LED Boost converter (aka “Joule Thief”) analysis

In the past I already tested a nice boost circuit that allows us to power a 3 V LED with a discharged battery (around 1 V or less). This circuit uses just one transistor, one resistor and a small transformer (1:1 ratio) and it is better known, at least on the internet, under the name of “Joule Thief”.

The theory behind this circuit seems to be obscure as we can find lot of comments and questions like: “can I add turns ?”, “can I increase the resistor ?”, “will it work with two batteries ?” and so on…

I decided to figure out the working mechanism of this smart circuit in order to understand the rules behind it and try to improve the efficiency. I’ll carry out a theoretical analysis, supported by some scope screen-shots.

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