Joule Thief Special There are thousands of different types of transistors, made by hundreds of manufacturers. But it takes a special kind of transistor to give good performance in a Joule Thief. The transistor should have high current gain at high currents and very low collector voltages. A maximum collector current of 500 milliamps is a good starting point, the more the better. Most transistors have a loss in current gain as the current increases past a certain point; for small signal transistors that point is about 100 mA or 1/10 of an amp. The datasheet for the (Vishay) BC337 shows the current gain beginning to roll off before 100 mA, but by the time the current gets to a few hundred mA, the gain is still 200, so it still makes a good JT transistor. The 2N3904 datasheet (Motorola or On Semi) graph shows the gain at 10 mA is around 240, but at 200 mA, it has dropped drastically to about 30. This is a poor choice for a JT.
The JT transistor must also have a low Vce(sat) (collector to emitter saturation voltage) at high currents. I look at the graph and see if the transistor’s collector saturation voltage is 1/4 volt or less at currents of at least 250 mA, but hopefully at closer to a half amp. The BC337 (Vishay) shows the Vce(sat) rising above 0.25V when the collector current gets up to more than a half amp. Every BC337 that I have used can handle current better than a PN2222A or 2N4401, so I recommend it for JT use. The same Motorola datasheet for the 2N3904 shows that the Vce(sat) rises above 0.25V when the collector current is about 75 mA, and at 200 mA it is over a half volt, and that is very bad, because it is wasted power heating the transistor and not lighting the LED.
The following is a table of transistors used in the same JT, and the supply and LED currents that I measured. The 2N3904 is very close to the BC550 performance.
Recommended:
2N4401, PN2222A, BC337, BC338, SS8050
Not recommended:
BC547 series, 2N3904 (the JT exceeds its maximum collector current of 200 mA.)