Plasma Speaker Schematic Draft -- Care to Comment? - Page 1
Forum section: electronics / projects

author: klr5205
author_rank: Regular Contributor
timestamp: September 30, 2016, 04:06:44 pm
content: Anybody care to look over this schematic for a plasma speaker? I'm just looking to have glaring errors or omissions brought to light.I decided to attempt a full bridge driver despite having never built one before. I implemented my own RC dead time control to prevent shoot-through, and added the connectors above the diodes as test points / a way to jumper it out. (There is a high side driver IC with the same footprint that includes its own shoot-through protection) I figure that with creative jumpering I can always just run it single ended if the full bridge doesn't perform as well as expected.The HV+ rail could in theory be as high as 170VDC, but I don't really have any intention of going that high, at least at first. I just wanted the capability to do so if the circuit seemed to function reasonably well at lower voltages. I'll check the gate signals w/ no HV+ first, and if everything looks right I'll probably start at 12V.Two notes:I'll probably go ahead and add dedicated flyback diodes across the FETs even though I'm not sure they're truly necessaryYes I know I had 5v available from the TL494 but 7805's are a dime a dozen and I was more comfortable using a dedicated regulator.This cell phone video wasn't really meant for mass consumption, but if you want to see the breadboarded version of this circuit running check it out here:

author: klr5205
author_rank: Regular Contributor
timestamp: October 04, 2016, 12:03:18 pm
content: One bump before moving on to layout...

author: Obi_Kwiet
author_rank: Regular Contributor
timestamp: October 04, 2016, 02:06:12 pm
content: You forgot to attach the schematic.

author: bktemp
author_rank: Super Contributor
timestamp: October 04, 2016, 02:16:59 pm
content: I don't think it is going to work this way: When changing the duty cycle away from 50%, you get a dc voltage at the output. If you are driving a transformer, the core will saturate and your mosfets will fail with a large bang. For a full bridge driver you need to either enable/disable the mosfets using PWM, or you need to phase shift one half bridge to get a variable output voltage.

author: klr5205
author_rank: Regular Contributor
timestamp: October 04, 2016, 02:59:26 pm
content: Quote from: bktemp on October 04, 2016, 02:16:59 pmI'm not sure I follow this, if only because I've proven in the video that this circuit works when using a single ended driver. As it is configured, the TL494 will nominally generate a ~50% duty cycle square wave (actually pulse wave), and the audio signal is used to modulate that. The full-bridge driver should just double the voltage swing I can get across the primary, unless I'm missing something. I don't think the transformer was saturating in my breadboarded circuit, although I suppose I could put a capacitor in series with the primary coil though to protect against that if it were an issue though.Quote from: Obi_Kwiet on October 04, 2016, 02:06:12 pmIt is attached as a pdf underneath the embedded video

author: bktemp
author_rank: Super Contributor
timestamp: October 04, 2016, 03:04:53 pm
content: Quote from: klr5205 on October 04, 2016, 02:59:26 pmDid you use a half bridge or a single transistor? If it is only a single transistor, it is a completely different SMPS topology.

author: klr5205
author_rank: Regular Contributor
timestamp: October 04, 2016, 04:33:09 pm
content: It was a single low side FET. The other side of the primary was tied to VDD.As I think more about your first comment I think I can conceptualize how the DC voltage you referred to could occur at duty cycles other than 50%. Are you aware of a decent app note that describes how to deal with this? Or would a series capacitor (perhaps w/ a bleed resistor to keep it from charging up) be adequate for a novelty device?

author: bktemp
author_rank: Super Contributor
timestamp: October 04, 2016, 05:20:33 pm
content: There is no appnote on how to solve this problem because it is simply the wrong circuit for driving a transformer.At 50% PWM, you get the maximum ac output signal. Going to 0% or 100% the signal turns more into a dc voltage with a lower ac component.This circuit could be used for driving a dc motor: At 50% the dc component is zero. Going to 0 or 100% gives either a positive or negative dc voltage, therefore the motor runs in either direction.I would use a phase shift PWM for driving the two half bridges: Each half bridge is driven at a fixed frequency and fixed 50% duty cylce. At 0%PWM both signals are identical, therefore the differential voltage seen by the transformer is zero. At higher PWM levels, the phase shift increases up to 180° at 100%: Now the transformer sees the full square wave.The only problem with phase shift PWM is how to generate the signals. It is not as easy as generating a simple PWM, because you need to delay the signal. But fortunally there are many phase shift PWM controllers doing this internaly.By the way: What type of transformer is this? It looks like an ignition transformer?

author: VintageNut
author_rank: Frequent Contributor
timestamp: October 04, 2016, 05:43:40 pm
content: That is very cool. I enjoy seeing a good corona or an arcover like a Jacob's ladder.I have built a rudimentary plasma speaker as a college senior project. It was a simple DC power supply and an modulating transformer. It worked enough to hear.The device will produce loads of ozone which is not good for your lungs.In the 1970s, Hill Plamatronics sold a system with a plasma tweeter. The tweeter had a helium tank inside to bleed helium into the arc to try to mitigate the production of ozone.If you smell the photocopier smell, you are breathing ozone.

author: CM800
author_rank: Frequent Contributor
timestamp: October 04, 2016, 07:00:08 pm
content: Nice job, I'm thinking of scaling something like this up and adding it to my X-Ray Transformer driver. (70kV & 150kV)I would strongly suggest changing the driving stage so that in the even the driving signal is lost, there is no voltage on the transformer, from the way I'm looking at it, (forgive me if I missed something?) if you stopped driving the switching line from the TL494 to the Inverters, then you would get full +HV DC voltage through the transformer with nothing to limit it or switch it the other way (no doubt causing it to fail)

author: viperidae
author_rank: Frequent Contributor
timestamp: October 05, 2016, 09:16:34 am
content: I think you should wire it up like a push-pull configuration. You can then use either the feedback or dtc pin to control the on time of each half of the h-bridge.

author: klr5205
author_rank: Regular Contributor
timestamp: October 05, 2016, 06:07:50 pm
content: Quote from: bktemp on October 04, 2016, 05:20:33 pmIt was actually harvested out of one of these:Quote from: CM800 on October 04, 2016, 07:00:08 pmGood point, I will come up with something for that.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: October 05, 2016, 06:56:57 pm
content: Is the ramp generator in the TL494 linear. If not I think it will make the input voltage to duty cycle a non linear function. And can tl494 be used without connecting the feedback pin. Looks good though..

author: klr5205
author_rank: Regular Contributor
timestamp: October 06, 2016, 02:05:27 pm
content: The ramp generator is a bona fide sawtooth from what I can tell in the datasheet, and I can say with certainty that the TL494 will run without the feedback pinEDIT: Actually thanks for triggering me to look at the unconnected pins of the TL494 - I noticed that my 10nF cap was mistakenly connected to pin 6 rather than pin 5 like it ought to have been!