Joule Thief

[MarkE]

The solution is really quite simple.  When you reverse C and E you change the circuit from common emitter with a relatively high output impedance to common collector with a low output impedance and in either case current flow stops when the base is not more than 1 Vbe above the emitter (NPN).  The transformer acts as a 2:1 autotransformer between the LED side and the transistor base side.  As the base voltage increases, the emitter voltage decreases and vice-versa.  As long as the initial voltage source is greater than Vbe, the circuit should be able to start. It should be able to hold down to Vbe/2.  Current through the E-C path of a forward biased BJT can flow in either direction.

[TinselKoala]

The solution is really quite simple.  When you reverse C and E you change the circuit from common emitter with a relatively high output impedance to common collector with a low output impedance and in either case current flow stops when the base is not more than 1 Vbe above the emitter (NPN).  The transformer acts as a 2:1 autotransformer between the LED side and the transistor base side.  As the base voltage increases, the emitter voltage decreases and vice-versa.  As long as the initial voltage source is greater than Vbe, the circuit should be able to start. It should be able to hold down to Vbe/2.  Current through the E-C path of a forward biased BJT can flow in either direction.

Yep. As soon as I posted that schematic the "bulb lit up" in my brain. My understanding must be improving, thanks for your help.

[Dave45]

http://www.learnabout-electronics.org/PSU/psu31.php
http://www.learnabout-electronics.org/PSU/psu32.php
The boost converter is constantly pulling from the battery whereas the buck only pulls during the on time of the transistor.

[MarkE]

http://www.learnabout-electronics.org/PSU/psu31.php
http://www.learnabout-electronics.org/PSU/psu32.php
The boost converter is constantly pulling from the battery whereas the buck only pulls during the on time of the transistor.

That is true, but the JT and its variants use either a flyback:  discontinuous current at both ports, or boost: continuous input current / discontinuous output current.

[Dave45]

So could we use this to take high voltage ac bring down the voltage and boost the amperage.