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SSTC performance issues; audio modulation, ballasting, and noise

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LAN_403

doctor electrons

Wed Mar 27 2013, 08:15PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Time to post your schematics!!

Sigurthr

Wed Mar 27 2013, 10:48PM

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

Alright! Here is my current setup:

Logic side of things is from Po210's driver board, and the half bridge is my own rip off of Steve Ward. I have both a 1:1 and a 10:1 transformer for the modulation. The 1:1 can handle the current all day long and is lower impedance, where as the 10:1 is about 9.6Ohms DC and gets hot fast.

The power audio signal is about 20Vp-p and is rated as 250W max.

doctor electrons

Thu Mar 28 2013, 01:39AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Ok, one thing i notice from that schematic. There doesn't appear to be anything to dissipate the energy stored in c8,c9,and c10, even when the variac is turned off. Which, in theory, if there is still a signal at the antenna your circuit will switch until the stored energy is gone. Try putting some bleed resistors across c9 and c10. That's just what i saw looking at it briefly but there are plenty of master coil guys around that may see something else. I would start there and see if that solves the problem of the coil still operating when you kill power.

How long did it run after you killed the power?

Sigurthr

Thu Mar 28 2013, 02:00AM

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

Lol, it ran about four minutes after I unplugged the variac. I didn't put it in the schematic but I have a 1MOhm resistor across C8 as well. I'm certain the "post mortem" power came from the bond between secondary LV side and DC negative bus; as soon as I severed this the output stopped. It takes about 30Vac at 1.5A to produce the arc that was going, so it wasn't from residual capacitor charge.

This coil runs beatifully aside from
1) popping sounds above a certain power threshold that coincides with a momentary lack of oscillation.
2) No method of audio modulation I have tried has been successful.

It's those two issues I'd love to hammer out.

RateReducer

Thu Mar 28 2013, 07:28AM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

I think it is not the best idea to modulate a "self oscillating" coil like yours.
Why donÂ´t you try to use a fixed frequency and PWM for modulation? For example TL494.

Steve Conner

Thu Mar 28 2013, 02:27PM

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

Obviously if your DC bus is not isolated from the mains, you shouldn't connect the negative rail directly to ground. It's against electrical safety codes. That's why I mentioned using a Class Y capacitor.

(Of course building a Tesla coil violates practically every known electrical safety code

)

Sigurthr

Thu Mar 28 2013, 10:32PM

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

@ Steve Conner:
Sorry, I misunderstood your phrasing when you mentioned the class-y cap. I took it as "if you want to maintain isolation then use the cap", not "you need it if it is not isolated". Being as it's already not isolated and I didn't see a need for isolation (as you said, TCs violate all codes anyway, haha), I figured I would give it a shot. It was interesting to see and no harm no foul (proper personal electrical/RF/HV safety is never overrated!).

Thanks again for all the info and help, I appreciate any and all advice.

@ RateReducer I've never been able to get a fixed frequency coil to run properly. I think it may have just been that I could never get close enough to resonance with 555 based oscillators. My best attempt resulted in a nasty harsh buzzing tone to the arc and only about 85% of power output compared to with feedback. Audio modulating the arc via PWM of the fixed oscillator worked but the distortion was so horrid you could not make out any of the audio. I attributed this to the mixing of the audio signal and the buzz.

Dr. Dark Current

Fri Mar 29 2013, 10:19AM

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

The change in performance might be due to the fact that the inductor alters the RF envelope, so it is no longer square but ramped. The operation may be similar to a"QCW" coil, but beware that the inductor might induce voltage peaks on your bridge and blow it up.

Sigurthr

Sat Mar 30 2013, 06:06AM

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

Seems my little audio amp cannot tolerate being near a TC so my idea to use a modulation transformer is out the window.

So, I switched to a 4046PLL driver and after a few hours of fiddling with the phase adjustment to get it just right I can now run at full power with no noise or popping in the arc, and output power is up about 10%. Finally found something that works just right! I even got the audio modulation working, though it doesn't sound great - probably too much ripple on the DC bus. The feta are definitely running cooler as well.

Thanks for your help everyone.

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