Joule Thief behavior question.

[crowclaw]

I may not be able to help you either but it took me a little over a year to understand semiconductors when it comes to transistors or mosfets when you look at the datasheets for each transistors you also need to look at the graphs  and the gains cause there all different.. at different frequency .. and voltages   so for a example is if you had 3 different transistors paralleled you may have 3 different frequencys running into you coil.. some transistors act different with different wave forms to the base and may draw more or less current ... also your coil inductance plays a big role in how much current passes and how much goes out.. ..  it also helps to see about how much resistance and capacitance the transistor has..   hope I could help some how..


robbie

I can't add anything extra either, both Xee2 and kooler have given good explanations. When youv'e played about with these circuits as much as we have you will come across many strange occurrences which are not easily explained mainly due to the JT's circuit altering parameters as you add extra devices. 

[TinselKoala]

OK.... here's another one.

I always test random transistors in my JT TestBed. This is a basic JT circuit that uses a 13-13 turn toroid, a 1 K base resistor, and has a load bank of 24 white LEDs in a 2S12P arrangement. It has a switch that can put a 70 nF capacitor across (in parallel) with the base resistor. It typically runs on a AAA battery. You all have seen this JT of mine, I'm sure. I even use it as sort of a transistor tester.

It runs on all kinds of NPN transistors, with slight differences or great differences in brightness and the response to switching in the base capacitor. I call the performance with a 2n2222a the "baseline standard".

Now that I have some BC337-25 transistors, I decided to try one in there. It works fine, about like a 2n2222, when it's hooked up properly, and it dims a bit when the base cap is switched in, and is silent.  BUT... when I flip it over, so that C and E connections are swapped, it works with Much Brighter LEDs in both modes and it makes an audible whine. Interesting, yes? Some other transistors will also work "flipped", but by no means all.

I also tried a 2sc4508... and it is _even brighter_ than the BC337, and hardly any dimming with the base cap. But it does not work at all in the "flipped" orientation.

MJE3055: in the standard orientation, very bright, with audible whine. Switching in the base cap causes no change in brightness but a change in pitch of the whine.  Does not work "flipped".

2n2369a: moderate brightness in standard orientation. Switching in the base cap is very interesting. The lights are very dim, but lit, and slowly brighten a bit. No sound. Does not work at all "flipped".

The monster 2sd1877... does not work at all in this JT but I will be trying it in Gnino's dual-incandescent-battery-charge circuit next.

So I'm liking the BC337-25, "flipped", and the 2sc4508. I have not yet scoped these different configurations.

[Legalizeshemp420]

Yes, I stumbled onto the flipped situation with the BC337-25 about 2 weeks ago now.  As I was removing my transistors I noticed it was reversed yet it still worked which is just odd.

[ddredar]

Hi all

I'm a JT noob, and have read through the JT 101 thread, and several others.  All this information is very interesting.  I have not been able to pull myself away from this stuff for hours.   I have very basic electronics knowledge from some classes I took in college many years ago.  So I know what this circuit is doing and how, but am amazed at the voltage output.  My first JT worked perfectly the first time.  However. I do have a couple of questions of my own.  hopefully one of you experts can educate me on this.

So my first JT I did with only one LED (3.3v 28ma) using an old AAA battery putting out 1.25v.  I thought the JT was providing close to the 3.3V for the LED.  But then I found a post on how to measure the output using a diode and capacitor.  With the LED in place, I measured the output at 1.8v.  With the LED removed, I was astonished to fine it measure at about 25v.

Now I understand that with the LED in place, it can act as almost a dead short.  Which would account for the difference in my voltage measurements.  But am I really dumping 22+ volts into my single LED?

In addition, I have added up to 80 LEDs in parallel, and they are BRIGHT.  But that would mean I'm drawing 2.24 amps from the circuit. I'm using an NTE293 transistor that's rated at 1A continuous and 1.5A peak.  How is that possible?

Also, can someone tell me if there is a way to measure the amp output of the circuit?  My meter just reads 0.0.  I'm assuming its because it cant keep up with the oscillation speed of the circuit.

[xee2]

ddredar


The Joule thief only lights the LED for a short time each cycle, most of the time the LED is off. You do not see this because your eyes can not respond fast enough. Thus it is only using power for short periods of time, not continuously. As a result  he average power is very low. An LED is a diode and will always have a fixed forward voltage drop when it is conducting, no matter how much voltage is applied.