With 10 turns wrapped around the core, that construction
would exibit a minimum of about 58uH, but the highest could be
20 times that value depending on the grade of core material.
Also, since the dimensions are very critical to the calculation
of the inductance, even a tiny error could lead to a big error
in determining the inductance. Thats why i will suggest the
Wrap 10 turns around the core and call it an inductor :-)
Connect a 100 ohm carbon composition resistor (5%)
(or metal film, 1%) in series with this inductor,
and power the two with a frequency
generator set to put out 1 volt ac at 100kHz.
Measure the output of the generator with a
high impedance digital voltmeter
first and set it to 1.000 volt on the meter.
Now using the same meter on the same scale,
measure the voltage across the inductor.
Here is a list of the theoretical ac voltage measured across
the inductor for various inductors close to 100uH:
L(uH) ...... vac
----- ...... -----
120uH => .602
110uH => .569
100uH => .532
90uH => .492
80uH => .449
70uH => .402
60uH => .353
50uH => .300
40uH => .244
30uH => .185
If your voltage reads low, add a few turns and measure again.
If your voltage reads too high, remove a few turns and measure
again. Anything between about .4v and .6v will work ok.
I used a 200uH inductor in the circuit (Brinkmann) and
it worked ok.
I hope you have a frequency generator, if not what test
you have? There are some other ways to measure the inductor
If you dont want to be bothered with measuring the inductance
perhaps you can start with 10 turns and see if that works in the
actual circuit. If not, add a few turns.
When you test the circuit for the first time, if you want to
a 5 ohm resistor in series with the inductor
that will protect the transistor in case something goes wrong.
When you are sure it works, you can always remove this 'test'
resistor and fire it up full steam :-)
In order to save on the more expensive white leds, if you would
to, you can do all your testing by replacing the white led with two
standard red leds in series. That way if something blows the leds,
you are only out two cheap red ones instead of a $3 white one.
When you are sure it works ok, pop in the white one :-)
The two cheap standard leds in series drop about the same
as one white one and are ideal for testing out new designs.
See that they have a forward drop of about 1.7 volts each.
Good luck with your LED circuits,
LED's vs Bulb's, the battle is on.