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> Plasma Speaker - Problem With Output, Having some problems with the output
TheComet
Posted: March 31, 2011 09:26 pm
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Hey everyone! I'm new here smile.gif


I'll get right down to the point. Here is the current schematic of my Plasma Speaker:

http://i254.photobucket.com/albums/hh100/T...asmaspeaker.png

FETs are irf530n and irf9530n. I know, not such a good choice, but I had nothing else lying around ATM. Circuit runs at 12 V DC, and I'm using a computer power supply (max 18 A). The PWM signal originates from a TL949 clocked at 30 kHz.

The problem I am having is that the flyback transformer (primary winding is 7 turns around the ferrite core) I connected only makes puny little 1mm sparks, and no music can be heard. The flyback transformer however works fine when I wire it up to a 6 kHz signal... I also tried a car ignition coil, but that failed as well. When measuring the output with an oscilloscope (nothing attached to output), I get a clean square wave signal, including the modulated signal. If I attach a speaker to the output, the music comes through nicely. Why doesn't this work with a transformer? The output signal is distorted a lot when a coil is connected, but not enough (in my opinion) to stop this from working.

If you spot any other problems with my circuit, please say so smile.gif


TheComet

EDIT: Big pic made link.

This post has been edited by tekwiz on April 01, 2011 07:41 pm
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tekwiz
Posted: April 01, 2011 07:58 pm
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I can think of a few issues. For one, flyback trafos are made to work at one frequency...15.7khz. Therefore they have a very sharp peak in performance at this frequency. Adjusting you frequency to a harmonic of this fundamental frequency may help.
Most plasma speaker designs operate at far higher frequencies...well into the mhz range.
Your flyback may also have an open secondary winding.
Another problem with flybacks is that they virtually all have voltage multipliers built in, & these tend to rectify & filter the output. The higher the frequency, the better this filter action. After all, CRTs require DC as the second anode voltage.
You might get better results from using the transformer from a solid state neon sign power supply. These are very similar to flybacks, except they have tapped secondaries & no voltage multiplier.
Often these power supplies can be obtained free for the asking from neon sign service shops after they fail & have been replaced. Note that it isn't usually the transformer secondary that fails, & even if it is, the failure only affects half of the winding. Any other failure cause doesn't matter, as all you need is the secondary & the core.
Now, for accurate music reproduction, your output must be at half power with no signal input. This because music has both positive & negative signal components & you cannot go lower than zero when feeding an arc. In fact, you don't even want to reach zero, as this will extinguish the arc & make re-striking it difficult...it takes lots of voltage to strike an arc, but very little to maintain it. A 1" arc can be maintained with less than 75 volts, whereas it may take 30kV or more to strike it.


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TheComet
Posted: April 03, 2011 11:50 am
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Thanks for your reply!

I tried lowering the frequency as you suggested, but it had no effect. I tried the 15.7 kHz, but it just had the same puny spark... I even went down to 6 kHz, still nothing. What is even more disturbing is that the flyback transformer worked in this configuration:

user posted image

6 kHz modulated input @ 12 V

The music was a little fuzzy, but recognizable.


I checked, and my output is at half power with no input, so there's nothing wrong there. It's something to do with the circuit in my first post... Is there something wrong in it? Is it OK that the n-channel FET is at the top and the p-channel FET at the bottom? Are the diodes correct?

TheComet
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tekwiz
Posted: April 03, 2011 05:53 pm
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I think most of the circuit looks OK...the main FETs & diode polarities are correct. The only thing I'm not sure of is the driver stage. Seems to me that the resistors & FETs should be interchanged...normally an N-Ch FET is used to switch the ground, not the positive.
Perhaps another member can clarify this.
That circuit, BTW is called an H bridge. There are lots of articles on MOSFET H bridge theory online. Google the term & have a look at some of them. wink.gif


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Sch3mat1c
Posted: April 05, 2011 03:07 pm
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Two things:
1. PWM means you're putting DC on the transformer. Thus, the transformer must be rated for the lowest frequency present in the audio, else it will saturate, drawing large currents through your transistors and potentially destroying them. (With only a 5V supply, mere IRF530s will probably survive as long as they are heatsinked (heatsunk?).)

2. Flyback works better because it's a flyback transformer. The moment the transistor turns off, a large spike of voltage is produced, which is able to breakdown the air gap. The amount of energy in that spark is roughly proportional to the time the transistor spent on, so it can be PWMed.

I think you will find better results building little more than an AM transmitter. This is pretty easy to do. First, you need a bridge, like in your first circuit, but one which can handle any supply voltage. Two copies of this circuit should do a fair job:
http://webpages.charter.net/dawill/tmoranw...Regen_Motor.png
Note that the transformer goes between +Motor on both halves, and -Batt goes to ground. +Batt goes to the supply, which will be modulated. (Since it will be modulated, omit the 1000uF bypass capacitors.) Note also that you need both halves driven opposite, so you need an inverter between the 5V Logic Inputs of each half. That way, one turns on and the other turns off, etc.

As for modulation, here's what you do. Find a power transformer with dual secondaries. It might have four pins, or four wires, or something like that on the secondary side. Hook up one secondary to an audio amplifier (any old ~20W amp will do), and the other in series with your inverter supply voltage. So when you drive audio through the transformer, now the supply voltage goes up and down, which means your output power goes up and down with the audio. Perfect. Hook up the transformer and oscillator and you should have a workable plasma tweeter -- add more power (supply voltage and audio volume) as desired.

Tim


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