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Effect of increasing secondary size on Steve's MicroSSTC?

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ahahn

Thu Aug 23 2012, 04:47PM

Registered Member #6075 Joined: Wed Aug 08 2012, 11:48PM
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I wasn't able to get the right size PVC for making a secondary with the precise dimensions of Steve Ward's MicroSSTC. Using what I had, I wound ~815 turns (instead of 595) on a slightly smaller diameter pipe. How is this going to change the performance of the coil? I note the design isn't really operating at resonance to begin with. (Presumably it'll mean a fair bit more tweaking when I upgrade it to Class-E, but I want to get the basic version working first.)

ahahn

Thu Aug 23 2012, 07:31PM

Registered Member #6075 Joined: Wed Aug 08 2012, 11:48PM
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I ended up messing with the primary geometry too as a result of the materials I had at hand.

Final dimensions of the secondary:
815 turns, 231mm x 75mm

Primary:
10 turns, 140mm x 30mm

Primary is wound parallel to the secondary, with the top of the primary ending where the secondary begins.

There's a grounded sheet of aluminum about the diameter of the primary about 5cm under the primary, and both driver and secondary have an RF ground in the form of a cold water pipe nearby. (ground connection via an alligator clip and 5cm wide aluminum tape strip in parallel)

I powered it up, and it worked. Discharge into free air was 1-2cm (~3/4"?), I was able to draw a slightly shorter discharge with a piece of aluminum foil on the tip of a plastic bottle. Operation was very nearly silent, a little bit of hissing from the breakout that went away as soon as I drew an arc.

Putting a 10MOhm input multimeter nearby with leads outstretched as a pseudo-dipole suggested a bit of RF output with no arc being drawn (~.2V RMS on the meter, 550kHz). Drawing an arc sent the frequency readout all over the place, as far down as single-digit kilohertz. A broadband diode detector hooked up to a pair of headphones indicated the coil was emitting plenty of hash with no load, but everything went nearly silent as soon as I drew discharge.

The 12V regulator with copper-clad PCB strip as heatsink got a little warm, the HEXFET (on a beefy aluminum one) not at all.

Right, any suggestions on improving it or tuning it to class-E? Right now C3 is the originally-specc'ed 0.001uF, but I've also got 0.0047 and 0.01uF caps that I can swap in.

ahahn

Thu Aug 23 2012, 10:40PM

Registered Member #6075 Joined: Wed Aug 08 2012, 11:48PM
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Followup. I tried calculating the secondary peak voltage and got about 600V, which seems very low given the discharge. What's the proper way to calculate that value?

Here's what I did. Plugging dimensions into JavaTC gives the following values, among other things:
Secondary--
13.226 mH = Lee-Equivalent Energy Inductance

Primary--
21.39 ÂµH = Ldc-Low Frequency Inductance

Vprimary = 24V

Vsec = Vprimary * sqrt( L sec / L primary)

--> 24V * sqrt (.013 / .000013 ) = 24V * sqrt (619) = 24 * 25

= 600V.

(boosting the primary capacitor to .0047uF and telling JavaTC to optimize it ends up giving me a calculated 767V secondary peak voltage; with .01uF on the primary that goes up to a hair under 1120V.)

Uspring

Fri Aug 24 2012, 01:27PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Followup. I tried calculating the secondary peak voltage and got about 600V, which seems very low given the discharge. What's the proper way to calculate that value?

The equation you used applies only to SGTCs. There is no simple way of calculating the output voltage of a SSTC. It involves the primary and secondary resonant frequencies, coupling, secondary spark load etc.

ahahn

Fri Aug 24 2012, 02:27PM

Registered Member #6075 Joined: Wed Aug 08 2012, 11:48PM
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Is there any documentation you can point me to on how to calculate the output voltage, or at least what's involved?

Uspring

Fri Aug 24 2012, 02:54PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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I not aware of any docs about this. You might want to try a Pspice simulation.

ahahn

Fri Aug 24 2012, 03:15PM

Registered Member #6075 Joined: Wed Aug 08 2012, 11:48PM
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I don't know enough about SPICE to simulate the magnetic field interactions of a Tesla coil. So:
How do primary & secondary resonant frequencies affect output voltage?
...coupling?
...secondary spark load presumably just affects the secondary resonant frequency.

When people talk of Class-E, is that merely driving a primary & secondary with equal resonant frequency at that frequency? Does that change the output voltage, or merely the amount of current available at the output?

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