for gate power.

[Alberto]

Since the power will be higher and the value of the coltage it hasn´t to be exact, I wont need a regulator right?

Correct.  15V supply current is low, ~0.3A with 60ohm gate resistors.  A simple cheap wall-wart supply should work fine for gate power.

Thabk you, but I meant the 35V power supply. Since a new one is a little bit expensive, I was thinking rewound the seocndary of a MOT, rectify and filter with a bunch of electrolytic capacitors. But do I need a regulator or with the full bridge rectifier and the capacitors is enough?

[petespaco]

Re: "But do I need a regulator ":
  Whether you need a regulator or not depends on how much voltage (and therefore power) you can afford  to lose as the current draw increases.
I rewound the secondary of a 600 watt MOT a few years ago and found a 15 percent voltage drop from about 31 volts OC to 22 amps.  And that's just loss at that transformer, of course.
I will try to attach a chart showing my results.

[Alberto]

Thank you. At 35V the current should be around 7-10 amps. So with that values, wich voltage and capacitance I would need?