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The best/easiest oscillator...

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Inducktion

Tue Mar 15 2011, 05:44PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

James wrote ...

They are, but mostly vacuum tube based circuits. The reason you don't see frequencies like 100 MHz is that the skin effect prevents the heating from penetrating deeply as the frequency goes up. In general you want higher frequencies for heating smaller parts. 100 kHz is typical of <2kW induction heaters. Huge ones used in foundries and such often run at 60Hz.

Is there any reason why those oscillators are only vacuum tube based? Or just because that is what they were designed off of, and it's not possible to exchange it with a transistor or MOSFET. Or, why not just use a radio transmitter circuit, but change the values so that it works at appropriate frequencies, with a power transistor?

Mattski

Wed Mar 16 2011, 06:59PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...

Is there any reason why those oscillators are only vacuum tube based? Or just because that is what they were designed off of, and it's not possible to exchange it with a transistor or MOSFET. Or, why not just use a radio transmitter circuit, but change the values so that it works at appropriate frequencies, with a power transistor?

I suspect vacuum tubes would be used in such applications because vacuum tubes can be made to operate at very high powers at high frequencies. That's why old radars (and some current ones) used tubes of various kinds, and still most microwave ovens are magnetron based. Of course semiconductors improve all the time, you can probably use transistors to do an induction heater with the specs you want.

Inducktion

Wed Mar 16 2011, 07:54PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Mattski wrote ...

But there still arises the same exact problem; what values do I use to make any of those oscillators work with the frequency range I want, with the semiconductors I want? :/

Inducktion

Wed Mar 16 2011, 10:30PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I attached my board I worked on a bit.
Hopefully that'll work for the Royer 2.
]supa_mega_royer.zip[/file]

Inducktion

Wed Mar 16 2011, 10:54PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Inducktion wrote ...

I attached my board I worked on a bit.
Hopefully that'll work for the Royer 2.

Sorry for Double/triple posting. Here's a board I recently made based off of Radhoo's oscillator for his small SSTC.

I hope it works, it uses the same principle as the ZVS driver behind the diode gate driving.
]oscillator.zip[/file]

James

Thu Mar 17 2011, 12:01AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Mattski wrote ...

As far as I know, *all* microwave ovens use magnetrons. There is simply no more efficient and cost effective method of producing high energy RF in that frequency range.

The frequency of any resonant circuit is determined by the capacitance and inductance of the components of the tank. Vacuum tubes have advantages in terms of robustness and power handling for high powered stuff, particularly when high voltage is involved. Semiconductors have largely taken over at this point, and I suspect the shift towards other topologies has to do with the way transistors behave vs tubes. These days high power semiconductors are cheap enough that for most applications it only really makes sense to use tubes for the cool factor.

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