Nice sine!
I've been trying to design a flexible full-power multi-channel EL inverter, but while the parts are on order this can be a testbed for the series resonant topology. Two inverters from the AC240 play the role of a full-bridge, driving 100 mH in series with 24 nF to +/- 5V. The resonance increases the voltage across the EL wire with each swing, limited in the end by the current that one output of the AC240 can deliver. I measured 18 mA RMS, 33 V RMS, 3200 Hz, which closely matches a theoretical ideal 100mH+24nF series resonant circuit. The neat thing about the resonance is that it turns 5V*40mA = 200mW of input power into about 600 mVA of "reactive power". The power dissipation could be much lower if lower-resistance switches were used for the bridge, or if it wasn't being driven at full duty cycle right up to the resistive limits of the AC240 chip. If we assume that the AC240 is a 20mA current source, then we'd expect to dissipate at least 100mW in the steady state even if every other component is ideal, because the AC240 only stops being able to deliver more power when the full 5V is dropped across it. 5V * 20 mA = 100 mW.
The feedback loop for this circuit is a bit dodgy. To drive a series-resonant tank using a switched voltage, you need to switch the driving voltage in phase with the output current. Because the load is almost completely reactive, the current is 90 degrees out of phase with the voltage. Either you could measure the voltage, then add 90 degrees of phase shift with some kind of filter, or you can measure the current. Many sine-wave oscillators use a transformer winding to sample the current; there's no transformer here so we can't do that. Instead, I measured the voltage drop of the AC240 inverter output, comparing it to another inverter with the same input but driving a light resistive load. When the current switches sign, the comparator trips and flips the full bridge.
In this scheme, the common-mode voltage swings from near 0V to 5V as the bridge flips, but in theory the differential voltage isn't suppose to change sign during the flip. In practice, extra transitions can be a problem. The first circuit I build, using a single inverter to drive side of the bridge, worked right away. When I tried to increase the output current by ganging together three inverters for each side, the circuit became highly unstable. The solderless breadboard probably plays a part, because the inductance of the tangle of jumpers created major ringing on fast transitions. I discovered that I could occasionally get the second circuit to work by touching the feedback network with my fingertips in just the right spot, adding some leakage resistance and capacitance that just happened to stabilize it. Unstable resonant circuits apparently make great proximity detectors and touch sensors.
It might be possible to regain stability with careful compensation of the comparator and feedback network, but I think it would be a better idea to redesign the feedback circuit, either with a sense resistor to make the differential measurement reliable, or with voltage measurement and a 90-degree phase shift. An inexpensive PLL like a 74HC4046 might help, too, by overriding temporary sense glitches and ensuring that the oscillator starts up.
So here's yet another use for the ubiquitous, BEAM-beloved 74AC240 chip.
