I removed the two core coil from the circuit of my previous blog and put the 550 turn coil in its place. In my haste to see how well it worked, I connected the power supply clip leads backwards, so positive was connected to negative, etc. But I really didn’t notice, because it was working. But then as I was working on it, I realized the mistake, and corrected it. But now it wasn’t working. Okay, so it worked with the power backwards but not with it correctly connected. Then I remembered that JFETs can be symmetrical: the source and drain can be interchanged. This must be what’s happening when I connected it backwards. So what I needed to do was reverse one winding to get it to work.
I unsoldered the fine wires of the feedback winding, and reconnected them the correct way. I turned the power on and it started working. But the LED started to glow at 87 millivolts, which was 6 or 7 higher than the original coil. And the oscillations showed up on the scope at 48 millivolts, about ten mV higher than the original. These are still quite low, but I was disappointed that they were not as good as the original. After all, I spent an hour or two winding all that wire onto the core – and got no benefit for it.
See the original blog here.
More on Low V JTs here in Ch. 3.
Back to experimenting…
