A few months ago I came across a 7 Year Joule Thief video from Xee2vids on YouTube (link below). It’s a conventional Joule Thief except that the resistor has been increased to 2 megohms, and a 100 pF capacitor has been added from the resistor to ground. He calculates that the LED should run for 7 years.
I decided to convert one of my conventional Joule Thiefs to this long running Joule Thief. But I really don’t want it to run for 7 years, I just want it to run for a few weeks.
I made the following changes. Instead of 2 megohms, I used 100k between the feedback winding and plus 1.5 volts. I put a 150 picofarad disk capacitor between the feedback end of the resistor and -1.5v. I changed the transistor to SS9014, A very low current, high gain transistor with a current gain of 400 to 1000. This would be a poor choice for a conventional Joule Thief because it cannot handle enough current, but for this low current Joule Thief it’s just fine. And this very high gain allows the use of a CdS photocell to turn it off during daylight. I soldered the photo cell on the base lead and to ground. I connected the circuit to a AAA cell holder. The tiny light green resistor is 100k. The larger 680 ohm resistor was from the previous circuit, and is optional – it can be zero ohms.
The battery current is about 4 to 5 milliamps at 1.5 volts. Assuming the battery capacity is 850 milliamp hours, the run time at five milliamps is 170 hours, which is 7 days. But we all know that as the battery gets depleted, the voltage drops and the LED dims. But the Joule Thief keeps on running for considerably longer than the rated battery capacity. Instead of 7 days it probably will run more like 14 days or even more, just a lot dimmer. And then I added the CdS photocell, which turns it off during daylight, so it should run brightly for at least two weeks. Then it should keep running until the light is no longer adequate, and it’s time to change the AAA cell. If it’s in a room where the lights are on in the evening, it will run considerably less than half the time, and the run time should then be a month or more.
LINK: www.YouTube.com/watch?v=a2SwZvE41Ok (ltr O, not zero)
I looked through other videos from Xee2vids and found at least one other JT that uses a 2 Meg resistor and draws very little power. One runs from a 10000 uF capacitor charged up to 6V and he said that it would run for 20 or more minutes. It really needs a super capacitor of at least 1 farad, or even more.