Joule Thief: PCBJT Wirelessly Powered by SassyClassE SSTC

I’ve dusted off the SassyClassE SSTC for some experimentation, and I happened to discover an interesting thing.

I can easily light up little LEDs near the coil using various hookups, but I was surprised at how well the PCBJT (printed circuit board Joule Thief) lit up. I discovered this accidentally. My workbench was piled with junk and projects new and old as usual, and I set down a peanut can capacity on the bench and must have made the correct connection to the JT completely randomly, because it lit up brilliantly for a moment, without being connected to anything. Then I discovered that I had to be touching the collector somehow, either on the backside of the board or the can of the transistor itself (2n2222a metal can has collector connected to case), along with something touching one or the other, or both, of the power input terminals to the board.

And of course the SassyClassE coil needs to be running.

The SassyClassE is running here on about 75 Watts input, so don’t go getting all excited. It’s not an especially “efficient” system, although when it’s running properly with a true Class E waveform it is a lot better. It uses a single IRFP450 mosfet to switch the DC input power to the coil’s primary, and the low aspect ratio secondary rings like a struck bell. (I may have given the wrong mosfet part number in the video; it is definitely IRFP450, no final letter).

The schematic I used for the SSTC is here:

I am using an IRFP450 mosfet, though, and I’ve never been able to run the input power up beyond about 50 volts, 6 amps from a DC power supply. The placement of the primary and the feedback loop are critical; it takes a bit of fiddling around to get them placed just right.

I’ve got 420 turns of #27 magnet wire on the secondary, and a 5-turn primary of #12 solid copper house wire. The feedback pickup loop is a single turn of the same, mounted directly underneath the secondary’s platform. The coil runs at around 800 kHz or so depending on surroundings and topload.

A fuse and/or current-limiting power supply will prevent blowing a lot of mosfets during tuning. I wish I’d had the discipline…

I also put in a small, separate circuit to monitor the temperature of the heatsink, using an op-amp comparator and a thermistor. This also works as an oscillation indicator, as the light turns on when the system is oscillating. If it stays on when the bias pot is turned down I know the transistor is getting hot.

The schematic for the PCBJT can be found in the videos specifically discussing that item; check my channel to find it.

Thanks for watching!

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