Joule Thief Testing

While trying to get my Joule Thief to work I brought in/wound several coils. Having these coils around I decided to try out various coil and transistor combinations to see what does and doesn't work. Maybe this will help out other people who are looking to build stuff (on the other hand maybe it won't, meh).

The Coils

From left to right:

• Big Clive Coil, 12 turns of double wound (both wires wound together, 12 turns on each) 38swg enamelled wire (e.g. YN91Y) on a small ferrite bead around 3.5mm OD, 1.5mm ID and 5mm long (N88CU in fact) on a pice of strip board for easy bread boarding.
• A common mode suppression choke (a 26-708), 37 turns of 0.5mm enamelled copper  on each side of a ferrite torroid 25mm OD, 14mm ID and 10mm long
• A salvaged ferrite (from a Lacie HDD cady) wound ala MAKE/EMS. 7 turns of 7/0.2 hookup wire on a 15mm OD, 8mm ID and 7.5mm long
• A Crude coil of 7 turns of PS2 keyboard cable on a ferrite (from a 26-8018) 27mm OD, 14mm ID and 11mm long. The PS2 keyboard cable has 4x 0.7mm wires in the middle with a foil and single wire shield. Because of the multiple cores they can be wired in two pairs (7 turns), or in series with another wire (14 turns)

Theses were paired with NPN transistors I happened to have knocking about:

The Transistors

From left to right:

• PN2222a (TO-92 case)
• BC107 (TO-18 case)
• BSS52 (TO-39 case)
• BFY 50 (TO-39 case)
• TIP31A (TO-220 esque case(may not be a 220 but a 257?))

The other components are a 1/4W (I think metal film) resister of unknown origin,  and a white LED (55-2484) the stupid brightness is probably not needed here but it's what I had to hand.

I  tried every combination of coil and transistor giving these results:

• PN2222a, all coils work well exception the common mode suppression coil wich gives a very dim light.
• BC107, all coils work and give good bright light.
• BSS52, all coils work and give good bright light.
• BFY50, all coils work and give good bright light.
• TIP31A, only works with the common mode suppression coil giving good bright light. Bupkis on and other coils.

Clearly there is some thing about the common mode coil that is ideally suited to the TIP31A but not to the PN2222a,  maybe something to do with the gate properties? The common mode coil has the most turns of wire but also probably the worst coupling (with the coils wound on oposite sides of the torrus instead of next to each other) wich is interesting, I might return to this when I have a scope.