As the waveform isn't important, use any small square wave oscillator and feed it to a MOSFET to switch the current on and off. Think of a power MOSFET as an almost ideal switch, when off the leakage is negligible, when on a good MOSFET is like a <1 Ohm resistor. Although they draw virtually no DC current into their gate pins, they do have a high capacitance (possibly >1nF) between the gate and source pins so you need to be sure you can produce enough current to lift the gate voltage when driven at 440KHz.
Beware that driving with anything but a pure sine wave will produce lots of harmonics, some of them falling inside the AM broadcast band so be prepared for complaints if the neighbors find out. Driving 440KHz sine wave would be considerably more complicated.
Brian.
As the waveform isn't important, use any small square wave oscillator and feed it to a MOSFET to switch the current on and off. Think of a power MOSFET as an almost ideal switch, when off the leakage is negligible, when on a good MOSFET is like a <1 Ohm resistor. Although they draw virtually no DC current into their gate pins, they do have a high capacitance (possibly >1nF) between the gate and source pins so you need to be sure you can produce enough current to lift the gate voltage when driven at 440KHz.
Beware that driving with anything but a pure sine wave will produce lots of harmonics, some of them falling inside the AM broadcast band so be prepared for complaints if the neighbors find out. Driving 440KHz sine wave would be considerably more complicated.
Brian.
Oscillators which use their MOSFETs in the saturation region are very inefficient e.g. Colpitts, Hartley (around 50% efficiency).I've looked at standard circuits such as Colpitts, Hartley & Wien and I wonder if a transistor of sufficient power rating and freq. will work, and if such a transistor exists. Some info. I've read suggests I may need to use MOSFETs? I have no knowledge of them or what circuit I could use.
Driving pulses into an inductor WILL cause harmonics to radiate, the real question is whether you can live with them or not.
I'd like to try H-bridge but would want to run with a 12V supply. Could you recommend a transistor type for the current / freq I need?
Unlike the demonstration circuit in post #7, the final circuit will rather generate square waves at both H-bridge legs and have the transistors controlled with required dead time. For currents up to 5 or 10 A, a 12 V supplied H-bridge can use transistors without heat sinks at 440 kHz.
Probably a TO-3 package and heat sink. Notice the sine wave suggests the half-bridges have moments of shoot-through (although there may be times the transistors switch clean On and Off). It's hard to be sure how to calculate dissipated watts, but at 12V 1A it could generate a lot of heat. Since I have several metal 2N3055 (a common type) I would start by using those for the N-device.
The PNP counterpart is MJ2955A or TIP2955.
This family should be able to switch at 440 kHz.
I think that FvM was saying that instead of using the self oscillating circuit from post #7, you can get rid of the "R" resistor and drive the transistors fully into saturation thus achieving high efficiency and square wave at both legs. You still need the LC series resonant circuit.That attracted me to this idea, but I presume there are other factors to consider that I don't yet understand?
There is not tool that designs an H-Bridge by itself specifically for this application, you need to use math as usually and then simulate it.Could you (or anyone reading) advise me of any tools I may use to calculate required components values etc. this would be a revelation to me.
using a large copper coil of unknown inductance and capacitance that I made as my primary.
My simulation does not make it obvious that my 420 uH value is selected so that it restricts current below 2 Amperes (with a 3V power supply). The capacitor is selected to determine resonance at 440 kHz.
Since your power supply spec is 12V, you'll need a larger Henry value than 420 uH, and a proportionally smaller Farad value. For a simplified schematic, here an op amp sends more than 1 Ampere through the series LC. A mere .07 ohm resistor can detect zero crossings, generating 0.1V swing. This is sufficient to cause a change of state from the op amp's output.
View attachment 144244
However the above is only theory and I suspect that it breaks down somewhere. My simulation develops half a million volts across the coil and the same across the capacitor!
I doubt that happens in real life. Only experimentation can confirm what is or is not possible.
A unit which expresses flux intensity is Webers. Calculated as Henries * Amperes.
You installed a resistor, which works to reduce current. And yes you can use a transistor or mosfet to achieve this. The method is similar to resistive drop. It generates heat the same as the resistor does. Nevertheless the method is used in many simple power supplies, including my homebrew supply.
It's worth a try for you to test high and low amounts of current through your coil. Be aware that by abruptly shutting off current in an inductor, it often generates voltage spikes. The greater the current, the greater the spike.
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