🚀 go-pugleaf

In article <asi6l201t31@drn.newsguy.com>, whill@picovolt.com talked 
about...
> Watson 'Atto Parsec' Name wrote...

[snip]
 
>  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


I came up with this recently.  It's the basic, simple way to make a current 
limited LED driver:


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

Most of the 2N7000s seem to need about 2.5 or so V to turn on, so I figured 
that a simple solution would be to do this:

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

           +---------------------------------+-----O +
           |                                 |  4 to 5.64V
           |                                 |  4 AA Ni-Cd
           \ R1                              |  Recharge-
           / 1.5k or so                      |  able Cells
           \                               -----
           /                                \ /  White
           |                                ---  LED
           |                                 |
           +-----------+----------+          |
           |           |          |       |--+ D
           |           |          |      |       Q3
           | TL431     \ R1       +------||<-+   2N
           |      +--->/ 25k             |   |   7000
           | K    |    \ Curr             |--+ S
         /---/  R |    / Adj.                |
          / \-----+    |                     |
          ---          |                     |
           | A         \                     |
           |           /                     \  R3
           |      R2   \                     /  10
           |      47k  /                     \  ohms
           |           |                     /
           |           |                     |
           |           |                     |
           +-----------+---------------------+-----O -


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.

Like you said, Win, the 2N7000 can make things a lot less complicated.  :-)


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