2018-12-04 FETs For Very Low Voltage Joule Thiefs

from FB group Building Transistor Radios 2018 Dec 4

Terry Harris
I didn’t try to mimic the ALD, I used the FETs for a voltage booster. The typical 5mm LED needs somewhere around 60 mW for Max brightness. If I was powering it with 60 millivolts, it would require over an amp to get the LED lit brightly. Since FETs might max out at a few tens of milliamps, there needed to be several in parallel. You have to remember that JFETs are already conducting with zero gate voltage, so they will oscillate with tens of millivolts supply voltage if the feedback winding is high enough turns ratio. Once the supply voltage gets below a volt, things that were once minor suddenly dominate the performance. Resistances of a small fraction of an ohm can cause a major drop in performance.

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