Joule Thief 101

[MileHigh]

A "fixed base resistor" in any transistorized circuit, particularly a "switching
circuit" is an industrial trade-off for ease of maintenance.  It is an operational
compromise which is reasonably close to but not at the point of maximum
efficiency.

Experimenters always use a variable base resistor to enable "tuning" of
the circuit under evaluation in order to attain the best possible operational
mode.  The variation in transistor characteristics from one to the next
makes it very difficult to come up with fixed values of resistance.

The Original Joule Thief circuit was made as simple as possible in order to
appeal to the vast audience of technically limited amateur "builders."

Then the Experimenters took up the challenge to make it better.

Indeed.

But of course since the base resistor is just for switching, and assuming that the real challenge is to choose the right fixed value for the base resistor, then indeed there are compromises to consider.  A lower value of the base resistor will allow for normal Joule Thief operation to a lower battery voltage, but considering the high proportion of time that the transistor remains switched ON, then a lower value for the base resistor will increase the drain on the battery.  Normally you might say, "Choose the right value of the base resistor to ensure proper Joule Thief operation with a battery voltage range from 1.5 volts down to say 0.75 volts."  Now, if you choose that base resistor value and throw in the usual 10% margin of safety, then perhaps doing some analysis on paper would make sense.  How much energy is drained from the battery for that value of base resistor per hour?  Perhaps it would make more sense to choose a base resistor value to ensure proper Joule Thief operation from 1.5 volts down to 1.0 volts instead, and the power savings associated with that higher value of base resistor will more than offset the reduced battery voltage range.

You could do some intelligent analysis along these lines and then perhaps try three base resistor values, low, medium, and high.  Then build three identical Joule Thieves except for the different base resistors, and then get three identical fresh batteries, and then run a test with all three at the same time.  At the end of that experiment you would end up picking the best value of base resistor with a high degree of confidence and the experiment would arrive at a successful conclusion.  More importantly, you analyzed the trade-offs and compromises and made some intelligent decisions and did the testing and analysis like a successful experimenter.

But of course with Brad that discussion never even had an opportunity to take place.

[MileHigh]

P.S. - Wattsup:  Ten years from now you will not have moved one inch, not even a millimeter, and the iPhone will be obsolete and everybody will be using something ten times better than an iPhone.  Then you can make the same posting but just worded differently and scowl at all the people that make the world work and keep 120 VAC humming in your wall outlets.

[tinman]

The phase shift video.


https://www.youtube.com/watch?v=EhpP7Bmcwhs



Brad

[Pirate88179]

Indeed.

But of course since the base resistor is just for switching, and assuming that the real challenge is to choose the right fixed value for the base resistor, then indeed there are compromises to consider.  A lower value of the base resistor will allow for normal Joule Thief operation to a lower battery voltage, but considering the high proportion of time that the transistor remains switched ON, then a lower value for the base resistor will increase the drain on the battery.  Normally you might say, "Choose the right value of the base resistor to ensure proper Joule Thief operation with a battery voltage range from 1.5 volts down to say 0.75 volts."  Now, if you choose that base resistor value and throw in the usual 10% margin of safety, then perhaps doing some analysis on paper would make sense.  How much energy is drained from the battery for that value of base resistor per hour?  Perhaps it would make more sense to choose a base resistor value to ensure proper Joule Thief operation from 1.5 volts down to 1.0 volts instead, and the power savings associated with that higher value of base resistor will more than offset the reduced battery voltage range.

You could do some intelligent analysis along these lines and then perhaps try three base resistor values, low, medium, and high.  Then build three identical Joule Thieves except for the different base resistors, and then get three identical fresh batteries, and then run a test with all three at the same time.  At the end of that experiment you would end up picking the best value of base resistor with a high degree of confidence and the experiment would arrive at a successful conclusion.  More importantly, you analyzed the trade-offs and compromises and made some intelligent decisions and did the testing and analysis like a successful experimenter.

But of course with Brad that discussion never even had an opportunity to take place.

This is exactly why most of my JT circuits, including the high voltage units, all have a VR on the base.  I tried to explain this earlier in the topic but, it really does work well for what you want to get out of the circuit.

Lidmotor even used this method on his replication of the Jeanna Light if I am not mistaken.  I actually used 2 VRs in that project...I think he used a rheostat if I remember correctly.

Bill

[Magluvin]

Its smart to use the VR. When using a regular resistor, once the battery wears down the light becomes less useful, and there is still usable current in the battery. So adjusting along the way is more bang for the buck. Err penny.     The VR compliments the Joule Theif and enhances its purpose. I wonder if there could be a simple transistor circuit that could replace the resistor and adjust the base automatically with the input voltage. TK used germaniums in one of his latest. Maybe those would be good to try on that idea.

Mags