Re: Joule Thief - still not working....

On Thu, 30 Jul 2009 13:01:09 -0700 (PDT), fungus
<openglMYSOCKS@xxxxxxxxxx> wrote:

On Jul 30, 12:40 am, Jon Kirwan <j...@xxxxxxxxxxxxxxxxxxx> wrote:

If it works, the next step would be to figure out what's
different about that bead.

Saturation level, mu, and l_m?

The real question is, if I were to order some from RS, what
would I order? RS stocks these: http://tinyurl.com/mpcjx7

Do I want low or high permeability? Maybe there's some
out there that work even better than the megabead...

{My criteria at the moment is "The harder it sticks to a
magnet, the better it'll work" ... but I'm not sure RS has
a search option for that... :-) }

Well, I'm just learning this stuff, myself.

Let's go over what we think we know (I'm a hobbyist, not an expert on
this stuff, so it's good to sit back for a moment and summarize.)

(1) Recoverable energy is stored as I^2*L/2.
(2) Saturation causes L to rapidly change from a high value towards a
very low (air core) value. The result of this fact is that the
current goes upwards __very fast__.
(3) After saturation the L is quite different, so blind calculation
of energy using a fixed L will yield the wrong answer once saturation
occurs.
(4) Power to the load appears to depend upon Ic_peak, not L.
(5) But frequency of operation does depend on L and we can't run the
BJT too fast (core losses, other than air core, become significant at
about 200kHz _and_ the BJT itself has 'turn around' difficulties,
too.)
(6) Most of the power losses (heating) in the BJT are due to the last
moments during its ON time because of Vce rise right at the same
moment the collector current is highest. Lost energy here is the
product of Vce and Ic and time.

Okay.

The upside of using air core inductors are (a) no core losses to worry
about; and, (b) no need to go buying something, as air is free; and,
(c) no saturation effect to complicate analysis. The downside of
using air core inductors are (d) lots and lots of turns to get the
frequency down to mitigate BJT losses at higher frequencies; and, (e)
poor flux linking resulting in lots of leakage inductance without
using special wiring techniques (and even then, expect more than what
you'd have with a manufactured core.)

Air core is acceptable and you've already got some thoughts on that
here. If you have ready access to that CAT5 or CAT6 cabling, that may
work. The downside there is that the wire itself is quite thick and
your transformer will be sizeable. If you have fine-gauge magnet wire
(did I read that you said you had some?), you might haul out a long
length of it, fold it over, and try and impart a uniform number of
twists per foot/meter in it and use it as a bifilar pair. Or use
google and see what other good ideas you can find out there for
winding good air core transformers with excellent flux linking. I am
not experienced at this (more experienced at winding on manufactured
cores, but still not very experienced there, either.)

Your question is about selecting a core, so let's go there and bypass
any further air core discussion.

One thing about anything other than air is saturation effects. Are
these 'good' or 'bad?' Well, to answer that I'm going to have to draw
a picture:

http://www.infinitefactors.org/misc/images/saturation.gif

I've drawn an energy curve, which is current verses volt-seconds. It
does NOT include core losses, but that's not helpful right now. It
also does not include the whole picture that lots of web sites and
books will include, as I've just focused on one aspect here.

The blue painted region is the recoverable energy -- what you get back
when the BJT turns off.

There are two noticeable parts to that energy. One that lies to the
left of a steeper sloped part of the red line and a tiny remainder
that lies also to the left of that almost imperceptably sloped red
line. The total area of the blue region is your energy.

Okay. So what happens under saturation? It's not entirely bad. There
is still energy to be recovered. The problem is that the current
rises upwards like a veritable rocket ship (causing the BJT to shut
off quicker) and for all that current there isn't all that much energy
added for the trouble.

Now, if you had access to super-conducting wires with zero ohms and
other idealist electronic parts like BJTs with an infinite capacity to
handle current, you wouldn't care. Air core would be fine. But
reality impinges. Wires have resistance, electronic parts that handle
huge currents are big and cost a lot, etc.

So current matters because real parts dissipate energy when passing
large currents. And so does frequency because real parts tend to
dissipate energy when dealing with high frequencies, as well as
behaving differently, too.

One thing we've already worked out for you is this:

Ic_peak = 2*Iout*(1+Vout/Vin)

So we know what that current should peak out at and we really don't
have a choice about this. Although it might be nice to figure out how
to use a lower Ic_peak, we don't have that option. It's cast in
stone. Frequency is something we can control, though. Via the
inductance of the collector winding in this circuit.

So what does saturation do? One thing is that it reduces the time
required to reach Ic_peak. In doing that, it increases the frequency
of operation. We generally don't want too much of that.

Are there any positive benefits to saturation in this case? Perhaps.
If it is a very controlled amount. Most of the BJT dissipation, in
watts, takes place towards the very end of the ON time. If we could
shorten up the ON time, then the total energy (watts times time)
wasted there would be less. If the transformer could be designed to
have a controlled point of saturation such that the knee (at point A
in the curve mentioned above) takes place close to Ic_peak, then the
current would rapidly rise and, in doing so, shorten the duration that
the BJT's Vce soaks up power and that would reduce wasted energy in
the circuit. The cost of being wrong, and having that point A be far,
far too soon, is that the frequency of operation would go way up and
we'd lose out on that score.

So perhaps it would be a good idea to use a planned design where the
Bmax value is used with the idea that if we are a little bit wrong
about it, we can adjust winding turns and tweak towards an even more
efficient design.

Where does point A take place? I'll use terms found at that Fair-Rite
web site thing you pointed at. It's about here:

Imax = Bmax * le / (u0 * ui * N)

where N is the number of turns you use. (ui is what they use for the
relative permeability of the material, u0 is the permeability of air
[4*pi*1e-7], and le is what they use for the effective magnetic loop
length.) In the above equation, using the u0 figure I gave, le should
be given in meters and Bmax in Teslas, not Gauss.

Note that larger N and larger absolute permeability of the core reduce
the current level we can reach before saturation. Longer magnetic
path lengths increase it.

Bmax is pretty much set. Ferrites will be in the 0.1 to 0.3 Teslas.
Iron cores maybe go up to 2T.

All this points up one of the reasons why I think that lower
permeability cores are better here. And it also shows why you
discovered that fewer windings worked better (when you had too many, N
was larger and Imax became less than your needed Ic_peak, the power
delivered went down, and probably your frequency went up, too.)

Let's call Ic_peak (the one you _want_ to have) is about 350mA. We
will set Imax to that value. Let's start with the number of turns and
set it to 50, for now. We will assume ferrite. Your web site uses
Gauss and Oersteds.

Take a look at their 601 sized type 67 core. I clicked on the first
tab at the site you mentioned (type 67 tab) and then looked down to
find their part numbers and the one ending in 601. Click on that.

Look down the page and see Hysteresis Loop chart. The 25C curve
starts to 'softly' bend over somewhere between 1000 and 1500 Gauss.
That's the beginning of saturation. On ferrites, this is usually a
slow process.

Now go back and pick out the same sized type 61 core. (Bigger
selection to choose from, size wise.) Find the 601 again and look at
the Hysteresis Loop again. Yup. About 1500 Gauss, again. But this
curve looks less gradual as it goes into saturation -- more
pronounced.

Now same sized type 43. Same area where it starts to bend over,
though there is 'something weird' going on at about 1 Oersted. Also
take a look at the Initial Permeability vs. Temperature curve. That
one shows a lot of change over termperature. Hold that thought, look
back at the prior two, and go over to type 75 and the 601 part there
and look at its temp curve, too.

I don't like that variation over temperature. So already besides the
fact that we already know that lower permeability allows a higher Imax
(which we may indeed want), temperature variation also adds another
encouragement.

By the way, if we use lower perm materials we will need more turns to
get the frequency we want. But this is fine because inductance goes
down proportionally to permeability but up by the number of turns
squared. So just a few extra windings can compensate, easily.

Let's focus on Type 61. It's temperature curve looks nice and its
saturation looks predictably smooth. Looking at the saturation curve
(and noting what it does at 100C as well as 25C), we need to figure
out H, in Oersteds.

H = 0.4*pi*N*I/le, using le in centimeters.

With le=5.2cm, we get 4.23 Oersteds. Definitely starting to saturate
there. But as I discussed above, that's not necessarily a bad thing.
It actually might help just a little in shortening up the time where
the BJT is dissipating faster. So let's say that looks pretty darned
good, as a first shot at this!

What's the relative permeability? Well, it's dB/dH in the cgs system.
At 4 Oersteds I read out 1900 Gauss and at 0 Oersteds I read out 1250.
This works out to (1900-1250)/(4-0) or about 162.5 (at 10kHz
operation.) So using the design figure of 125 and our computed
estimate aren't too far apart.

What's the inductance for this core with 50 turns for the collector
winding? Well,

L = u0*ui*Ae*N^2/le

But the figures are in meters, etc. So from the web site we see that
Ae=.243 cm^2, which is 2.43e-5 m^2. Also, le=5.2cm or .052m. So, L
is about 185uH, using the ui=125 figure. Or getting close to 240uH
with the estimate we made at 4 Oersteds. Turns out, this is probably
pretty good for a usable frequency, too.

The only question now is ... can you actually wind 50 turns for the
collector and still have room enough for your secondary winding.

PS: Is RS "Radio Shack"?

I don't know. It's what I'd assume. But I can't recall them having
any cores. Or if they do, it'll not be very many types or sizes.

Jon
.

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