Need advise for a Joule Thief with 20ma 1,2v power source

[forest]

Can you also post schematic of JT with mosfet transistor ?

[Groundloop]

Can you also post schematic of JT with mosfet transistor ?

Forest,

Let's say you want to light a LED spot light (12 Volt 3,5 Watt) from a 1,5 Volt input (battery  or power supply)
using normal n-type mosfets. Your problem will be that mosfets needs a fairly high gate voltage to switch,
typical +15 to switch properly on and -15 volt to properly switch off again. So you will need to make that high
voltage swing locally in your circuit to do so when your input voltage is so low. You can use a IC that converts
your low voltage to 15 volt and then run a mosfet one transistor circuit from that voltage. Or if you do not want
to use a IC but only discrete components, then you can convert only the gate voltage to a higher voltage and
run the mosfets switching process from your low voltage input. And there is probably other ways to do it
by using special mosfets that are default on or have low gate voltage threshold etc.

Attached is a circuit that will switch a normal n-type mosfet on and off. The circuit uses two normal npn transistors
to make a simple oscillator. This oscillator runs two small toroid transformers so that we get a +15/-15 volt
gate voltage for the mosfets. Then the mosfets switches another toroid so that we can transform the low
input voltage up to +12 volt to drive our LED spotlight. The winding ratio of the PTRF1 transformer can be
anything you want, so it is theoretical possible to drive a 230 VAC LED bulb from a low voltage input.

GL.

[Groundloop]

Can you also post schematic of JT with mosfet transistor ?

Forest,

This circuit will probably also work. A low volt gate threshold mosfet is used in this circuit.
(I have not build and tested this circuit, but I see no reason that it will not run.)

GL.

[Groundloop]

Here is my Eagle CAD implementation of a mini JT.
The pcb is approx. 10mm x 20mm and still solder-able
by hand.

GL.

[ikram85]

Hi,

The attached oscillator can be built with SMD components. The two coils does
not need any coupling between them at all. The coils can be separated in distance.
The resistor is there to give the transistor a small voltage bias so that the oscillator starts
and can be from 100K and up to 1M, 1 mega Ohm is typical used here.

Much of the oscillator frequency is determined by the value of the coil and capacitor at
the base of the transistor. 1 to 100 nano Farad will work OK for a LED driver. You can use
almost any type of LED on the output. Good luck with your project.

GL.

Thank you very much to all of you, the first one works pretty well, we are learning pretty well how these amazing circuits work, I'm going to try this one, maybe it glows more, I'll use 1M ohm then, thank you all again