🚀 go-pugleaf

Watson wrote...
>
> whill@picovolt.com wrote,
> 
>>  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 = R2/R1 * r_e for Q1 at its 1mA operating current, or 36 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

    More accurately, R4 = R2/R1 * r_e for Q1, or 36 ohms.

> But Win has given us a simple solution (above) that can be built with BJTs 
> available at Radio Shaft.  And no pots to adjust(!)  The only problem I see 
> with his circuit is that with three or more LEDs, the base currents for the 
> drivers may add up to more than the constant voltage source can supply.  So 
> I have to look at increasing the current to a couple mA, or else use more 
> than one constant voltage source.  Because just about everything that I've 
> built has a half dozen white LEDs or more in it.

 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.  :>)

 Thanks,
    - Win