An earlier blog about very lowpower Joule Thiefs is here.
I experimented with a very low power Joule Thief using a SS9014 high gain, low noise NPN transistor, a red high brightness LED, a T231212T toroid with two 12 inch (300mm) lengths of 30 AWG (.25mm) solid enameled wire, and a 1k resistor in series with a 100k pot – the 1k was there to prevent the resistance going to zero, which would put the battery directly across the base to emitter and causing damage. I put a 68 pF disk capacitor in parallel with the pot; without it the circuit wouldn’t oscillate.
With the pot set at minimum, the red LED was very bright; with the pot set at maximum, the LED was still bright. The battery current was still too high, more than 8 milliamps. This indicated that the S9014 was indeed very high gain – 600 or more. It also indicated that the 100k was too low, so I proceeded to increase it. I removed the 100k pot and soldered in a 470k pot.
I powered it up, and the battery current was a lot lower, but still more than what I wanted it to be, which was about a half milliamp or less. So I knew my job would be to get an even higher resistor and put it in. I removed the 470k pot and soldered in a 1 meg resistor. Now I was getting down below the half milliamp point. The battery current measured about 375 microamps, or about 3/8 of a milliamp. The red LED wasn’t very bright, but it was clearly visible.
