#### bug13

Joined Feb 13, 2012
1,892
Hi guys, I am writing a technical report on How to make a Joules Thief for my year 1 Engineering Literacy, in the report need to explain how it operates, please have a look it for me see if there is any error, thanks

schematic used:

1. When the circuit is first switched on, the electronics current goes from positive of the battery to the resistor and to the secondary winding, and then goes through the base of the transistor, finally come out from the emitter of the transistor and goes back to the negative of the battery.
2. Because the electrical property of the transistor, when there is a current flow from the base to the emitter, the collector and the emitter of the transistor will be start to switch on. The transistor here acts as an electronic switch.
3. When the connection between the collector and the emitter electrically switch on, the can current goes from the positive of the battery to the primary winding and through the collector to emitter, then goes back to the negative of the battery, the winding connected to the collector of the transistor is call the primary winding;
4. A positive voltage is induced in the secondary winding, which makes the connection between the collector and the emitter switch on harder;
5. Depends of the material of the toroid, the ferrite saturates at some point, and the magnetic field stop increasing. The inductive current at the secondary winding stop increasing as the result of no magnetic field change in the primary winding;
6. The reducing current through the base of the transistor drive the transistor staring to turn off the between the emitter and the collector;
7. The magnetic field start to decrease and driving the current to the LED;
8. The decreasing magnetic field induce an opposite current in the secondary winding, and turning the transistor off harder, and most of the current round to drive the LED;
9. That's one cycle, and whole process repeat in hundreds and thousands time per second, its blinking so fast that its not noticeable to human eyes hence the LED looks like it's on all the time
.

#### Jony130

Joined Feb 17, 2009
5,186
As for point 5.
You don't need "core saturation". This circuit will also work when we use ideal coils in the circuit.
When BJT is "ON" his base current remains constant.
And at the beginning, BJT is in saturation and almost all supply voltage is apply to secondary winding. So current in secondary winding start to rise up progressively but Ib remains constant. Collector current also increases with the time. So BJT operation point is move from saturation to the linear region (comes out of a saturation) and at the same time Vce voltage rise to.
And when BJT entering linear region the Ic current stop increasing.
So the BJT him self caused secondary winding current and in the same time magnetic field stop increasing any further.

#### hgmjr

Joined Jan 28, 2005
9,029
BUG13,

Your statement in Step 2 says:

Because the electrical property of the transistor, when there is a current flow from base to the emitter, the collector and the emitter of the transistor will be start to switch on. The transistor here acts as an electronic switch.
My recommendation is that you change it slightly.

Because of the electrical property of the transistor, when there is a sudden current flow from base to the emitter, the transistor in this circuit is almost immediatedly driven into saturation. The transistor here acts as an electronic switch.
The reason that the transistor can enter into saturation is that the primary of the transformer immediately following the application of power will appear as a high impedance collector load.

The secondary will similarly appear to be a high impedance but since collector current is controlled by the base current, the base current must build up to the point that it can turn on the transistor before any activity on the collector circuit can begin.

hgmjr

#### bug13

Joined Feb 13, 2012
1,892
Both Jony130 and hgmjr point out something I don't quite understand about this, I assume it's something non-liner property of this circuit, maybe a concept I don't know or something else.

Can I know what key words should I Google it, so I can understand it better?

Thanks

#### jegues

Joined Sep 13, 2010
733
Both Jony130 and hgmjr point out something I don't quite understand about this, I assume it's something non-liner property of this circuit, maybe a concept I don't know or something else.

Can I know what key words should I Google it, so I can understand it better?

Thanks
Look into the operating modes of a BJT transistor and the conditions associated with each.

When being used as a switch the BJT will be switching between the cut-off and saturation region.

#### hgmjr

Joined Jan 28, 2005
9,029
Both Jony130 and hgmjr point out something I don't quite understand about this, I assume it's something non-liner property of this circuit, maybe a concept I don't know or something else.

Can I know what key words should I Google it, so I can understand it better?

Thanks
A couple of rules to always remember when you are dealing with capacitors and inductors is:

Voltage across a capacitor cannot change instantaneously and the current flowing in a inductor cannot change instantaneously.

hgmjr

#### hgmjr

Joined Jan 28, 2005
9,029
A key equation that plays a role in understanding and explaining the "joule thief" circuit is:

$$\Large \frac{di}{dt}\ =\ \frac{V}{L}$$

In words, this equation states that the change in current flowing through the inductor with respect to time is equal to the voltage across the inductor divided by the inductance of the inductor.

If you apply a step change in voltage across an inductor, and if that voltage remains constant then the current flow in the inductor will increase linearly until it reaches a maximum current whose value will be that constant voltage divided by the DC resistance of the inductor.

hgmjr