🚀 go-pugleaf

Watson 'Atto Parsec' Name wrote...
>
> Early this year or last year we had this newsgroup discussion about making 
> a simple current regulator for LEDs.  You will have to put your LEDs in 
> series in place of the single LED and adjust the resistor values for the 
> higher voltage.  For a start you will probably want to put a pot in place 
> of the 2.2k resistor.  Here's the simple version of the schematic.  To view 
> this you must use Courier font.
>
> Rich Grise wrote:
>
>> If I were building it, I'd lose the 2N3904, both of the 1N914's,
>> the 3.3K, and the 22K, and just connect the wiper of the pot to
>> the base of the 2N4401.   ...
>
> Thanks for the idea... Do you mean something like the following???
>
>> Watson A.Name wrote:
>
>>>      Current Limiter, Low Dropout, for LEDs
>>>      by Watson A.Name  Jan 16, 02 rev 020116A
>>>
>>>         +---------------+--------------+
>>>         |               |              |
>>>         \               |              |
>>>     2.2 /              --- White       |
>>>      k  \              \ / LED         |
>>>         /              ---             |
>>>         |               |              |
>>>     +---+               |              |
>>>     |   |               |              |
>>>     |   |            |  /              |
>>>     |   \            |/              ----- four
>>>    ---  / 470   +----|  2N4401        ===   Ni-
>>>    \ /  \       |    |\              -----  MH
>>> D1 ---  /       |    |E \             ===   AA
>>> 1N  |   |       |       |            -----  cells
>>> 914 |   +-------+       |             ===   4.0
>>> D2  |   |               +-o T.P.     -----   to
>>>     |   |               |             ===   5.5V
>>>    ---  \               \            -----
>>>    \ /  / 2.2k          / 10           |
>>>    ---  \               \ ohms         |
>>>     |   /               /              |
>>>     |   |               |              |
>>>     +---+---------------+--------------+
>
> Jan 20, 2002  Measurements taken with B&K 2707 DMM.
> At 5.5V, T.P. V = .325V.   At 4.0V, T.P. V = .271V.
>
> This is about 20 percent variation in LED current over the full range.  
> This is not as good as the original adapted circuit that had about 13 
> percent over the full range.  ...

 Generally a transistor beats a few diodes, and takes about the same
 amount of space.  In the circuit below a Vbe-multiplier reference
 has replaced the diodes.  The adjustable nature of this type of
 reference allows us to run the current-sense resistor at 0.2V, and
 a properly-chosen collector resistor R4 allows us to cancel out
 the effects of changing supply voltage.  To a first approximation,
 R4 = r_e for Q1 at its 1mA operating current, or about 27 ohms.

.       ,----
.       |
.      R3
.     2.2k   white   white
.       |     LED     LED
.  ,----+ x    |       |
.  |    |      |       |
.  |    R4     C       C
.  R2   +--- B ----- B
. 1.0k  |      E       E
.  |    C      | Q2    | Q2'
.  +--B        |       +---
.  |    E Q1   |       |
.  R1   |      R5     R5'
. 2.2k  |      6.8    6.8
.  |    |      |       |
.  '----+------+-------+----
 
 A quick calculation shows the voltage at point x should be about
 0.95 volts, setting the value for R2/R1 at 0.95/Vbe - 1  = 0.46

 Thanks,
    - Win