Step 6: The Usual Suspects

For the Joule Thief to work well, it is important to choose the right transistor; it has to handle relatively high currents and switch quickly on very small signal changes. It will also have to work with low voltages at frequencies beyond the audio spectrum. If you look at the data sheets of the transistors, the key pieces of information will be Ic (or Max Collector Current), HFE (the Signal Gain or Amplification factor), Vce (the Collector-Emitter Voltage drop) and ft (Frequency). Successful candidates should have a low, low Vce (less than 0.2v) and high Ic, HFE and ft.

The following table lists some of the popular makes with graphs for them below. The 2N3904 values are always shown on Ch1, in yellow:

2N3904 QS = 3.14
MPS2222 QS =3.32
2N4401 QS =3.38
BC337-25 QS =3.73
BC337-40 QS =4.02
MPS651 QS =4.12
Dual 2222 QS =4.20
2SC2500 QS =5.00
2SD965 QS =5.17

According to these results, the 2SD965's output should be vastly superior to the 2N3904, and the picture below shows this. It is even more apparent if we put a sheet of paper above the lights, as the second image shows.
<p>What happens to your website? It's not accessible anymore.</p>
Thank for the great instructable! You mention that your inductors measure to within 2% of eachother. How is this measurement made? Is there such a thing as an inductance meter?
One of the best bargains around is the capacitance &amp; inductance meter from<a href="http://www.aade.com/lcmeter.htm" rel="nofollow"> Almost All Digital Equipment</a>. I've had their IIB meter since it first came out 15 years ago and it's never let me down.<br> <br> Amazingly their prices have not changed in all this time - their 'kit' is still available for under US$100.
It was from this Instructable that I learned of Almost All Digital Equipment. <br>Almost All Digital Equipment also sells a frequency meter. <br>I bought one and am quite happy with it. I use it to measure the frequency of the &quot;Joule Thiefs&quot; I've been working with. These &quot;Joule Thiefs&quot; use only a four legged ic, a tiny inductor (as shown above), a battery and a LED. More info about this at: <br>https://www.instructables.com/id/Joule-Thief-Circuits-crude-to-modern/ <br>I wonder how the circuit I use would stack up against the variations revealed in this Instructable... <br> <br>
Thanks for the info, I ordered a kit yesterday!

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