LED Driver delivers constant luminosity

[Simon Dyer]

I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its
ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with
very minimal and inexpensive components. Since it pulses the LED, greater
perceived brightness can be acheived at great efficiency. See
http://www.reed-electronics.com/ednmag/article/CA302239?pubdate=6/12/200
3&spacedesc=designideas

According to the formula, changes to R, L or C could thoretically provide
more current to the bright white LED - I am looking for 30ma average, but in
practice I have not been able to acheive this.

Can anyone tell me where I am going wrong? I would be happy to use three
cells if it helped.

--

 

[Activ8]

I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its
ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with
very minimal and inexpensive components. Since it pulses the LED, greater
perceived brightness can be acheived at great efficiency. See
http://www.reed-electronics.com/ednmag/article/CA302239?pubdate=6/12/200
3&spacedesc=designideas

According to the formula, changes to R, L or C could thoretically provide
more current to the bright white LED - I am looking for 30ma average, but in
practice I have not been able to acheive this.

Can anyone tell me where I am going wrong? I would be happy to use three
cells if it helped.

By believing that pulsing an LED produces greater perceived
brightness.

 

[Ban]

I very much like the circuit by Israel Schleicher in EDN Jun 12 2003
for its ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5
volt cell with very minimal and inexpensive components. Since it
pulses the LED, greater perceived brightness can be acheived at great
efficiency. See
http://www.reed-electronics.com/ednmag/article/CA302239?pubdate=6/12/200
3&spacedesc=designideas

According to the formula, changes to R, L or C could thoretically
provide more current to the bright white LED - I am looking for 30ma
average, but in practice I have not been able to acheive this.

Can anyone tell me where I am going wrong? I would be happy to use
three cells if it helped.

Simon,
if the LED current should be 30mA average, then it has to be with 20% duty
cycle(which is varying with battery voltage) 150mA. For this kind of current
the right transistor should be something like a BC337 with more current gain
at high currents. The inductor probably needs to be of bigger size and less
inductivity too and you can try to solve the equation given, which IMHO is
too simplified. The best would be to simulate the circuit with the free
LT-spice.
I think the first article gives a better circuit, in the pdf the connections
to the driver transistor are not drawn or got lost in some pdf-conversion,
but it is clear where they go.

 

[Simon Dyer]

Bonjorno
Thanks for the practical response Ban.
I'm not up with Spice models, but will try swapping out Q2.
Can you suggest values for the inductor?
Simon

 

[Tam/WB2TT]

Simon,

If all else fails, try this: 3 AA batteries in series with 10 Ohm resistor
(for current limiting) and 50 Ohm potentiometer (connect to 2 terminal only)
to LED. 30 ma corresponds to the potentiometer at about the center of its
range.

Tam