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Build Log - Modulated SGTC

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Sigurthr

Thu Apr 07 2016, 04:46AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Thin, long wire connections, gator clips, possibly poor ground, lots of "stuff" around the coil... all these things detune it. You've got losses and failure points stacked up pretty high. I'd try improving the physical set up and then measuring things again.

Art

Thu Apr 07 2016, 01:42PM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

Hi :) It was pretty much the same setup for the primary,
except what has to be changed.
Itâ€™s the first YouTube hit for â€œTesla Coil Tuningâ€ that I followed.

It just seems to me that was a dramatic difference to use less than half the primary turns for resonance.
I have got performance back up to my initial bench testing, so thatâ€™s good :)

Art

Sat Apr 09 2016, 05:51PM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

hen918 wrote ...

It might be easier to start / stop the oscillator/driver by pulling the gates of the MOSFETs low via a couple of diodes. Will have to try this out at some point. May be possible to turn a Mazzilli driver into a Voltage controlled power supply with pulse width modulation. It might be quite slow to react though.

Edit: I presume it's a Mazzilli and not some other ZVS driver...

Thinking about this today when I ran the ZVS driver a little higher voltage than itâ€™s rated,
I felt RF on the positive battery terminal for the first time.
Maybe itâ€™s back EMF from the flyback transformer, or maybe from the Tesla coil primary from too high voltage for the diode in the flyback.
Now I worry that connecting an IO pin from a 555 or microcontroller, will wobble itâ€™s supply
through itâ€™s diodes and such. If a microcontoller supply line is wobbled too much it makes it impossible to use itâ€™s inputs.

Thereâ€™s no telling the DC solid state relay will isolate the load from the silicon controlling it either,
at least until I find a data sheet on it.

I do have a test oscillator ready for it now that can strobe at visual frequencies, and then use a second 555 to
modulate the strobed signal at audio frequencies. It also has a 1A H-Bridge in case it helps to drive the DC SSR
control input with current, and then pull down (or go high impedance) to turn the SSR control input off.

Art

Sun Apr 10 2016, 01:22PM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

Well that worked :)

hen918

Wed Apr 13 2016, 08:25PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I've just built an uzzors2k style capacitor charge controller:

, and when not attached to any charging capacitor, the feedback loop turns it into an interrupter in the mid audio range.
A proper modulator could be built quite well using diodes to disable the driver, I think.

Art

Thu Apr 14 2016, 10:20AM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

Hi :) That does seem the most streamline way to go, but Iâ€™ve moved to an idea using three identical SS DC relays to
switch in the battery array at three different taps.. 12V (for tuning with a narrow gap), then 24-36V during operation
which could be sequenced by the microcontroller. Hopefully also with some frequency feedback to turn it off if it stops working.

hen918 wrote ...

I've just built an uzzors2k style capacitor charge controller:

Art

Fri May 06 2016, 04:24PM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

Hi Guys :)
This is kind of what I meant by â€œModulated SGTCâ€.
It shouldnâ€™t be possible to modulate it at a higher frequency than the spark is arcing.

It was the perfect project to try an oscilloscope with a HD44780 LCD controller.
The keypad is connected with Toslink hardware talked about in another thread.
Cheers, Art.

Art

Sun May 22 2016, 04:49AM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

That was hair raising to glue the primary form together: 20pm Zpsor6lrki9

Now I have 3 DC SSRs at different voltage taps so it should actually be possible to make a very crude sine at the controller end.

Sulaiman

Sun May 22 2016, 06:53AM

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

Very nice !

A late answer to your question
" If the 13 turns primary was maintained,
which of decreasing the coil pitch or increasing the coil diameter
would be expected to increase primary inductance further?"

for a LONG solenoid L = u.Area.n^2/height,
so inductance is proportional to r^2
and inversely proportional to height.

for shorter coils this is not correct but similar, L = (r^2 x N^2)/((9 x r) + (10 x h))
but not quite so easy to conceptualise.

for a given length of wire or copper tubing,
maximum inductance is achieved when the radius is approximately equal to the height.
(conductor length = 2 x pi x r x n plus the above formula allows you to prove it ! )

so increasing the coil diameter will have the greatest effect, but needs more copper,
and of course coupling will decrease much more compared to shortening the primary.

"It shouldnâ€™t be possible to modulate it at a higher frequency than the spark is arcing"
true,
but the audio modulating frequency needs to be less than half of the spark repetition frequency
to avoid severe distortion

For audio modulation you intend to use DC SSRs,
is the frequency response of your SSRs adequate ?

Art

Sun May 22 2016, 07:17AM

Registered Member #57369 Joined: Fri Sept 18 2015, 01:24PM
Location:
Posts: 66

Hi, & Thanks :)
Itâ€™s a bit late to make changes now. I wish Iâ€™d gone a little wider,
but the problem remains Iâ€™d need a wider form to wind it.
This primary form was made so I could pre-wind the copper tube
onto a stock section of PVC, and then thread it after.

Thereâ€™s more room to move with tuning than I expected though,
since my measurements with signal generator & scope werenâ€™t consistent with the unit in operation,
and the error worked in my favour.

Itâ€™s not audio frequencies Iâ€™m hoping for right now, just strobe and pulse effects.
The spark gap operates at an audio frequency, so I canâ€™t expect to modulate that with another audio frequency.
The DC SSRs appear to handle audio speeds at the current theyâ€™re switching though.

The whole base part including the driver stuff which is not pictured here is being built modular,
so one day it would be possible to keep the transformer, and update the rest :)
Itâ€™s taken a very long time considering I wound the secondary in October â€™15!
Itâ€™s never going to impress with massive arcs, so the plan has always been to be distinguished in other ways.

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