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One-Transistor Voltage Converter

Taking apart a solar-powered lamp revealed a single-transistor voltage converter circuit that allowed an LED to be driven from a 1.2-V cell. The upper diagram shows the circuit (with slight modifications). The circuit oscillates at about 500 kHz and, at a cell voltage of 1.4 V, draws 11 mA with a respectably bright LED.

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Colin55 2 years ago

You may find the two inductors have the same inductance but completely different resistances. From memory one had about 4 ohms and the other about 33 ohms.

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Johan Benko 2 years ago

Glad you pointed to this simple but very useful schematic! It can also be a life saver in case you only have only "empty batteries" at hand.

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Johan Benko 2 years ago

also known as Joule Thief

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SVETOSLAV PIREV 2 years ago

What is the efficiency ? I expect no more than 60%-70%

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Happy Harry 2 years ago

According to the first referenced articles, a typical Joule thief is at 40% to 60% but the article shows and explains an update to give 80%. The main advantage of the Joule thief though is that it works down to a very low voltage. The second reference states 0.6v using a silicon transistor, but also shows a modified circuit using a germanium transistor which is supposed to work as low as 0.3 V, but even works as low as 50-100 mV.

efficiency: https://devopedia.org/joule-thief-circuit

voltage: http://tefatronix.g6.cz/display.php?page=ulv_jt&lang=en

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