![]() 2) At lower voltages, the collector current through the transistor is limited by the base current configured with the base resistor. This is why it is suggested to use a ferrite core versus some other core material with a higher magnetic flux capacity. ![]() I found some references that alluded to the original Joule Thief circuit having two operating modes, which I think reduce this variation somewhat: 1) When the battery has lots of life left, and the voltage is high, the trigger for the transistor to switch off is the core reaching saturation. I won't talk more about the circuit he describes, but you can view the video here if interested: I set up an LTSpice simulation of both circuits, and after playing with those for a bit and doing some reading on the topic, discovered that, in the simulation at least, the current through the LED varies greatly over the life of the battery. I suspect it was chosen due to the fact that it does not require winding a custom inductor. w2aew's video describes a slightly modified version of this circuit, and one that is neither as efficient nor able to operate to as low a source voltage. It has the ability to operate down to very small source voltages (I measured start-up voltages as low as 0.54V, and operating voltages as low as 0.44V). The circuit is rather clever in its simplicity. The "joule thief" is a very simple switching, boost voltage converter that allows a near-dead AA or AAA alkaline battery to be used to power an LED that normally requires a 3-4V supply. I watched one of w2aew's videos which was about a little open source hardware project, specifically a version of the popular "joule thief" project for the "Ears to Our World" charity.
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