Talk:Joule thief

Please Clarify this Article

Hi, It's very strange that nobody really talks about JT to say what's really going on. Even wikipedia does not say, doth not seem to comprehend. The voltage spike is the Back EMF biasing or amping caused by the back stroke diode effect through the transistor B-E and the resistor. Negative entropy, it's shorting, biasing, amping. The spike is also juicing or at least assisting the battery. The Joule Thief is no Joule Thief at all. Some Geek likely called it JT just to get past the naysayers, the reptilians ;) Sirmikey1 (talk) 10:00, 20 January 2010 (UTC)

The above terms amping, juicing, etc. are non-technical jargon and should not be used in a technical article. 208.127.8.41 (talk) 08:44, 24 December 2010 (UTC)Watson

http://www.nifty-stuff.com/LED-boost-circuit-joule-thief.php seems to claim that the circuit is just for feeding the LED. Therefore, I guess, the end terminals have been drawn into a very wrong place: rather, the LED should be replaced by terminals and the current ones should be cut? I don't comprehend the circuit fully, so I don't dare to fix the article. Anyone? — Pt _(T) 23:14, 9 March 2010 (UTC)

@Sirmikey1 -- What are you arguing, exactly? Are you saying that the article is incorrectly named? That the description is insufficient? That the device's name is nondescriptive? Please clarify. Tylerl (talk) 06:09, 17 April 2010 (UTC)

The circuit in the link to "Nifty Stuff" above is quite clearly not the same circuit as in the article, though very likely the same function. SpinningSpark 09:43, 3 May 2010 (UTC)

The article is correctly named; but needs a lot more explanation before it will be easy to understand. if i build the referenced circuit, what will it perform like? where are the actual outputs? what controls the switching speed? for the values given, what is the nominal switching speed? does it drive the LED only? does it give stable voltage output? so many questions are unanswered. -- 99.233.186.4 (talk) 01:47, 1 September 2010 (UTC)

Dubious

What do a Joule thief and a nickel-cadmium rechargable battery have in common?! There's no inductance nor an oscillator in the latter. Also, the claim needs a reliable reference. — Pt _(T) 11:03, 8 April 2010 (UTC)

Though I don't know for certain, I assume the author meant that circuits using NiCd batteries employ this type of DC-DC converter to lengthen the useful life of the battery. I've never heard of it being the case, but that's the only sensible way I can read the statement. Tylerl (talk) 06:21, 17 April 2010 (UTC)

I've deleted the claim, cleaned up the article, added a description of its operation and amended the diagram. SpinningSpark 15:26, 8 May 2010 (UTC)

Operation Principle

I consider the description of the operation of that circuit to be misunderstandable or even wrong. The winding at the base of the transistor should be considered the secondary and the winding at the collector the primary winding in a model of a nearly unloaded transformer, as the base current is low compared to the collector current. From the transformer law, the voltage across the base-connected winding equals the voltage across the collector-connected winding (if you ignore ohmic losses).

On start-up with a 1.5V battery, some current goes through the resistor (which is around 1k typically) into the base of the transistor, turning it on and giving rise to a collector current. The collector current causes a voltage drop over the collector-connected winding, which will cause a voltage rise on the base-connected winding, turning the transistor on even harder, so that the transistor enters saturation with a collector-emitter-voltage of around 0.2V and 1.3V across the transformer. This causes the base current to be (1.5V(from the battery)+1.3V(via the transformer)-0.6V(transistor base-emitter voltage drop) = 2.2V)/1kOhm = 2.2mA. The collector current rise is limited by the inductance of the transformer. As typical small-signal transistor has a current gain of around 200, so the collector-emitter path keeps saturated until the collector current has risen to around 400mA (this is the on-state of the blocking oscillator) or the core saturates and ceases to produce the base voltage boost. At that point, the collector-emitter voltage starts to raise, and thus the voltage over the collector-connected winding decreases. This also decreases the extra voltage induced in the base-connected winding to decrease and thus the base current to drop. A dropping base current on the other hand makes the collector-emitter voltage rise even higher, which will further reduce the base current, and quickly turn off the transistor. Now the energy stored in the ferrite bead is discharged through the LED, the voltage across the collector-connected winding now is around 1.7V that gets added to the 1.5V from the battery to obtain the 3.2V forward voltage of the LED. The result is 1.7V also induced in the primary of the transformer, which pulls the base voltage down to -0.2V, keeping the transistor turned off (this is the off-state of the blocking oscillator). At the time the energy of the coil is discharged and the current drops down to zero, the oscillation restarts like on power-up. —Preceding unsigned comment added by 87.123.67.77 (talk) 16:56, 21 November 2010 (UTC)

Operation theory has been described correctly by "Vintage Dave" in the mean time. Thanks! —Preceding unsigned comment added by 87.123.119.98 (talk) 16:04, 23 December 2010 (UTC)