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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
@Daedronus Do you mean that little spring thing that slides onto the end of the probe? I tried that as the probes ground return but it made no difference.
Well I think I have made as much improvements as I can with it mounted on stripboard, mainly using lots of de-coupling capacitors (including on the bridge) and refining the layout of the bridge section. I also added a 470R variable resistor in series with the audio source since my phones internal volume control increases in large increments and easily drives the circuit into over modulation.
Also I found using a pointed edge for the lower electrode and a suspended keyring as the top one helps reduce electrode related distortion. Here is what I got with 24v on the bridge drawing about 2.5 amps. 12v for the logic.
You might have to turn your sound up since my DSLR doesn't do a really good job at audio (seriously the mic is a small pin hole on the side of the cameras body).
In reality the sound from the arc is about as loud as a small speaker and its pretty low distortion. The arc will ignite at around 3-4 inches on 36v but the LOPT arcs over becuase of the AC primary waveform. So I keep it loaded at all times with much smaller arc as seen in the video.
I had it running for about an hour and a half last night and the heatsink only really got warm on the one side where the FET's are mounted. The RC snubber helps too, I am not sure if the values are right but a 560 ohm resistor in series with a 270nF capacitor in parallel with the primary coil is what I used, they only get warm too. I think if I used a lower value cap I could get slightly more volume from the arc, since having the RC snubber there does reduce the volume slightly.
There was still some gate-source waveform distortion once voltage is applied to the bridge (even with no LOPT connected). I think it could be the FET's gate-drain miller capacitance causing it? who knows, perhaps I just need lower V-DS FET's for their lower parasitic capacitances.
But I am relatively pleased with how this one has turned out so far, plus its turned out to be a good first project with my scope.
Anyway, I am sure this is all old news to most of you guys.
Just one question though, what actually happens when you over-modulate? is it the chip that gets overloaded or is it to do the bridge/core of the LOPT saturating? remembering this is frequency modulation and not PWM.
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How to improve gate drive waveform's?
