Re: simple current source for LED and 3 cells?
On Tue, 31 Jan 2006 18:24:57 +0000, Peter Heitzer wrote:
> Hi folks,
> I have some cheap LED torches operating from 3 AA batteries that I
> would like to improve. The curcuit in the torch consists of a 22 ohm
> resistor for each white led giving about 20mA @ 3.5V but ca. 55mA @ 4.5V.
> I'd like to have a rather constant current of 20 mA over the whole range
> of 4.5V to 3.5V.
> Any idea for a simple discrete solution?
Build a current source, and use it to drive a mirror. That way, the
compliance of the source won't be as much of a problem.
batt ---o--------------------------------o--------o--------.
| | | |
| | | |
| --- --- ---
| \ /==> \ /==> \ /==>
| --- --- ---
| .------. | | |
| | | | | |
'---| 317 |---[62]---o--o---. | | |
| | | | | | | |
| | | c | c c c
'------' | b--o--b--------b--------b
| | e e e e
'--------------' | | | |
| | | |
batt ----------------------------o-------o--------o--------'
You can also hack up your own current source, but it won't be as
temperature-friendly...
The 317 may have too much overhead for this application, so an
adjustable LDO might be a better choice. Using a transistor array
would make it more consistent; or, you can match beta by hand.
If the current overhead is too much, you can decrease the wasted
current by increasing the 62 ohm resistor to 1k, and putting a 62 ohm
resistor between the emitter and ground of the leftmost NPN. The
problem with this is that the temperature coefficient of the current
through the LEDs will suffer.
--
Regards,
Bob Monsen
"we can allow satellites, planets, suns, universe, nay whole systems
of universe[s,] to be governed by laws, but the smallest insect, we
wish to be created at once by special act"
-- Charles Darwin
.
