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Looking for quick and dirty HF oscillator for Class E SSTC

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Dr. Shark

Mon Mar 13 2006, 09:16AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

This is still my Class-E SSTC endavour. I have played around with CD4046s, NE555s, antenna feedback, but nothing so far has worked well. The feedback approach has the drawback that I cannot test the driver on its own, but only connected to the tesla coil. The CD4046 is a way too complicated chip for me, I just can't figure out how to set the center frequency, locking range and this funny low-pass filter feedback thing correctly.

Therefore I am now thinking about putting together something like a Colpits or Hartley oscillator to provide a stable 4MHz or whatever sine-wave which I can then square up and amplify with a gate driver chip. It would of course be nicer if could tune the frequency with a pot, but I don't want things to get too complicated and I can live with tuning a cap of inductor.

Is it completely stupid to use a valve-era oscillator to drive a SSTC, and I should either get accustomed to todays digital stuff or leave it completely? Or is this a remotely feasible idea?

ragnar

Mon Mar 13 2006, 10:30AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

A joule-thief circuit with a tiny inductor will get to those frequencies quite easily.

I might also mention that high-speed phase-locked-loop chips are very nice cheap oscillators, considering they're always running, and you can kinda control them with a pot.

Alternatively, you might be inspired by my 'tiny royer' oscillator, built with 2N7000 FETs and 1N4148 feedback.

Try joule-thief HFPLL, or tiny royer, for those frequencies, I'd say. =)

For reference, I've had a joule-thief-esque circuit running at a memorable 3.6MHz. Wind your inductor from very fine wire around a chip of ferrite or something or half a ferrite bead, not a whole one =P

Sulaiman

Mon Mar 13 2006, 11:28AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have been considering an LC oscillator (variable + fixed capacitors)
along the lines of a regenerative radio receiver/tuned-base feedback oscillator.
A small aerial to the inductor, or a sample of the primary (or secondary) current
should be able to lock the oscillator - haven't got round to building it yet
but the Idea seems good.

HV Enthusiast

Mon Mar 13 2006, 12:24PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Just use a crystal oscillator. They are cheap and easy to use. You can also get ones that can be tuned slightly around its fo if needed.

Dr. Shark

Tue Mar 14 2006, 04:20PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I think crystal oscillators are not tuneable enough for my application. I really like the tiny royer idea because you drive the gate directly and eliminate any losses from the gate capacitance. Together with Class E, this could give insane efficiencys. I am not sure if I have any diodes that work at 5MHz though. Is the 1N4148 common enough that I could salvage it from an old TV or so?

Steve Conner

Tue Mar 14 2006, 06:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, the 1N4148 is a very common diode. Even Maplin sell them

The datasheet for the Philips 4046 is very confusing. I think they're mainly trying to hide the fact that the frequency and tuning characteristic vary a lot with supply voltage, temperature, and between different chips. That's not a problem when you use it in a self-tuning circuit like mine, but it makes the chip look cheap and nasty (which it is)

I think the idea of using a Colpitts or Harley oscillator is a good one and ought to be explored further, I just never got round to trying it

As for crystal oscillators: Richie Burnett used one to drive his Class-E SSTC. Even tunable crystal oscillators have too little range to do anything useful in the context of Tesla coil tuning, so you have to tune your coil to the crystal.

Dr. Shark

Wed Mar 15 2006, 07:47AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Ah, finally I don't feel like an idiot anymore for spending hours trying to make sense of the CD4046. Cheap and nasty. Right!
I think I will try out the Hartley circuit, it seems a lot simple than using the Royer flyback circuit to resonate the gate capacitance, and the gate losses are only 10W or so anyway, so it's not worth the extra effort for me. I'll let you know how it goes.

kaboooom2000uk

Fri Mar 17 2006, 01:18AM

Registered Member #324 Joined: Thu Mar 16 2006, 10:28PM
Location: UK, Midlands, Buckingham, MK
Posts: 6

i got a simple circuit based upon opamps and a handfull of resistors and a few capacitors.

you will need a dual supply to run this circuit, +12 0v and -12, the beauty of this is that u can use the negative going pulse to discharge the gate of a FET or IGBT,

aternativley you can pass the (positive) signal through logic to clean up the signal and to go into a opto or a GDT driver, il post the circuit, if its unreadable i can email a copy if needed.

**link**

Dr. Shark

Sat Mar 18 2006, 06:30PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

That circuit is a bit complicated for my taste. Actually I now went the simples route possible and used antenna feedback straight to the gate driver - basically Steve Wards Micro SSTC.

I don't seem to be able to get the proper Class E waveforms to appear though. Instead of the drain voltage decribing the positive half of a sinusoid as it should, it ramps up exponentially and only starts going down when the FET is turing on, causing massive losses. I am using pretty much exactly the component vaules Richie Burnett gives in his HF-SSTC schematic, and for the shunt capacitor I have tried values from 0-10nF.
I wonder what is wrong this time?

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