[858] Not A Joule Thief

Date: January 7th, 2011 | Comments : [13] | Categories: DIY.

Not a Joule Thief

This picture shows a depleted AA battery driving a white LED, with nothing between the battery and the LED except for the wires, which should not be possible since the forward voltage of a white LED, that has to be exceeded to the device to light up, is at least 3 volts!

There is no circuit hidden in or behind the battery and holder. It’s just a plain old battery holder with a spent AA battery in it.

The component that looks like a resistor is actually a small inductor, and it was blocking the view of the smd integrated circuit chip that is soldered directly to the wires of the white LED.

Not a Joule Thief

The mini SOT23 chip is step-up converter that has an internal mosfet driver transistor, so the only external part that is needed is an inductor.

Not a Joule Thief

The ZXSC380 IC (from Diodes Inc.) is very tiny and incredibly hard to solder to the LED. The surface tension of the liquid solder will pull it out of position and I ended up having to use small copper alligator clips to hold it down while I soldered.

I should have cleaned up the excess solder but I wanted to get some pictures before I took the chance of de-soldering it by accident while reheating.

Joule Thief schematic
The chip is soldered onto the legs of the LED and then the wire between pins 1 and 2 is cut out so there is no connection. The inductor is then soldered so as to bridge pins 1 and 2. I soldered the inductor on before cutting out the wire but it might be easier to try some other method.

As I said previously, this is a tedious solder job, and the SOT23 IC is easily damaged (ask me how I know!) It takes patience and a steady hand to assemble these few parts.

This circuit would work for the LED indicator in the 1.5v boosters and would most likely be a better choice than the Joule Thief that I posted previously.


13 Responses to “Not A Joule Thief”

[2035] Acme Fixer Says: 11:12 am, January 17th, 2011

You’ve got more patience than I have. Great job! But it still has some issues that are just like the Joule Thief. I wrote a blog on it here.


What are you using to take the closeup pictures? They look good.

[2083] Jay Says: 9:02 am, February 25th, 2011

Use much more solder flux and you won’t get spikey solder thingys

[2904] dollarbil Says: 4:12 pm, June 18th, 2012

hey can u tell me what is written on that sot32 cuz i have a board from a cd-or dvd rom and it has a couple of sot’s or sob’s like i use to call them 😛 and they have some small writhing on thme on most of them is written 26 24 26 beeing written with bigger letters than 24 and i was wondering if one of thouse sot’s could do the trick ? or i have to get one just like yours and if so tell me what is written on the surface of ur sot .

[451282] Allen Says: 8:58 am, December 13th, 2014

can you pleas tell the marking in the transistor

[456572] admin Says: 12:01 pm, December 15th, 2014

The ZXSC380 IC (from Diodes Inc.)

[530366] Greg Says: 10:45 am, January 19th, 2015

What are the specs for the inductor? It looks like 100 Microhenrys, /-5%.


[531874] admin Says: 8:09 am, January 20th, 2015

yes, 100uH was what I had around the lab, so that’s what I used.

[534258] Greg Says: 2:08 pm, January 21st, 2015

Whats the effect if you use a higher or lower inductor? Just curious before I buy the inductors.


[537938] admin Says: 6:24 am, January 23rd, 2015

Greg, the best idea is to get the datasheet for the ZXSC380 IC and read up on it to see how/why the inductor is chosen.

[581431] Greg Says: 5:17 pm, February 19th, 2015

Thanks. Checked the specs and that value for the inductor appears the most efficient. Just made one, worked first try! Now I’m gonna leave it running on an old button battery and see how long it lasts.

[582772] Adam Says: 8:19 am, February 21st, 2015

Hi Greg, please let us know the results…

[762894] Ron Says: 10:44 am, December 11th, 2015

I discovered this circuit from Diodes Inc’s website so was glad to find it used here. Any test results on how long it lasted? I would like to run this with 2 batteries instead of 1. Any ideas on how to do this? It is my understanding that 2 alkalines in parallel is a bad idea because one could overheat. Thanks.

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