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Mini Audio Modulated Tesla Coil

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Goodchild

Tue Sept 20 2011, 08:39PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

That is a really good question, Yes I believe that this is may be partly the case. That is why Litz wire is so awesome at high frequencies. Though because the wires share the same core, the effect may not be as substantial.

Though I would go with a single conductor in this case. It will be very hard to tune a DR with the primary setup the way you have it. I would also still consider doing a ZVS driver over a ZCS. At least at this frequency you would have a lot to gain.

Tetris

Wed Sept 21 2011, 12:29AM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

OMG ITS ADORABLE! (I love all tiny things) Its so cute! :D You need to post a video when you are done! LOL touch the sparks! Aww lol put it on a hat. :D Hat coil.

currentkills91

Thu Oct 06 2011, 01:22PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Okay, so I looked over everything and thought about it, and i'm just gonna stick to what I know. (for now anyways) I'm using Steve Ward's DRSSTC .5 driver. I am going to just grit my teeth and make it a DRSSTC after all (:

I have a question though...With the operating frequency being this high, can I interrupt the coil at 30kHz? The primary circuit is gonna be a fairly high impedance setup so the actual operating current doesn't blow my IGBT's. But even with that i'm not sure if 30kHz is a good idea. I want the high frequency so I can do the audio modulation, and not get the harsh noise from the carrier frequency, if that's the appropriate term to use?...

I also made ALOT of progress, having finished the driver board, interrupter, GDT, and the CT, along with the actual base for everything to be mounted on. I'll try to throw some new pics up of everything sometime today(:

Until then, any response to my question would be much appreciated :D

currentkills91

Tue Oct 11 2011, 11:26PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Okay, since nobody answered my question, I'm gonna add to it.

Will the coil work with a max duty cycle of around 8% and an interrupt frequency of 20-30kHz?

EDIT: Pics added to my Flickr account.

currentkills91

Sun Mar 25 2012, 09:20AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Started working on the coil again, alot of things got in the way x.x I have also decided to make this a SSTC to get use to the operating frequency of this small of a coil. I will later convert it to a DRSSTC.

So I started work on the half bridge tonight, still waiting for the rest of the parts though. They will be here hopefully Friday, if not then Monday. I have the capacitors but the way this will be layed out I need the zener diodes first. (sorry about the quality)

Datasheets are listed for in depth part specs:

The MOSFETs are STP30NF20 - 200V 30A 65mohm 98ns rise/fall times total
The Diodes are STTH10R04 - 400V 10A 15ns recovery time
The Capacitors are ECW-F2564JAQ - 250V .56uF (Didn't pay any attention to the datasheet when I bought these, and they will most likely fail)
The Zener Diodes are 1N4746A - 18V 1W
The Resistors are ERX-2SJ6R8 - 6.8ohm 2W (Had them on hand, testing will determine the final resistance)
The heatsinks are rather small I know, but with a low duty cycle and maybe a fan they should work OK. The diodes are only mounted on them to make it look neat!(:

Steve Ward

Sun Mar 25 2012, 05:57PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Well ok if you want to be technical about it's 270* out of phase or you have a 90* lag in primary vs secondary current

I think you are wrong Eric. The reason secondary feedback is a poor choice for a *DRSSTC* is that during the transient response, the phase between primary and secondary currents will typically drift between zero and 180 degrees (which is why the waveforms "beat" as they add and cancel). Eventually, even for a DRSSTC, the primary and secondary currents should settle out to 0 or 180 degrees, shifted only slightly by the real resistance of the circuit (or the sparks)

For a SSTC, there is the magnetizing current (the current that would flow in the primary inductance with out a secondary present) will be non-sinusoidal and "90 degrees" lagging the voltage. Generally this current should be small relative to the current thats driving the secondary coil, which is IN PHASE for a single resonant SSTC. The net result is some small phase shift, not 90*.

How will having my primary consist of two wires effect my primary inductance? The calculated inductance of a one wire primary would be 2.22uH. So if I have 2 of these in parallel will they act as resistors do when put in parallel? Like, will the inductance decrease down to 1.11?

Nope, you gotta add back in the mutual inductance term. Your coil will be really close to the original 2.22uH, only slightly less because a twisted pair of wires like that arent perfectly coupled to one another, so the mutual inductance between them is slightly less than 2.22uH. If you care to see the math:

M = k*sqrt(L1*L2) = 1*sqrt(2.22*2.22) = 2.22 [here k is nearly one given the physical twisting of the 2 wires]
Ltotal = 1/[1/(L1 + M) + 1/(L2 + M)] = 1/[1/(2.22+2.22) + 1/(2.22+2.22)] = 2.22.

Now, if k were zero because these 2 coils were physically isolated, then M = 0 and Ltotal drops to 1.11. The reality is that it should be really close to the inductance of just 1 wire because the 2 wires share the same field, and its the field that matters.

currentkills91

Sun Mar 25 2012, 11:15PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Thanks for clearing that up Steve, I'll be using 2 wires for the primary then as I like the look. That equation is being saved too for future reference :)

currentkills91

Sun Apr 01 2012, 06:24AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

I decided to rebuild the driver board, however the layout is terrible. I imagine it will still work though, I don't see why it wouldn't? I have yet to make sure I have all the wire jumpers correct. I will get around to it before it's ready for testing though.

It's based on Steve Ward's DRSSTC .5 Driver, with the zener and schottky feedback regulator, so all credit goes to him. The whole board is only 60mm long, by 32mm wide, by about 28mm tall with the heatsink. The TO-220 package is a 15V 1.5A regulator, the TO-92 package is a 5V 100ma regulator. I have decoupling caps on both regulators, and one for each chip as close as I could fit them to the chips themselves. The driver chips only have 10uF tantalum caps for now, but I will swap them for 33uF ones when I get some. The output cap for the GDT is a .27uF cap, but I think it should probably be bigger? It will use secondary feedback, and it will be an ISSTC. Yep, finally decided on a final choice, lol

Bottom of the board, with the nasty wire jumpers...I tried somewhat to the best of my ability to route them in a way where things aren't overlapping too much... (Will those jumpers cause any major problems, I know at high frequencies things get tricky..)

Well, what do you think, any input is appreciated. I will have the interrupter rebuilt hopefully tomorrow

currentkills91

Wed Jun 27 2012, 12:32AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

First post updated with new info!!

currentkills91

Sat Jun 30 2012, 02:44AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Progress Report:

Today I built the driver, the half-bridge (minus GDT) and started getting the primary and secondary ready to wind. Interrupter will be built sometime tonight, but that's the least impressive thing.

My goal was to have the layout as small as possible while having no jumper wires on the bottoms of the boards. I so far have pretty much accomplished this, the driver board could be smaller yes...but I like it. There is one jumper on the bottom of the half-bridge board however, because I didn't want to put a solder track there. Also I tried to eliminate ground loops, didn't really pay attention to how well I did this though.

(Sorry for pic quality, Ipod is all I have right now)

1341023098 3831 FT124461 Securedownload 2

Driver Board, based off Steve Wards DRSSTC Driver .5
(blue jumpers are just covering the input pins for the interrupter and GDT output)

1341023098 3831 FT124461 Securedownload 3

Bottom of driver board
(yes I know I forgot to solder the other GDT terminal to the driver output x.x )

1341023098 3831 FT124461 Securedownload 4

Half-bridge board, the GDT will be mounted on here as well when it's "perfected"

1341023098 3831 FT124461 Securedownload 5

Bottom of half-bridge board

Progress is good so far, power supply is still a work in progress in my head...not 100% sure how I'm gonna do it yet.

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