🚀 go-pugleaf

In article <at0rth01cgq@drn.newsguy.com>, whill@picovolt.com 
mentioned...
> Winfield Hill wrote...
> >
> > The circuit can be easily scaled to handle many LEDs.  One possibility
> > is to operate Q1 at higher currents, scaling R1 to R4 appropriately. 
> > Another is to add a buffer transistor between Q1 and the bank of current-
> > sink transistors.  We can do this because the Vbe reference is adjustable
> > and can easily accommodate another Vbe drop. 
> >
> >=    1.69V               power supply
> >=    ,----+-- R3 --+---- 3.4V to 6V or more
> >=    |    |  1.8k  |                            multiple 30mA
> >=    |    R4      R6                            white LED driver
> >=   R2b   62      27                            using 2n4401
> >=   68    |        |       white    white
> >=    |    | 1.63V  |       LED a    LED b ...
> >=    |    |        C         |        |
> >=   R2a   +----- B           | 30mA   | 30mA
> >=   3.3k  |    Q2  E         |        |
> >=    |    C        | 0.96V   C Q3a    C Q3b..
> >=    +--B          +------ B ------ B ------- many many more...
> >=    |    E Q1     |         E        E
> >=    R1   |        R5        |        | <- 0.20V
> >=   2.2k  |       1.0k      R7a      R7b
> >=    |    |        |         |        |
> >=    '----+--------+---------+--------+--- etc
> >=                           all 6.8 ohms
> >
> > As before R4 cancels changes in Vbe-Q1 with supply voltage.  If you
> > desire, R2 can be trimmed for exactly 30mA with your transistors,
> > this worked out to 3.368k for Spice's 2n4401 parts.  R6 protects Q2
> > by limiting the transistor-array base drive to 100mA, which should
> > be enough for perhaps 500 LEDs.  :>)
> 
>  Or better yet, use a trusty LM358 (miniDIP version of the 324) for
>  a simple yet accurate low-dropout multiple-LED current source.
> 
> =                             power supply
> =    ,-----------------+---- 3.3V to 6V or more
> =    |                 |                          multiple 30mA
> =    R3                |       white    white     white LED driver
> =   22k                |       LED a    LED b
> =    |           LM358 |        | 30mA   | 30mA
> =    |                _|        |        |
> =    +-- R2 --+------|+ \       C Q1a    C Q1b..
> =    |  36k   |      |   >--- B ------ B ------- many many more...
> =    |        |   ,--|-_/       E        E
> =    | LM385- |   |    |        |        | 2n4401
> =  \_|_1.2   R1   '----|--------+        |
> =   /_\     4.7k       |        |        | <- 0.14V
> =    |        |        |       R5a      R5b
> =    |        |        |        |        |
> =    '--------+--------+--------+--------+--- etc
> =                               all 4.7 ohms
> 
>  The LM385-1.2 is one of my favorite parts, a jelly-bean reference
>  IC that should be in every parts cabinet.  If more than 50 LEDs
>  are driven, Q2-style buffer can be added as in the previous design.
> 
>  Thanks,
>     - Win

Thanks for the latest, Win.  I built your upper circuit and it's working 
okay with four LEDs, on a set of four AA Ni-MH cells.

I can get the TL431s or TL430s, but I've never seen the LM385-1.2, and 
probably the closest I could come is the following.  Maybe you'd like to 
comment on this one.  It sems to give constant voltage, but it 
apparently doesn't have temp compensation.  I guess it works similar to 
your upper circuit, but wih two transistors?  



         --+-----  Unreg V          
           |
           \
           / To give a few mA, maybe 1k
           \
           /
           |
           +-------->  Constant V, about 1.4V
           |           
            \E |
    PNP       \|
    2N         |-------+
    3906      /|       |
            /  |       |
           |           |
           |           |
           |           |
           +--/\/\/\---+  Less than 10 ohms
           |           |
           |           |
           |       |  /
           |       |/     NPN
           +-------|      2N
                   |\     3904
                   | E\
                       |
                       |
                      ===
                      GND

 

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