Joule Thief 101

[SeaMonkey]

The first illustration below is the original Joule Thief which
was developed by a Gentleman in England. It represents
the basic, non-optimized circuitry.

The second illustration below is an optimized version of
the circuit which replaces the fixed 1.0 KOhm base feed
resistor with a variable of 1.0 KOhm or slightly more.

Additionally, a suitable capacitor may be placed across
the base feed resistor to enhance switching characteristics
of the small transistor.

Transistor = Transfer-Resistor

Yes, there is indeed a type of resonance involved in its
operation.

The discussion thus far has been quite interesting, both
in terms of the subject matter and the various personalities
on display.

Carry on gentlemen!

[picowatt]

https://www.youtube.com/watch?v=v9tsvbkOeW8

Tinman,

Do you have the 'scope channel displaying the base/emitter voltage inverted? 

PW

[minnie]

    Junkie,
           have you looked at Joe Flynn's parallel path?
I was quite intrigued by it.
                  John.

[minnie]

Oh,that was in the wrong slot.

[sm0ky2]

Brad:

A typical standard Joule Thief has a 1K-ohm base resistance, correct?  A standard Joule Thief is designed to have the transistor act as an ON-OFF switch.

That means that with a 1K base resistor that the transistor will be fully ON, and the collector-emitter voltage will be at a minimum.  If you reduce the base resistance to 700 ohms, then you will increase the base current, but the transistor as a switching device will still be fully ON, with the same minimum collector-emitter voltage.

When the transistor is acting like an ON-OFF switch, when it is ON and the battery voltage is constant, in the case of a 1K-ohm base resistor, or in the case of a 700-ohm base resistor, there will be no increase in current flow through the inductor, it will be the same. The factor that is limiting the current flow is the resistance of the inductor, it has nothing to do with the amount of base current flowing into the transistor.

In addition, if the battery voltage is 1.2 volts, then the maximum current flowing through the coil will be proportional to (1.2/coil_resistance).  If the battery voltage decreases to 1.0 volts, then the maximum current flowing through the coil will be proportional to (1.0/coil_resistance).  Therefore, there will be a decrease in the maximum current flow through the coil when the battery voltage is lower, resulting in a decrease in the initial current flow through the LED and therefore a dimmer LED for a lower battery voltage.

MileHigh

[transistor 101]

1kOhm?  hmm....  2N2222? (yup,.. "standard"... uh-huh)

ok let's roll with that for a second shall we..

What is the current through the transistor?

Ie = Ic + Ib

So,. what happens when we increase base current, while collector current remains the same?
Current at the emitter increases!

...there will be no increase in current flow through the inductor

Where does inductor current come from?
The emitter? hmm