Discussion in 'Homework Help' started by bug13, Apr 17, 2012.

1. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
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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:

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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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.

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3. ### hgmjr Moderator

Jan 28, 2005
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BUG13,

Your statement in Step 2 says:

My recommendation is that you change it slightly.

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

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4. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
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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

5. ### jegues Well-Known Member

Sep 13, 2010
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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.

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6. ### hgmjr Moderator

Jan 28, 2005
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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

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7. ### hgmjr Moderator

Jan 28, 2005
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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

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