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PLL SSTC, how to set it up?

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Dr. Dark Current

Fri Sept 18 2009, 12:30PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think I'm not sure how the PLL SSTC works. I think I should be using something like this schematic

, but how to choose the component values around the 4046?

How does it behave with weak feedback signals, when does it switch from locked operation to internal oscillator?
Can it sustain direct ground arcs without the bridge exploding, when using CT from the secondary base?

Sorry if the questions don't make much sense, I'm pretty much lost on what goes on with this circuit...

Arcstarter

Fri Sept 18 2009, 08:34PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

The 12k and 16k resistors on pin 11 and 12 chose the operating frequency, and the range. Change the to match your secondary accordingly. The 330pf capacitor on pins 6 and 7 is the timing capacitor. Change that to match your secondary accordingly.

Weak signals are fine, as long as the voltage is high enough. Unless you meant too low voltage? It required no real current for feedback, and as long as the feedback is at least 5 volts, it is ok. It is clamped to your supply voltage anyway. The PLL did much better than a mini sstc did with direct ground arcs, and the CT helped further to keep giving feedback while there are no sparks. So that helped keep it running in resonance, even when it is loaded massively.

To explain operation better, visit

. Under 'circuits' in the applet window, go down the list, and 4 from the bottom is PLL. Look under that. That will explain what is going on in there with the phase comparators and VCO and all.

Hope it helps, good luck :D.

PS, on that circuit with the PLL driver, you can insert a resistor that goes from the 10nf cap on pin 9 to ground. Where the resistor and capacitor connect is audio + in, and the common ground is your audio ground. Then you have nice sounding audio mod :D. For the resistor i used 160 ohms.

Avi

Sat Sept 19 2009, 12:29AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

This example

used the method explained by the 4046 datasheet, however it still needed some additional tweaking.

Dr. Dark Current

Sat Sept 19 2009, 07:19AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Okay, thanks for the replies. With weak feedback I meant what happens e.g when you're drawing a ground arc and make it too short that the secondary resonance just starts disappearing (the sec. base current never returns back to zero), does the output behave erratic when just losing the feedback signal?

...
Well, I think I'm gonna buy some cheaper FETs for experimenting, which I don't mind blowing up...

Arcstarter

Sat Sept 19 2009, 05:50PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

If the feedback fails, the VCO is still operating. So, essentially, as long as you do not have a driver failure, you will always have oscillations. But, without feedback, it will operate slightly out of tune, and cause a bit more heating. But, it won't cause the bridge to blow up. Also, with a CT and the topload shorted to ground, i have never lost feedback, on the mini sstc or PLL. So that should not be a problem.

Even when the output is shorted, like i said, you should get oscillations at the frequency of the VCO. When you have feedback, it kinda 'overpowers' the VCO, and runs on feedback. Just set the VCO as near resonance as you can, and you will be good.

Dr. Dark Current

Sat Sept 19 2009, 05:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thanks Arcstarter.
I kind of understand now how to set up the resistors from pins 11 and 12, but I don't understand what's the purpose of the two 120k resistors and the 10n capacitor. How are the values chosen? Can anybody explain? Thanks

Arcstarter

Sat Sept 19 2009, 08:21PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

The 120k resistor and 10n capacitor can be kept those values for any frequency, at least in the normal Tesla secondary range. Pin 2 is the output of phase comparator one, and 9 is the VCO input. The pot on pin 9 with the 120k resistor controls the oscillating freq of the VCO, and the 10nf capacitor is maybe to 'smooth' the VCO's input voltage, for stability, or maybe to prevent the oscillations from skipping around if the pot does not change resistance smoothly. Not sure what the other 120k resistor is for, from pin 9 to 2...

Also, make sure you choose the pin 11 and 12 to ground resistors wisely. Make sure the ratio is something like 1.5:1 or something, to ensure it does not lock to a harmonic frequency. Mine did that before, the mosfets do not run very happily like that... A scope is almost essential for this, so once running, you should scope *everything*. Make sure the phase of the output is correct, in relation to the gates on the mosfets, minimize ringing, blah blah blah.

Steve Conner

Sat Sept 19 2009, 08:24PM

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

Those resistors set the time constant of the PLL loop filter so that it locks without hunting. It took me about two pages of math to work it out.

Yes, it should stand direct ground arcs no matter what feedback method you use. You can see me on Youtube shorting the topload of my old SSTC to ground with a chicken stick while it's on.

The required feedback voltage is a few hundred millivolts IIRC.

Dr. Dark Current

Sat Sept 19 2009, 08:36PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thanks.
I just don't feel good about the 120k resistors near the huge RF field from a TC. Would the result be the same if I used 12k resistors and a 100n cap?

Arcstarter

Sat Sept 19 2009, 11:52PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

120kohms will be fine. My driver was 100% unshielded, about 8 inches from the secondary. It ran fine. Frequency did not vary more than .5KHz, as long as there was not a grounded object moving around it, such as my hand. That is around .1% of the total operating freq.

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