2011-12-20 Joule Thief Mini Maglite

I recently found this article on how to convert a Mini Maglite to use a LED instead of the incandescent light, and the author does it by using a Joule Thief.  This has been an inspiration to me, because I have thought about converting several flashlights to LEDs or to a single battery but I have always found some difficult to surmount obstacles that prevent me from proceeding.  Another factor is that I usually (especially at this time of the year) give myself a gift of a couple new flashlights every year.

One obstacle I’ve always run into is the on/off switch, which in the case of the Mini Maglite, is different than other flashlights.  Most flashlights with a screw type switch tighten the threads to turn the switch on; the Mini Maglite tightens the threads to turn it off.

Brightness  He complains about the brightness saying that it is barely usable.  I agree that the 3mm LED may be blamed for much of the problem, but I think that a good part of the problem is the coil.  He said he used very fine wire, 40 AWG, which according to my wire table has about 1 ohm of DC resistance per foot.  If he used about a foot of wire to wind the coil, then the primary winding has about an ohm of resistance, and this is much too much resistance for a decently performing Joule Thief, and will also lower the efficiency, which he claimed was very low.  In the tiniest of the coils that I wind I use 30 AWG wire, which has 1/10 ohm per foot, and consequently would have 1/10 the amount of resistance in his coil, and also 1/10 the amount of losses and higher efficiency.  He mentions, but doesn’t give any details about changing the “transformer” (more on this here).

Transistor  The type of transistor used is very important.  He said he used the “PN222A” but I think he meant PN2222A, which is a better choice than the 2N3904 (but see my note below).  He didn’t say anything about the resistor he used, but the standard JT value is 1000 ohms or 1k.  One way to increase the output of the transistor is to decrease the value of the resistor.  Typical values are 820, 680 or 470 ohms, but I would use 470 and probably 680 only on depleted cells; the current would be too high on a fresh cell.

For getting more current out of a JT, some good choices for super beta transistors are 2SC2500(D), KSD5041, and NTE11.  Any of these will boost the output more than double, to the point where you must use more than 1 LED to prevent the LED from having excessive current.

Back to experimenting…

Note: This not became so long that I had to give it its own blog.

One Response

  1. Antoine says:

    Thank you for your post on my blog. I recall reading your old blog while I was building that project.

    You are most likely correct, the path’s resistance to the LED is most likely too high; even though it’s 3mm, it’s much brighter when I hook it up on a 3.3V supply.

    When I get my hands back on that project, I’ll investigate.

    Thanks for sharing your knowledge.

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