2015-12-16 JFET Joule Thief

I captured this off a video and tried to make it a bit more readable.  I think it was from Sanjev.


I built it and it does work at 40 millivolts.  The 2SK170 JFETs are not common, and may be difficult to obtain.  Other JFETs can be used, but I haven’t tried them.  I wrote about this in the updates in a previous blog. The reason there are so many turns on the feedback winding is the input voltage is very low, and the JFET’s gate is controlled by voltage, not current like a regular BJT transistor.

2 Responses

  1. Telmo says:

    Hi. What are the dimensions of the toroid?

  2. admin says:

    In order to get 100 turns of thin wire onto a toroid you need something like an OD of 0.87 inch or 22 mm, an example is FT87-43. Another that I found that works is this tiny inverter transformer.


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