My last plasma speaker used a single output mosfet, and that hard-switched FET got hot. To cut down on the heat I decided to use a half bridge to drive the flyback this time. In essence it’s a circuit which two flip-flopping mosfets; each taking a turn conducting the load current. Kind of like a push-pull amplifier.  Current spikes are lower, the FETs switch better, less heat is made and everyone’s happy

Below is a pictorial schematic of the circuit, while a regular schematic can be found here for those who prefer that. For people who like instructables, here’s one. Essentially it’s a high voltage class D amplifier.

By sending more current through an arc you produce a hotter arc. Hotter arcs will have a higher pressure, and by varying the current through an arc you create varying pressures –something which is commonly known as sound.

Now this circuit uses PWM to accomplish that. By shortening the width of the pulses to the mosfet’s gates you can reduce the current that flows through the flyback. By changing those pulses to the tune of music, the arc’s current and pressure will vary in tune with the music, thus producing music!

Some important notes about plasma #2:

• The 10kΩ pot still controls the frequency, and a lower frequency means higher voltage. The sweet spot was around 50kHz for my flyback but it may be different for yours. It’s a trivial matter to turn a knob though.

• There is a gate drive transformer. What this does is provide an isolated gate drive for the upper mosfet and inverts the signal for the lower one. A GDT is simple to make: all you do is take a bundle of 3 wires and wrap them around a ferrite toroid. I used 14 turns on a 2.5 centimeter toroid, but other designs may work better for you. Ferrite toroids can be bought for about a dollar online if you don’t have one laying around.

• The 1uF cap must be either an MKP, mylar or foil cap. No electrolytics! Its function is to block excess DC current from flowing through the gate drive transformer as capacitors block DC and pass AC.

• The GDT must be phased correctly for the bridge to work otherwise your mosfets switch on simultaneously and blow up. What is phasing you ask? Phasing is the direction of the coils in a transformer. For this circuit to work, one of the coils on the mosfet side must be ‘reversed’. On schematics phasing is marked by dots, and you can get a better picture of what’s supposed to be done by looking here. A properly phased transformer looks like this on a scope; one wave is inverted.

• Too loud a signal can and will overload the chip. When you input your audio set the volume to 0 then slowly turn it up. When the music becomes distorted that’s as loud as the speaker is getting. It’s not terribly loud, but it is still easily heard in a quiet room. You must have a clean audio source too, no electrical noise.

Plasma #2:

Youtube video of it playing some supertramp. It sounds much better in real life (more bass). Sorry Germany, UMG blocked the video in your country.