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Cute little FBT

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Mark-H

Sun Nov 14 2010, 02:15PM

Registered Member #607 Joined: Tue Mar 27 2007, 10:39AM
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Posts: 64

I built a few of these using a ZVS and just modulating the DC supply with a 1Watt amp via a 2N3055.
The volume isn't startling but it's an easy way to make one.
My videos are here...

Cheers. Mark.

Adam Munich

Mon Nov 15 2010, 01:51AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Well the 555 circuit failed, so I tried the modulated zvs idea. After I built the mini zvs, I appllied power into the wrong place by mistake and blew up my only TO-220 mosfets. Until I get more, this project's on hold.

Arcstarter

Mon Nov 15 2010, 02:31AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Maybe instead of giving up you could have just found the problem.

The audio mod ZVS will be very inefficient. Using a transistor to vary the voltage to the driver will be mostly linear, dissipating like 50% of the power. And, it does not sound as good as the 555. What is your audio source?

Adam Munich

Mon Nov 15 2010, 02:41AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I didn't get to the audio source yet, there wasn't even an arc from the FBT with the 555 circuit. The 555 was oscillating alright, but the IRF640 I used got really hot. It was still working though, because I did get a few little flashes when I touched the FBT HV wire to ground. The same kind of spark you get when you short out a 9V, barely visible. That suggests some switching though, but nothig spectacular. It was the same problem I had before with that circuit.

I kept killing the power before the fet got hot enough to burn, so it should still be good.

Edit: Instead of spending 10 bux on mosfets that I will likely burn out, I'm going to build that FM circuit instead. I still have 4 IRFP260s that need a home.

Adam Munich

Wed Nov 24 2010, 05:34AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Yay! It works!

I used the simple design, but there's a few problems with it. #1 Is that the mosfet pumps out a bunch of heat at only 12V. A whole bunch of watts. It still works, but I'm afraid to run it higher than 12V because bad stuff may happen.

Here is a video of it running at a low frequency, somewhere around 30kHz. This gives it very high voltage, so high in fact that corona forms around the FBT when it's open circuit. The arc makes quiet music at this freq, but since the voltage is high I can move the electrodes apart to make an unstable arc. This makes loud crackling music that the camera can't pick up right.

At higher freqs the voltage is lower, but it makes a nice stable arc that is relatively loud. About as loud as ipod headphones on full blast, maybe a tad bit louder. At higher freqs though I get squeaking sounds when there is a loud beat on the music. 50kHz seems to be the sweet spot.

I think the half bridge would solve many problems, but I don't have a gate drive transformer. For those that made this circuit, what did you use?

Freitsu

Wed Nov 24 2010, 08:17AM

Registered Member #3147 Joined: Sun Aug 29 2010, 10:53AM
Location: Finland
Posts: 56

I got a crazy idea, how about if you wound a secondary on the inductor from ZVS and then feeding an audiomodulated signal into that secondary (without ofcourse frying the amplifier then...). Wouldn't that then effectively vary the impedance thereby varying the frequency of the driver?

Thing is I can't come up with a driver for this...

Adam Munich

Thu Nov 25 2010, 04:51AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Well I solved one problem, the distortion one. Turns out that after 3 hours of scratching my head all I needed was a better power supply. 18A from the ATX wasn't cutting it. So after hooking it up to a gel cell I got great results. The audio is still a little crackly though, maybe some capacitors will fix that.

As you can probably tell by the video, heat is still a major issue. If anyone did the halfbridge, did the heat problem vanish, and if so what did you use for a gate drive xformer?

Adam Munich

Fri Nov 26 2010, 10:07PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I made a page:

Adam Munich

Fri Dec 10 2010, 10:34PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Anyone want to help me make a gate drive transformer? I've never made one before...

I want to put this circuit on a PCB but ice baths aren't too potable. I figured the half bridge would cut on the heat. Also, where do the 18v zeners go?

Edit; Just found a ferrite toroid. 23.6mm OD, 12.6mm ID, 9.6mm thick. I don't have an L meter, so how many tuns do you think would suffice?

Adam Munich

Wed Dec 15 2010, 04:58AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

This FBT has a permanent home. I used the half bridge design and it definitely cuts don on the heat, A LOT. It still can't run continuously because I used those 460's I pulled from the PSU. (460's have a high rds-on). If I use FETs with a lower resistance it should run forever without a problem.

There's another problem though, even though I can draw the arc to 1.4", any more than .5" and the music gets quiet and distorted. Maybe I need more capacitance on my half bridge, (I've got 14,000uF and the circuit runs at 24V). I really have no clue what's wrong, but I guess that's what an oscilloscope is for.

Video tomorrow, pix today.

Edit: The IC only runs at 12V (via the 7812 in back). Maybe the speaker will preform better with a 7815 in there. I just don't ant to blow up my fets! I'll also need lager zeners; I used 12v ones and I'll need 18V ones if I go for 15V.

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