Hello again Ian,
Oh ok, i think i see a few possible problems...
First, probably the biggest error in measuring
eff is assuming the LED always drops 3.0 volts.
This can easily lead to errors of 10 percent or
more in some efficiency calculations.
Normally, the voltage across the LED would be
measured also and the actual reading would be
used in any calculations.
Second, i think you'll see much better accuracy
using a digital type meter set to a scale close
to what you are measuring. Analog meters lead
to errors for a bunch of various reasons.
If you can get ahold of two inexpensive meters
you might be happier
Third, when the current is measured in a switching
regulator like this it's best to insert a small
non-inductive type resistor in series with the
load, measure the voltage across the resistor,
and use Ohm's Law to calculate the current.
The value could be 0.1 ohms to maybe 1 ohm,
which would make calculating the current from
the 'across' voltage very easy to do in your head.
For your circuit probably 1 ohm would be best,
then whatever voltage you read is also the current:
If you read 20mv then the current is 20ma, etc.
Be aware that meters (even digital ones) sometimes
switch rather high value resistors in series with
the load to measure current, especially on the
low current ranges (20ma, 2ma, 200ua, etc.)
This could lead to VERY high errors in the
Fourth, i wonder what the series resistance
of your inductors are? i would guess 8,000uH
would have a pretty high dc R value...perhaps
you could measure this value using your ohm meter.
Fifth, a good operating frequency (if you can
set it) would probably be around 50 to 100kHz.
Small value inductors (uH) dont work well
at lower frequencies because the peak current
goes too high (leading again to eff issues) and
at higher frequencies other factors become a
pain in the neck (skin effect, lead lengths,
stray capacitance, inductor core losses, as
well as rise and fall times). Heck, at some
upper frequencies the transistor may no longer
be supplying a true 'pulse' to the circuit, but
rather a 'triangular' shaped wave which eats
up efficiency very fast (due to long rise and
Hope this all helps some...
LEDs vs Bulbs, LEDs are winning.