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Tonskulus

Fri Mar 13 2009, 10:33PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

New experiment is here, dielectric heater. It can heat almost any nonconductive materials with high dielectric losses. Including wood, paper, some plastics, fabrics...

My design is simple hartley oscillator using high power tetrode. Material to be heated is placed between plates of tank capacitor. Plates can be almost any size, it can be tuned easily by adding or removing some series capacitance and/or inductance to get desired heating effect.

And here it comes..
schematic:
http://www.elisanet.fi/tonskulus/skema.jpg
system photo:
http://www.elisanet.fi/tonskulus/dheat1.jpg
Video:
http://www.youtube.com/watch?v=6TOwSp3oOy8&feature=channel_page

This seems to be very fast way for drying some wood etc.. Maybe i shoud try to cook some pizza with this?

Any ideas are welcome :)

Proud Mary

Fri Mar 13 2009, 11:22PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

78A for heater current alone! A tetrode with a great appetite!

...

Sat Mar 14 2009, 01:51AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Looks awesome!

although it looks like those plats are floating at 3kv with a ton of rf on them, might not be the safest system in the world

As a side note, I one had dinner with a man that worked at a composites company, who had a dielectric press which was basically a gigantic dialectic heater that heated the resin used for the products they made. He mentioned that the transmission line to the tank capacitor was made of out 2 pieces of concentric copper sprinkler pipe

He offered to let me come see it, but never followed up on my e-mail

Tonskulus

Sat Mar 14 2009, 08:05AM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

Yes, there are some really highpowered dielectric heaters used by many kinds of industry.

I had weird problem with this circuit, parasitic choke on grid 1 was always burning out in a minute. I used 5..10Watt resistors, seems like its not enough. There was 5turns of 1mm wire wound over resistor.
Anyway, without this pfc, circuit is not working at all..plate current drops down to 100mA.
Without load plate current was around 0.4-0.5Amps and rises up to 1.2A with some heavy load in work capacitor.
Can I say work capacitor?:D Because this is just vice versa compared to induction heater using work coils..

Proud Mary

Sat Mar 14 2009, 01:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What is the screen grid current Ig2?

What is the self-resonant frequency fo of L5?

What is the voltage across R2 in the two conditions you have described?

Couple very loosely a small loop of two or three turns containing a low voltage torch bulb to the hot end of L2. It should glow brightly if oscillation is present.

Alternatively, make a field strength meter from a short wire antenna of 15cm or so, a signal diode, a 1nF capacitor and a 1mA meter or thereabouts This should give a very strong indication of oscillation within several metres of such a powerful oscillator, and will help you adjust it for maximum power.

Remember that this circuit, as you have drawn it, with so much power, will be rich in harmonics, and will radiate strong interfering signals all the way into the VHF.

My advice: Make a proof-of-concept model using a cheap RF tetrode like 807 or whatever you have to hand, and work on the model until you have learned what are the best conditions of oscillation with your L & C, and RF choke. Very difficult to get a very large valve to behave as you want without some prior study!

Tonskulus

Sat Mar 14 2009, 03:42PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

Well, this is not my first tube oscillator, i think its about 100th one so i have learned what i need to :)

And it is working just fine. There is parasitic choke to prevent unwanted oscillations. But i have some more ideas to get even more efficiency..we will se that later.

I havent measured G2 current, voltage is 500V as it should be.

resonant freq of L5 is not critical if its not near Fres of tank circuit and its harmonics. It stays cool so its not resonating.

I have not measured voltage across gridleak resistor.. So yes there are lots of things to be optimized.
This is just very first version, i did calculate necessary values for components to get it working even a little bit. Tank impedance is about 1/10th of tubes load impedance.

Another modification to do is to change tetrode->triode to get rid of one transformer. I know, triode requires more driving power but that is not a problem. I want this to be as simple as possible,

Proud Mary

Sat Mar 14 2009, 05:02PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If the two metal plates shown in the video form the capacitance of the tuned circuit, they will certainly cause severe interference in the 14mHz band.

Tonskulus

Sat Mar 14 2009, 05:14PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

Harry wrote ...

If the two metal plates shown in the video form the capacitance of the tuned circuit, they will certainly cause severe interference in the 14mHz band.

Yes, indeed. As almost every dielectric heaters does. Usually those heating plates are open without any shielding.. But i have seen also 100% shielded ones too.

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