🚀 go-pugleaf

In article <at12n201e0@drn.newsguy.com>, whill@picovolt.com mentioned...
> Watson 'Atto Parsec' Name wrote...
> >
> > whill@picovolt.com mentioned...
> >> Watson wrote...
> >>> whill@picovolt.com wrote,
> >>> 
> >>>>         ,----
> >>>>         |
> >>>>        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
> >>>>    |    |      |       |
> >>>>    '----+------+-------+----
> >> 
> >>     More accurately, R4 = R2/R1 * r_e for Q1, or 36 ohms.
> >> 
> >> 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.  :>)
> >
> > 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.  
> 
>  Nope, that's a problem or potential problem at one level or another
>  with all of the shared-reference circuits, because they rely on only
>  driving Ic/beta for each LED-transistor combination, rather than Ic.
>  The circuit above, capable of driving 100mA into the base-bus line, is
>  less vulnerable because in applications with say up to 100 LEDs it has
>  left-over capability to handle the situation of a few missing LEDs.
> 
>  You do have the option of using more Q2 transistors, so that each one
>  is responsible for fewer LED drivers.
> 
>  One "solution" is to add a separate base resistor with each LED-driver
>  transistor, thereby limiting the current it can draw if the collector
>  is open.  But be careful in choosing the resistor's value, because it
>  introduces an error related to the transistor's beta. 
> 
>  For example, assume a beta of 150 creating a base current of 200uA.
>  Adding a 470-ohm resistor would create a 94mV drop; we would correct
>  for this by raising the reference by 94mV.  However for a transistor
>  beta of say 250, drawing 120uA for a 30mA output, and dropping 56mV,
>  we'd see an emitter voltage of 238mV instead of 200mV and thus get a
>  current of 36mA (actually 34.5mA, but you get the idea).  So adding
>  this resistor can create a likely non-trivial error.  In the absence
>  of an LED, the 470-ohm resistor limits the extra base current draw
>  to about 0.4mA, which is nice.  Perhaps one could be satisfied with
>  a smaller base resistor, say 220 or even 100 ohms as a compromise.
> 
>  Alternately a different solution could be used, such as adding a
>  second transistor to provide a current path in place of an open LED.
> 
>  Thanks,
>     - Win

I thought about the added stuff.   But then I started out with the other 
solution, which I posted previously.  No base current, because no base!  
No problem!  Hey, it was Win who said that these 2N7000s make things a 
lot simpler.


LED Current Controller Basic (Block) Diagram:


                 +--------------------+------O + 4 AA Ni-Cd cells 
                 |                    |             4V to 5.6V
                 |                  ----- White LED
                 |                   \ /  3.5V at 30 mA
                 |                   ---
          ,-------------,             |   
          |             |             | D
          |  Constant   |          |--+
          |  Voltage    |         |        Enhancement
          |   Source    |---------||<-+    mode MOSFET
          |  Equal to   |       G |   |    2N7000, BS170,
          | Vgs(th)+.3V |          |--+    NTE490, etc.
          |             |             | S
          '-------------'             |
                |                     \  R3  10 ohm
                |                     /  Current
               ---                    \  Setting
               GND                    /  resistor
                                      |
                                     ---
                                     GND



For FETs with Vgs(th) 2.2V and higher:    

           +---------------------------------+-----O +
           |                                 |  4 to 5.65V
           |                                 |  4 AA Ni-Cd
           \ R1                              |  Recharge-
           / 1.5k                            |  able Cells
           \                               -----
           /                                \ /  White
           |                                ---  LED
           |                                 |
           +-----------+----------+          |
           |           |          |       |--+ D
           |           |          |      |       Q3
           | TL431     \ R1       +------||<-+   2N
           |      +--->/ 25k             |   |   7000
           | K    |    \ Curr             |--+ S
         /---/  R |    / Adj.                |
          / \-----+    |                     |
          ---          |                     |
           | A         \                     |
           |           /                     \  R3
           |      R2   \                     /  10
           |      47k  /                     \  ohms
           |           |                     /
           |           |                     |
           |           |                     |
           +-----------+---------------------+-----O -


 

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