Joule Thief

[tgraca]

can't use a typical LC meter to measure a coil's C... it's only good for a real capacitor that after charging has infinite resistance... a coil will always be less than infinite resistance.... and not the parallel capacitance of the coil..

once you know the L though if you have a signal generator you can approximate the coils' C by finding it's resonance as an open circuit and calculate C from L and F

If you can send a YouTube video of someone doing that, that will save me looking for a new LC meter. Thanks!
PS - I understand the math, have a signal generator, but haven't tried this, so even an article would help... In the mean time, thanks for
reminding me, and I will also look for articles and videos on this. Nice catch!!!

[d3x0r]

If you can send a YouTube video of someone doing that, that will save me looking for a new LC meter. Thanks!
PS - I understand the math, have a signal generator, but haven't tried this, so even an article would help... In the mean time, thanks for
reminding me, and I will also look for articles and videos on this. Nice catch!!!

Well... in the case of your prefab inductors... maybe a couple turns of thin wire... connect signal generator ground and signal to the wire.... and scope to inductor.... ground and signal on each end... 
2 problems 1) the scope probe has a capacitance that is significantly larger than your inductor  (25-250nF)
2) the ratio of turns between primary and secondary affect the resonant frequency... 
so it's only an approximation in any case...
might be better to use a LED load or something and tune for max brightness... even then LEDs are like 200pF.


it works better for larger coils....


I guess typically they will add additional series or parallel capacitors of a known value and then you can get deviations from known values... it's a lot more work and haven't found a general calculator to approximate it... so if you had 10pF 100pF 1nF 10nF caps you could approximate as L * x+10pF = yF L * x+100pF=y2 F  ...

[MarkE]

I got a Fluke 6 months ago.. it couldn't even take a volt reading... I will never buy another one... I'll check into Keysight.
Note that these are standard inductors I am testing and they do show a lot more capacitance than 50-100 turns of 30 gauge
wire around a bolt core, so at least that gives a certain amount of reference. Although the simplified joule thief schematic does
work with standard inductors and without a resistor or capacitor, a resistor or resistor and capacitor IS necessary when working
with the hand wound coils. I am just trying to figure out why, and it seems that the resistance is a major factor. I would like
to think there there is a bit more capacitance to them too, which would make sense. The meters at least show this...

If the inductors are catalog items then if you let me know the Manufacturer and their part number I will look into this.  It should not be a surprise that your circuit just using a coupled choke and a transistor oscillates reliably.  Winding resistance changes the Q o the inductor.  Too much resistance and the transistor will just bias to a stable operating point.

If you are keen on experimenting with that circuit, you might want to try placing a small capacitor across the coil on the base side of the circuit.

[tgraca]

If the inductors are catalog items then if you let me know the Manufacturer and their part number I will look into this.  It should not be a surprise that your circuit just using a coupled choke and a transistor oscillates reliably.  Winding resistance changes the Q o the inductor.  Too much resistance and the transistor will just bias to a stable operating point.

If you are keen on experimenting with that circuit, you might want to try placing a small capacitor across the coil on the base side of the circuit.

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

[gyulasun]

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