Joule Thief

[Vortex1]

Farmhand:

I've been optimizing on the bench and in simulations a similar circuit that I posted here: http://www.overunity.com/14591/lasersaber-strikes-again-a-joule-thief-king/msg402950/#msg402950

But that was the elementary schematic, I have made many improvements that lower power drain etc.

This circuit has a number of advantages over a blocking oscillator, one of which is better control of inductor current and switch off point.

The disadvantage is base switching current for the NPN does not come from the current step up feature of a well designed single transistor blocking oscillator's coupled inductor, rather it comes directly from the supply, but there are many interesting circuit mods in the two transistor circuit to minimize this current.

This  two transistor topology can be arranged to provide a very wide pulse repetition rate, making brightness control over a wide range easy.

I prefer this single inductor two transistor circuit topology over blocking oscillator types for most applications but each has it's place, depends on what you want to do.

Regards, Vortex1 (ION)

[Farmhand]

Yep I seen your circuit and that's why I decided to try some things.

It seems for running down to lower input voltages the coil has a lot to do with helping, I found a coil with more self inductance will work for much better output at lower input voltages, although the frequency is lower, I'm using the full resistance in series with the PNP base. It's running down to 0.8 volts and lighting one led with 1.33 volts RMS and 4.32 volts max at 8.5 kHz. 

Coil is a 3 mH or so on ferrite.

In the low power mode with more base resistance it can light one LED real bright with 1.2 volts and uses a lot less power but the light gets low when at 0.8 volts.

Still, it can boost a very old leaky 200 uF capacitor to 6 volts from 0.8 volts input, in a second or two which might be useful as well. 


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[MarkE]

Yep I seen your circuit and that's why I decided to try some things.

It seems for running down to lower input voltages the coil has a lot to do with helping, I found a coil with more self inductance will work for much better output at lower input voltages, although the frequency is lower, I'm using the full resistance in series with the PNP base. It's running down to 0.8 volts and lighting one led with 1.33 volts RMS and 4.32 volts max at 8.5 kHz. 

Coil is a 3 mH or so on ferrite.

In the low power mode with more base resistance it can light one LED real bright with 1.2 volts and uses a lot less power but the light gets low when at 0.8 volts.

Still, it can boost a very old leaky 200 uF capacitor to 6 volts from 0.8 volts input, in a second or two which might be useful as well. 


..

That's probably because each time that the circuit switches it suffers switching losses.

[Pirate88179]

Does anyone know if I can put two 47 uH axial inductors (like the ones in a solar garden light) together in series or parallel to get a 94 uH inductor?

Thanks,

Bill

[TinselKoala]

Does anyone know if I can put two 47 uH axial inductors (like the ones in a solar garden light) together in series or parallel to get a 94 uH inductor?

Thanks,

Bill

Yes, series.
Like resistors: inductors in series simply add. Inductors in parallel do the inverse thing: L_final = 1/(1/L₁ + 1/L₂ + ....)