Joule Thief

[MarkE]

Yeah... I've done that in the past with no success. Bill suggested it to me again last week and I rand about 20 capacitors on that specific
circuit yesterday and per Bill's suggestion, I have had a 20F cap on it for several hours and the voltage reading never got past the source
voltage, which is about 450 mV. The larger capacitance cuts the electric from my earth battery to that circuit's LED. I was thinking of
demonstrating this on a short video later today.

As far as these little inductors/chokes, I measured and noted the resistance, which is significant. I am pretty sure these LC meters can
measure resistance of these things much better than they can measure the capacitance of them. I tore a few apart a few months ago.
They have a tiny ferrite core and very thin wire, which looks like copper, wrapped around them. The covering, which you can see, is
some kind of insulator. I love these things!

I have about 50 circuits all over my place with these chokes/inductors of various inductance - I got a 20-value set of 200 on eBay
earlier this year on eBay for less than $5 USD delivered from China early this year. The 1 mH's show about 30-31 ohms and work well
with the 3904/3906 transistors without the need of a resistor or capacitor in a joule thief. The 1 mH version of this JTC also works very
well with fully charged AA batteries all the way down to dead batteries with about 400 mV, 0.5 mA. The 10 mH seem to choke the light
out of the LED with batteries that have too much power.

I'll try to find and demonstrate a "simple" way to test capacitance on these and my other inductors/coils. In case you haven't seen this,
here's a fun view of my mess (I mean lab)! lol

http://youtu.be/_36V25kDTEU

What I meant was to put a small value resonating capacitor directly across the left hand coil.   For a 1mH coil 1nF should result in oscillations around 150kHz. 

The steel bolts that you are using for cores are very lossy.  You can get more efficient operation with ferrite cores.  If you really want to hand wind, you should buy some ferrite stock or reclaim it out of old electronics.  If you just want coils with well controlled properties then you can buy from places like Coilcraft. 

Do you have any particular goals like:  Longevity from a AA cell, or minimum operating voltage, or brightness from your LED?

[gyulasun]

lol... damn... I guess I am in the market for a descent LC meter... Any suggestions? I am pretty sure these work well with ohms and
mH, but I would be willing to find a better one just to make sure... The LC meter is the most important tool to me...

Unfortunately, the LCR meter I would draw your attention is expensive but has useful features like the choice of choosing between some test frequencies, it can measure the impedance of an L or C or R component, it can show on display some automatically calculated data of the components, see info and the type of the meter here:
https://bkpmedia.s3.amazonaws.com/downloads/datasheets/en-us/87xB_datasheet.pdf

Gyula

PS I am not an agent for those meters... 

[tgraca]

Hi tgraca,

Member conradrelektro showed methods how to measure the self resonance of pancake coils and of course the methods are useful for quasy any coil shape, including your Joule thief coils.  Here is a link to the videos in 2 parts
https://www.youtube.com/watch?v=fC84W0PIZoE  part 1
https://www.youtube.com/watch?v=spQ9yLdb7v4  part 2

Notice for the pitfalls like the input capacitance of a scope probe may cause.

I would show you another video that shows the use of a grid dip meter for checking the self resonance of coils,  from time 13:10 : https://www.youtube.com/watch?v=vi24SpKYYoQ     
His video #100 tutorial shows a method  for measuring capacitors self resonance, it can also be used for coils too.

Allow one notice, prompted by seeing your coils: I would not use bolts and washers in or near coils at all, even if they are stainless steel materials, they may introduce unpredictable eddy current losses at the several kHz frequencies or higher.  I had bad experience with bolts as cores in coils, though not in Joule thief cirsauits, that is.

Gyula

Gyula, thank you so much... that's exactly what I needed when I needed it. I can run this tests with what I have, but that dip meter
would really save a lot of time. I need to know what i am working with and I need to be able to check many components quickly.
I'll look into getting one of these. -t

[tgraca]

What I meant was to put a small value resonating capacitor directly across the left hand coil.   For a 1mH coil 1nF should result in oscillations around 150kHz. 

The steel bolts that you are using for cores are very lossy.  You can get more efficient operation with ferrite cores.  If you really want to hand wind, you should buy some ferrite stock or reclaim it out of old electronics.  If you just want coils with well controlled properties then you can buy from places like Coilcraft. 

Do you have any particular goals like:  Longevity from a AA cell, or minimum operating voltage, or brightness from your LED?

My original goal was a bright light that works with a single crystal battery and a wide variety of crystal battery builds and a JTC that
I could build quickly and at a very low cost so I can fill my home with little night lights that run for a long time.
I think two of these 1 mH inductors and the 3904 does the trick, so I have met that goal.
My new goal is to understand why these inductors work so well. The videos Gyula shared lay out succinctly every question I need to ask
and then answer. Wow... thank you Gyula and thank you Mark and Bill and Stefan for hosting this forum. This thread has moved me forward
very quickly in a very short time. Namasta everyone!

PS - I'll continue to share my many failures and rare successes on YouTube and here... tune in if you want a good laugh once in a while. lol

[TinselKoala]

You may want to test different transistors. I have tested many, and I've found that MPSA18 and BC337-25 work well, and even the 2n2222a works well. In my testing the 2n3904 actually had the _highest_ start and stop threshold voltages in the basic JT test circuit I used. Depending on your circuit component values you may get different results. I tested only the "start oscillation" and "stop oscillation" voltages; other performance parameters like current drain and LED brightness may not follow the same pattern. But experiment, you never know unless you try.

Here's my "data table" giving the start and stop threshold voltages for various transistors.  These are the supply voltages. "Start" is the lowest voltage at which the JT oscillations will start and light an LED, and "Stop" is the voltage when the oscillations stop. The LED may go out at higher voltages, many JTs will continue to oscillate well after the LED is actually out.