Yes i think the "Brinkmann" circuit will
work for up to maybe 6 LED's wired in
parallel. The resistor R2 has to be lowered
in order to supply the extra current.
The coil L1 has to be a bit bigger though.
You can probably get away with a hand wound
toroid with 10 to 20 turns.
I did a simulation on that circuit with
1 to 6 white LEDs in parallel, but i put
one 10 ohm resistor in series with each
LED. Also, i used a Schottky diode on the
output with a 5uf output filter capacitor.
I found full brightness to be easier to
attain with the diode and output cap.
The simulation showed that if you can get one
LED to conduct 20ma ( by adjusting R2 ) then
you can get 6 LED's to conduct 20ma by
dividing the value of R2 by 6 and connecting
5 more LEDs in parallel (with a series 10 ohm
resistor for each LED).
The transistors used were 2N4403 PNP and
2N2222A NPN.
The only thing i couldnt determine was
whether or not the 2N2222A transistor
would get too hot or not. It looked ok
with 6 LEDs and R2/6, but i would have to
build the circuit to find out for sure.
Should work fine for 4 LEDs, with the value
of R6 for one led divide by 4.
Replacing the 2N2222A transistor with a
2N3055 general purpose audio transistor
and the circuit doesnt work at all.
I suspect its because the transistor gain
and switching speed arent enough.
Because of this, i would think other high
powered general purpose transistors wont work
either (because of low gain), unless you
get a special type that has high gain, fast
switching speed, and fairly low Vsat.
Zetex makes some some good transistors, but i
havent been able to get ahold of any yet.
Regulation isnt all that good, so brightness
diminishes as battery voltage falls.
I ran a simulation on another circuit which is
very similar but has feedback. The only drawback
is that it takes 2 more transistors to provide
the regulation circuitry, and a few more resistors
and one more capacitor. Im not really sure
the extra complexity is worth it.
I'll have to build this circuit up to make sure
it behaves like the simulation did also.
Can you get LM339 ic's? These chips can be
quite useful in these kinds of apps.
They are dirt cheap too :-)
The data sheet says they will work down to
2 volts also.
Im thinking of making a circuit up using
one of these too, with super regulation
and multiple leds etc.
Zetex has some specialized chips out just for white leds,
but again i would have to get ahold of some before
i could use them. The other drawback is the
best one only comes in surface mount package.
I did get to try out a MAX756 ic chip, and its
pretty good for 2 cell apps, but is quite a pain
in one cell apps even though the data sheet says
it works down to 0.7 volts. With only one AA cell,
you can only really drive 1 LED because of the
very unusual way the chip works. Yeah it 'works'
down to 0.7 volts or so, but at hardly any
output current.
For 2 cell apps though, you could probably drive
20 leds or something. Once you get about
2 volts input, the chip runs smoothly.
Below 1.2 the output current gets very
skimpy. Unfortunately, there is also no
apparent way to fool the chip with a
pre-regulator/booster either, as the same pin
that is used for the power supply input is
also the voltage feedback pin! It bootstraps
the MOSFET drive voltage for 'normal'
mosfet drive once the output voltage ramps up
to par. The only trouble is, if you have a
significant load on the output, it never
ramps up to par :-) so you never see
the benefit of the bootstraped drive voltage.
In any case, for one cell apps i would stay
away from this chip except for driving only one led.
Even then, i would go with the MAX757, which allows
for output voltage adjustment with two resistors.
(Oh yeah, the MAX756 has two preset output settings:
either 3.3 volts or 5.0 volts, so you also cant
fool it into current feedback mode of operation
either :-( Instead, you have to settle for setting
it up at 5 volts and use a current limiting resistor.)
The nice thing about discreet transistors is you can
use them down to about 1 volt any way you care too :-)
Summing up:
If your in a hurry for a simple circuit to drive 4 leds
then use the brinkmann circuit with resistors and Schottky
etc as above (two cell app) and home wound coil.
If you can wait a little and you dont mind using
4 transistors i'll be working on the regulator version,
which provides 20ma to 4 LED's (probably 6) from an
input of 2 to 3 volts or so (probably regulates down
to 1.5 volts input and can take up to 4.5 volts input).
Good luck with your LED circuits :-)
--Al