Joule Thief

[TinselKoala]

PNP Joule Thief

The standard JT circuit using NPN transistor will also work with a PNP transistor, by reversing polarity of the Battery and the LED, and connecting the PNP collector to where the NPN emitter was connected, and the PNP emitter where the NPN collector was connected. I've just confirmed this on my Testbed JT, which uses a 1:1 toroid winding of 13+13 turns and a 2s12p series-parallel array of 24 LEDs, running on a depleted AAA battery.

I just got a package of 50 ea. BC556B transistors for about $3.50 US  and I need to find something to do with them, since I only needed one for the reason I ordered them.

[Dave45]

PNP Joule Thief

The standard JT circuit using NPN transistor will also work with a PNP transistor, by reversing polarity of the Battery and the LED, and connecting the PNP collector to where the NPN emitter was connected, and the PNP emitter where the NPN collector was connected. I've just confirmed this on my Testbed JT,

Interesting
I built the pnp joule thief as well but just changed out the transistors from npn to pnp and it worked well.
The transistor legs went back into the same positions, emitter to emitter, base to base and collector to collector.
Of course the battery and led were reversed.
I didnt think to turn the transistor around.

[TinselKoala]

I've found that some NPN transistors will also work "reversed" in the JT testbed with no other changes from the normal NPN configuration. Only some, not all. 

The BC337-25 is one such NPN that works both ways, although the "correct" way results in a brighter LED array and an audible singing sound.

The BC556B PNP also works both ways, I found, with the battery and LED polarities reversed from the normal NPN circuit.

Bear in mind that my testbed uses a 13+13 turn winding. It might not be the case that a reversed transistor will work with other winding ratios. I'm using a AAA battery that reads about 1.31 volts unloaded, and I'm driving a 24 LED array of white LEDs, 12 in parallel, in series with another 12 in parallel.

[Dave45]

I find it interesting that the high side setup is a start stop reaction whereas the low side setup continues to circulate the electron and current.
It would seem the low side setup would be much more efficient

[Dave45]

Its not really the transistor type but the topology