2013-08-10 Very Low Power Joule Thief Update

DSC_0464S3I blogged my experiments with this very low power JT back in April, and I connected it to a cheap ‘heavy duty’ AA cell and left it running for a month or two.  I added updates to the end of that blog I made one mistake: I should have taped a post it note to the AA cell with the starting date and followup dates and measurements.

As can be seen in the photo it’s still running – the LED is still lit – but the cell voltage is down to 0.9V.  These ‘heavy duty’ cells have a lot less (about half – see here) capacity than the alkaline cells, and the shorter run time and lower voltage seem to show this.  This just makes it easier to do tests because I have to wait half as long for the cell to run down (grin).  Thus I can say that the run time on an alkaline cell will be about twice the time it runs on a ‘heavy duty’ cell.

On the right can be seen the 470k resistor with the tan colored 47 pF disk capacitor across it.  This is the very high value resistor that throttles down the transistor’s base current to a very low value, causing the Joule Thief to run at much less than a milliamp through the LED, and greatly extending the AA cell’s life.  The AA cell has to supply less than a milliamp, too, something around a third of a milliamp, if I remember correctly.


2 Responses

  1. xiaolaba says:

    that is very interesting topic, good job.

    do you have any JFET version such blocking oscillator with experiments to lights up LED, any story with success could be posted?

    • admin says:

      Yes. Do a search for J105 Joule Thief and 2SK170 Joule Thief.
      Also TN0702 Joule Thief.

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