I am away from my shop but I miss electronics if I do not take it with me. A week or so ago there was a question on the Forum about the Joule Thief Circuit. I became interested in the circuit as it looked simple enough for me to fabricate with the limited resources that I brought with me. My first challenge was to make a small toroid transformer with 2 windings of 20 turns each using 38 Ga wire. I went to a local "Old School" electronics shop and found a small toroid. I would have bought the wire too but it was only available there as a 3 kg Spool. I decided to stop by a computer repair shop near my hotel and see if I could get an old computer switching power supply. The owner went into the back and brought out a power supply with the wires cut off. I offered him some money but he insisted that I just take the supply. Inside the supply I found a small 3 winding transformer that looked promising. After a bit of pounding, breaking, twisting, and some careful unwinding I was able to salvage several meters of approximately 38 gauge wire. After winding two sets of 20 turns I tried patching the Joule Thief circuit together.
Unfortunately when I set up a test circuit it didn't work very well. I decided to double the windings on the transformer. I slid the two existing windings together and wound a second 40 turn winding with a tap in the middle. I also mounted the finished transformer onto a small piece of circuit board that would plug into my breadboard. Here is a copy of the simple schematic that I was attempting to build.
The next step was to breadboard the circuit to see if I could get it to work. I found that I had to play with the value of R1 to get a good strong oscillation. I put in a variable potentiometer and this allowed me to control the output. I determined that 1.3K was optimal for making the finished circuit function. It was also necessary to put a 100 uF electrolytic across the 1.5Volt battery to stabilize the power supply. One LED worked fine so I added 4 more LEDs for a total of 5 in series. At this point the circuit was generating about 10 volts across the LEDs at 475 kHz. The current meter showed a 63.9 mA draw on the 1.5 volt battery. As you can see the battery is badly drained at 1.1 volts but the Joule Thief is still working. Here is what the set up looked like on my hotel room table.
Here is a picture of the bread board. I apologize for the quality but the lighting is bad and I do not have a macro setting on this camera.
This little project was extra fun as it involved the first time I have wound a transformer for one of my projects since I wound a coil for my Boy Scout Crystal Radio 55 years ago. Another thing that made it fun was the challenge of finding the raw materials which made it a bit of a treasure hunt.
John
Top Comments