🚀 go-pugleaf

In article <at0a6l01j0s@drn.newsguy.com>, whill@picovolt.com 
mentioned...
> 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

Well, thank you for the improvement.  I found one design deficiency(?) 
with the original circuit.  I found that if one LED fails open, the 
other LEDs dim substantially.  The 30 mA is no longer flowing thru the 
open LED's 6.8 ohm emitter resistor, so there is no IR drop across it.  
This allows much more current to flow thru the B-E junction of that 
transistor, hogging the base drive current away from the rest of the 
transistors.  Hopefully, keeping the LED current at 30 mA will prevent 
that open LED from happening.  


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