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


"Paul E. Schoen" <paul@xxxxxxxxxxxx> wrote in message
news:%wqdm.353$Jg.293@xxxxxxxxxxxxxxxxxxxxxxx

"fungus" <openglMYSOCKS@xxxxxxxxxx> wrote in message
news:128b0b87-6c98-4e8d-8271-049d63d2e223@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Aug 1, 9:13 am, "Paul E. Schoen" <p...@xxxxxxxxxxxx> wrote:

Not in direct reply to this thread of the post, but I simulated the
circuit
and I was able to add current regulation so that LED current stays
within
20-27 mA from about 2 volts to 6 volts and efficiency is about 80%.
I plugged those results into my graph:
http://www.artlum.com/jt/jt_outputs.gif
By extrapolated the line using my Winnie The Pooh ruler I
can see that crosses the y-axis of the graph at 16mA.
ie. with zero volts input you have about 16mA output.
Maybe you should patent this circuit...

Well, I never claimed it was linear.

I was just kidding...

I googled for LTspice to see what this "simulation" thing
is all about and had a look at your circuit - you added a
few extra components!

So what you've done is regulate the current better so the
output is a lot flatter as voltage drops then it suddenly falls
off at the end.


Maybe I should build this circuit. I have made an LED
flashlight circuit for about 40 watts at 12 VDC using a
PIC and a boost converter. But this is so much simpler!

I haven't got enough parts here to make one but if it works
as claimed then the extra efficiency could pay for the extra
ports quite quickly (with non-rechargeable batteries).

------------------------------------------------------------

If you have installed LTspice then you should try their jig for LT1932
which is an LED driver that works down to 1.5 VDC and has built-in current
regulation. It runs at 1 MHz so the inductor can be tiny, and there are no
big spikes in the LED current as there are on the JouleThief without a
capacitor.

Also I tweaked the original circuit without regulation and achieved 90%
efficiency at 3 VDC and 87% at 2 VDC. The output current is roughly
proportional to the square of the input voltage, and the circuit works to
1.5 VDC. Some of the components might not be necessary, but I did find a
lot of variation with different BJTs. A 2N3904 seems to work best.

Nice analysis. I've tested many other types, "switchers" work best (2N1711
and 2N2019) on the original circuit, but with the R1 cap then almost any BJT
will work. There is more than hfe involved, transistor capacitance seems to
be significant, and frequency. I've never reached 50 percent efficiency,
though.

The Schottky diode D5 and filter capacitor on the output limit the high
current spikes that otherwise go through the LEDs. Another Schottky D7
limits the reverse voltage spikes. These spikes could damage the LEDs.

I've not tested these. I've not noticed LED damage after several days. I did
try hanging a 6 volt zener on the base, which does protect the BE junction
but cuts efficiency.

R1 does control the output current, and the small capacitor across it
greatly speeds up the transitions and increases efficiency. I added an
input bypass capacitor and realistic source resistance. If your voltage
source is not close to the circuit, a bypass is needed.

Yes, but it is not linear. Transistors are current devices.

It seems to work OK with equal values (10 uH) of L1 and L2, but with L1=40
uH I was able to get it to work at 1 VDC input, and 69% efficiency with
about 3.4 mA output. The frequency is 800 kHz, which might be too high for
a successful deadbug prototype.

100 uH coils brings the frequency down to 135 kHz and about the same
efficiency, but less output. I boosted R1 and I ran some more simulations
with even better results, so I left it at that. It should be interesting
to
build and test an actual circuit.

Paul

My coils seem to be around 40 to 50 uH. About 30 turns on a 1/4 inch diam
ferrite rod salvaged from an old TV. Symetric winding is not optimal, I
think there is improved output with 20 turns on the collector side and 10 on
the base side, but I've not checked the waveform. The original circuit with
a 3300 pF cap across R1 gives a fairly good waveform at around 100 kHz and
10 percent duty cycle. 1 1/2 volts input gives 12 volt peaks. Very
non-linear voltage response.


.

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