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

calculated efficiency.

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

fall times).

Hope this all helps some...

Take care,

Al

LEDs vs Bulbs, LEDs are winning.