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Mini Audio Modulated Tesla Coil

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currentkills91

Fri Sept 16 2011, 05:49AM

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

Okay, so this whole thing is getting rebuilt from the beginning...mainly due to a blown driver board (always check polarity before connecting power x.x ) Also, my cat got ahold of the secondary...

So here are the final specs:
Driver - Steve Ward's DRSSTC Driver .5, leaving out the primary cap making it an ISSTC.
Primary - 40mm X 25mm wound with 10 turns 18 gauge litz wire
Secondary - 26.8mm X 85mm wound with 1200 turns of 44AWG (.0635mm) wire. Resonant frequency: ~1.22MHz
Topload - 57mm X 27mm (2 red bull can bottoms) Calculated capacitance: ~2pF
Secondary resonant frequency with topload: ~800kHz
Switching MOSFETs - IPP110N20N3 - 200V 88A 11mohm Rds, 96ns rise and fall times
Interrupter - Steve's 555 interrupter, eventually audio modulation

Progress:
6/25/12 - Bought primary and secondary forms, secondary wire, sandpaper, and epoxy. ~ $20.00
7/5/12 - Attempted to wind the secondary twice, will have to build a better winding jig later.
7/22/12 - All new parts ordered for driver board, half-bridge, interrupter, and power supply. ~ $80.00
7/22/12 - Designed layout for driver board, half bridge, and power supply.
7/28/12 - Bought acrylic for case, still trying to find a way to cut it ~ $20
8/2/12 - Built driver board, and half-bridge

I am on my final revision of the driver board, and since I foolishly posted pictures throughout this thread I will be uploading all progress pics to my Flickr, and then posting links to them on this first thread.

Driver Board -

Half-Bridge -

Inducktion

Fri Sept 16 2011, 04:06PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Instead of using op amps to modulate the TL494, why not just use what they do with plasma speakers? It would be a lot easier, and plus there's already schematics out there showing you how.

Also, you don't even need an interrupter, especially if you want to play music.

ANDDDD, do you still need mosfets for your TC? You haven't replied to my email yet. :U

currentkills91

Fri Sept 16 2011, 08:54PM

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

I'm gonna be using the interrupter no matter what, because I don't wanna run CW. And I thought I did reply o.o hmmm, weird

Goodchild

Sat Sept 17 2011, 07:55AM

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

You know you don't need an external Op amp, the TL494 has 2 built in already. Also if your going to be using a DRSSTC driver you need the resonant cap or else the driver will not function. This is because the driver needs feedback from the primary tank circuit. With no resonant cap there will be no natural oscillation in the primary circuit and the driver will not oscillate.

At 2.2MHz your switching losses are also going to be significantly high due to capacitance in the switch storing voltage at turn off and turn on, wasting energy. I would suggest looking into a ZVS type driver such as a class E driver. This way you can turn on and off with zero voltage across the switch meaning zero switching losses (or at least close to it).

You have a great start on a resonator you just need some clever electronics to drive it

Reg,
Eric

currentkills91

Sat Sept 17 2011, 08:26AM

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

First thing, thanks for the compliment, and that driver actually relies on secondary feedback, so I'm in luck (: I've been going through the design and I realize I don't need the op-amps anymore. I was originally just gonna leave the interrupter out, but I need it in there for something I'm doing.

Also, if you missed it, the topload will bring the frequency down to around 1.3MHz. So driving this thing will be a little easier, I know it's gonna be tricky...but I'm good with tricky :)

Goodchild

Sat Sept 17 2011, 04:54PM

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

Wow it took the Fres down that much? Usually the toroid will only take the freq down by 10% or 15% at most. Also using secondary base feedback will hard switch your switches!!! This is because the secondary current and primary current are out of phase by 90* so as your switch turns on at the secondary current zero crossing the primary current is already at 25% of it's cycle, same goes for turn off. This will make for some nasty losses especially at 1.3MHz!!

1.3MHz is still ungodly high for most MOSFETs. You will still need some sort of ZVS topology. ZCS would be impractical because of the devices internal capacitance.

My recommendation for driving a coil like this would be to use a class DE half bridge and add a resonant cap in the primary. As ZVS relies on a resonant circuit to operate. I've tried the whole ISSTC and SSTC thing. It will be really dismal compared to what you could get with a resonant circuit and ZVS.

Two paper I highly recommend for class E and class DE operation:

Once you realize how awesome ZVS is you will never go back to the old SSTC/ISSTC method of driving a coil.

Inducktion

Sat Sept 17 2011, 06:00PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I thought secondary and primary currents are out of phase 180 degrees, not 90, in any case, using two schmitt trigger inverters would get rid of that problem anyway...

Goodchild

Sat Sept 17 2011, 09:08PM

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

Inducktion wrote ...

I thought secondary and primary currents are out of phase 180 degrees, not 90, in any case, using two schmitt trigger inverters would get rid of that problem anyway...

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. Adding a schmitt trigger would not fix this problem because the primary current is leading the secondary current by 90* (usually). this can change over the period of a burst as the loading and tuning changes because of added sparks. You can get a shift anywhere between 90* and 180*. Because the amount of phase between the two signal changes with time, there is no way to compensate without introducing a reference to the primary current.

As seen here you can see the phase relationship between the primary and secondary current changes.
6156854482 E612423f17 B

There is a reason that everyone stopped using secondary base feedback, that reason is that it will hard switch the switched 90% of the time resulting in very high switching losses.

currentkills91

Mon Sept 19 2011, 02:08AM

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

Well, you learn something new everyday O.O Thanks for the info guys. I've decided to go with making it a DRSSTC, I've already been ordering parts for it, and I already made the current transformer and the gate drive transformer. I also redesigned the primary and made a few un-important changes to the secondary. I'll post pics to my Flickr as soon as the batteries for my camera are done charging. (:

currentkills91

Tue Sept 20 2011, 02:07AM

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

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?

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