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Author Topic: Induction Heater as a Tesla Coil driver + magnetic modulator  (Read 2370 times)
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HenrySWL
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« on: March 22, 2017, 01:32:08 PM »

My initial attempts at constructing an audio modulated tesla coil are mentioned here

http://amfone.net/Amforum/index.php?topic=41655.0

I've decided to largely depart from my design and much of what I had constructed because the 75khz grid driver remained plagued with parasitic oscillations. The secondary is still useful, and even if I end up winding a new one, the point of this is to use a frequency low enough that ferrite transformers can be used to drive it.

During the attempts at constructing this, I began thinking about the possibility of combining a plate modulator and a plate power supply; basically a much larger version of the grid modulator I had been constructing with a ferrite transformer from an inverter-microwave oven power supply. This solid-state supply would drive a ferrite transformer at the resonant frequency of the tesla coil secondary, the tube would be operating at half-wave, and the audio modulation would be applied at the grid.

Where to find a large power supply or amplifier that can operate at the required frequency between 70 and 80 khz? An ideal range is between 60 and 120khz, leaving me freedom to use a different secondary if needed.

The closest thing I've found to such power supplies are induction heaters. These can actually be obtained quite cheaply from china.

One I have been looking at is the LH-15A; a 15kw heater with a frequency range between 30 and 80 khz which is less than $1000
EDIT- I've since abandoned considering this unit in part because 15kw is the peak rating
To explore the feasibility I have ordered some smaller MOSFET inverters which are sold as "induction heater boards"
Their rating is "1kva 50 amps 12-36 volts"


It would be preferable if I had a way to set the frequency of this power supply exactly; without precise regulation of the output frequency it would be pointless to attempt to modulate a stage which derives it's power from this one. It may not be too difficult to do this, but the drive requirements may be substantial.

I've also reconsidered my use of tubes to accomplish modulation. What I was building before was not going to have enough power to satisfy me anyways. OPCOM suggested modulating the source voltage (drain voltage? to the mosfets, but I had another idea for modulation which is mentioned in my reply

Attached is a picture of the induction heater/inverter boards I've ordered


* w.jpeg (84.8 KB, 1000x1000 - viewed 228 times.)
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W2PFY
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« Reply #1 on: March 23, 2017, 10:12:49 PM »

Quote
Where to find a large power supply or amplifier that can operate at the required

What are your requirements just for the power supply? Perhaps low voltage? For example 36 or 48 volts maybe up to 400 amps or so? I may know of sources for that voltage and amperage range..

Over

Terry
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The secrecy of my job prevents me from knowing what I am doing.
WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #2 on: March 24, 2017, 10:19:10 AM »


My personal beef with the Tesla coil crowd is their failure to understand that these are transmitters!
As such - for display purposes they need to be in a GROUNDED and SHIELDED enclosure - like a metal building for example.


There are a bunch of sites and people who have driven large Tesla coils with solid state and tube gear...
...speaking only my opinion, if you have to ask these questions, you're better off not messing around with HIGH VOLTAGE & HIGH POWER.

                   _-_-
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WD5JKO
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WD5JKO


« Reply #3 on: March 24, 2017, 10:37:09 AM »

...speaking only my opinion, if you have to ask these questions, you're better off not messing around with HIGH VOLTAGE & HIGH POWER.

What could possibly happen?   Cry

Jim
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HenrySWL
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« Reply #4 on: March 24, 2017, 06:18:39 PM »

Quote
Where to find a large power supply or amplifier that can operate at the required

What are your requirements just for the power supply? Perhaps low voltage? For example 36 or 48 volts maybe up to 400 amps or so? I may know of sources for that voltage and amperage range..

Over

Terry

I'm just looking at wattage and frequency so far with the solid state approach. I've wanted to make something large. I need something that can drive a large tesla coil secondary. Depending how I wind it, anything from 70 to 120khz.

Lower frequencies will allow me to use ferrite transformers in the modulation stage. But I don't really want to go much below 80khz resonant frequency on the tesla coil secondary because if it gets too close to the audio spectrum in it's resonant frequency it won't sound good when it's audio modulated.

The voltage isn't too critical... it's going to be a pain because I'm either going to be dealing with high current or high voltage to obtain high power levels anyways.


My personal beef with the Tesla coil crowd is their failure to understand that these are transmitters!
As such - for display purposes they need to be in a GROUNDED and SHIELDED enclosure - like a metal building for example.


There are a bunch of sites and people who have driven large Tesla coils with solid state and tube gear...
...speaking only my opinion, if you have to ask these questions, you're better off not messing around with HIGH VOLTAGE & HIGH POWER.

The bottom of the secondary needs to be well grounded. As far as shielding, wind is a bigger issue than RF, at least at these frequencies. Are you doing something below 1mhz that I'm going to mess up if the building that holds this isn't metal? I doubt my 75khz tesla coil is going to bother anyone on the air from afar. It may bother the neighbors or interfere with electronics locally.

It's not like I'm trying to run a spark gap coil... those can make some real interference.

-----------------------------------------------------------------------------------------------------------------
THOUGHTS ON AUDIO MODULATION

I had an interesting idea for how to modulate this tesla coil that I definitely will need some advice on.


Rather than having to come up with a filament transformer and water cooling for a tube, I'd rather use a simpler means to accomplish the audio modulation...

the magnetic amplifier

further reading can be found here:

http://teslapress.com/magamp.pdf

http://www.tuks.nl/pdf/Reference_Material/Magnetic_Amplifiers/Platt%20-%20Magnetic%20Amplifiers%20Theory%20and%20Application%20-%201958.pdf

http://arizona.openrepository.com/arizona/bitstream/10150/319332/1/AZU_TD_BOX2_E9791_1956_56.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/021818.pdf

http://sparkbangbuzz.com/mag-amp/mag-amp.htm
http://sparkbangbuzz.com/mag-audio-amp/mag-audio-amp.htm

Really what I'm after is a magnetic modulator.

The primary of this modulatior transformer is run in series with the 1kw induction heater output. The inductance is high when there is no signal, and lowered by the addition of a signal in the other winding. Bias is controlled by the DC voltage which is applied to the "modulation winding" (unless anyone knows the actual term for this).

 They actually work exactly like tubes in a certain sense, except relying on magnetic rather than electrostatic principles for switching. An image of the modulator circuit I intend to use is included, as well as a direct comparison of the magnetic amplifier to a tube.

Some design considerations for this transformer.. The primary winding must handle 36 volts at 30 amps.
I'm unsure of what the other winding should be. According to my reference material (sparkbangbuzz) a gain of 10-15 was observed in a magnetic amplifier circuit that did not use a diode on the power coil, and a gain of 1000 was observed with the addition of the diode in series with the power winding. It's impossible to know just what the gain might be until it's built. My design consideration will assume a maximum of 100 watts drive, with extra taps because I intend to use a setup which includes diodes.

The control winding on this transformer would see a DC voltage and a signal from an audio amp with 8 ohms output. So if I assume the control winding would need to handle a maximum of 100 watts and it acts as an 8 ohm load, that would be approximately 28 volts at 3.5 amps. To avoid induced AC voltage, this control winding can be two 14 volt windings with two legs hooked together such that the remaining two legs show 0 volts AC between them. DC voltage will be applied between these two points for bias, and audio will be coupled through a capacitor into these windings.

Here are some vendors I've found who sell cheaply customized ferrite transformers. Their cost is very economical for the amount of power they handle:

https://www.aliexpress.com/item/Customize-Vertical-Type-Horizontal-Type-EE55B-Primary-Pure-Copper-Inverter-High-frequency-Transformer-12V-24V-1000W/32755579418.html

https://www.aliexpress.com/item/ER3501-PTH-vertical-high-frequency-switch-power-supply-transformer-set-up-transformer-push-pull/32728017774.html


Any input is appreciated. I know this may seem like a bit of a hair-brained scheme but I think making a few custom transformers for modulation may be the easiest and cheapest way to accomplish reasonably high quality audio modulation.


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* magnetic amplifier as a modulator.png (27.49 KB, 471x260 - viewed 216 times.)

* ns2093-cr-33-l.jpg (51.22 KB, 484x219 - viewed 238 times.)
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