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I was wondering what is the role of this type of circuit connected to LEDs. I found it in a LED torch supplied by 2xAA batteries. Is it an oscillator? Is it only an energy trick? What are the advantages of driving a LED with high frequencies?

(LEDs emit white light, BJT transistor NPN is 8122 or equivalent NTE108, the value of inductor is 10u about).

Thanks for replies

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Is Q1 upside down? \$\endgroup\$ – AaronD May 1 '15 at 20:18
  • \$\begingroup\$ I think Q1 must be upside down. I don't see at all how it would work with Q1 like that. But the catch is, I don't see how it would work the other way, either. \$\endgroup\$ – mkeith May 2 '15 at 4:57
  • \$\begingroup\$ I'm doubtful indeed. The 8122 (or NTE108) transistor has TO92 package and my doubt is that on its datasheet (NTE108) the pinout is E-B-C (looked from the flat side), whereas others transistors (like BC547) has C-B-E pinout. However I drew the schematic considering the first case (NTE108 datasheet). If Q1 was upside down, it would have Vbe voltage of 3V, then it would work very beyond saturation. \$\endgroup\$ – Overlord87 May 2 '15 at 9:07
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This circuit is a variant of so called "Joule Thief" which is a variant of "Blocking oscillator". The main use of it is to increase the battery voltage output, which is needed when the battery is considered to be "dead" by regular means.

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8122 is not a BJT, but a white led driver YX8122!

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