What is the fastest Joule Thief in the world? I have had one operating at 13 MHz, believe it or not. What is the slowest Joule Thief? I built one with a large coil, and the high inductance caused it to operate at 1200 Hz, well within the audio frequency range. I think that that is the slowest Joule Thief I’ve had operating, however it’s relatively easy to get one to go slower. It’s more difficult to get it to run at very high speed.
In the case of the 13 MHz JT, I had one of the windings connected backwards, so it was 180 degrees out of phase and theoretically it should not have done anything. But it took off at 13 MHz instead, and even put out light from the LED. It was a real screamer. The fastest JT I have had with the windings correctly connected was 2.5 MHz, with very few turns on the air core coil. I suppose I could have removed more turns and made it run even higher. But as the frequency increases, the losses increase until the JT is wasting most of the power and gives very little light. It’s better to run the JT at a frequency where the losses are a small fraction of the total losses.
Back to experimenting…