Joule thief general question on how they work

[Sceadwian]

How long does the coin cell last?

 

[audioguru]

How long does the coin cell last?

The datasheets of coin cell batteries show an extremely small current that lasts for many hours. A few simply say that they can provide occasional short bursts of a few mA.

 

[Sceadwian]

I didn't analyze the circuit in detail but the coin cell isn't there to provide current, it's there to provide a bias voltage for the mosfet gate which allows the ultra low voltage. It's only capacitively coupled to the rest of the circuit so the active current draw could be quiet low to non-existant, just enough to provide gate charge to the fet. I've seen crystal radio sets that 'cheat' in this manner where a small coin cell is used to provide bias to a hot carrier diode, allows the detection of signals that would otherwise be too weak to detect, the current used by such circuits can be lower than the self discharge of the cell.

 

[acmefixer]

The coin cell lasts for its shelf life. The MOSFET gate draws picoamps (too small to measure) once the gate capacitance has charged up. If the trimmer pot is a 470k, then the coin cell supplies a current of 1.5V/470k, or 3.19 microamps. If you use a 1 meg pot, it will draw 1.5 microamps. In either case, that's so low that it will take years to drain the coin cell. Say, for instance, the coin cell is rated at 150 mA-hrs. The 470k will drain the coin cell in 47 thousand hours, or more than 5 years. The 1 meg pot will drain it in over ten years.