>>I need to supply power to a large number of LED's in a vehicle, 
>>where all
>>LED's will be on or off simultaneously, and with 2 brightness 
>>levels.

>>  I came up with this circuit, though I'm not sure if it
>>would work properly...
>>         http://www.narwani.org/neil/electronics/Const-2Cur-Ckt.gif
>
> What is the voltage at the top of the LEDs?  In other words, how 
> much base current will you have when the brake line goes HI?
>
> If these LEDs have the usual 1.7V forward drop each, your circuit 
> should work just fine.  Base resistor value needs to be fairly low 
> or, better yet, perhaps consider the use of a logic-level FET 
> instead of the transistor.

Above is good, but more thought is needed yet.

If this is automotive you have say 12v available. usually more a motor 
running when brakes are usefully used but allow some wiring drop etc.

LM317 is far from being an LDO alas - drop across regulator is 1.75v 
semi-worst case at 200 mA, possibly 2v in some applications (see data 
sheet) . (Depends on how cold you want the mornings to be able to be 
;-) ). lets say 1.7v v. Then there's the 1.3v worst case drop across 
the sense resistor (see data sheet). These two add up to 2.05v 
available for LEDS. Next look at your LEDs and see what the WORST CASE 
drop is at rated current. May LEDs you can run is < (12-2.05)/Vled. 
You mention series strings of 6 LEDs which would allow (12-2.05)/6 = 
1.7v / LED which MAY be enough. If you give the LM317 some headroom 
you may need 5 or even only 4 per string. Or select a good LM317 on 
test :-)

The switched resistor idea looks good, although there can always be 
something one misses.

Using two LM317's makes the job very simple.

The idea of using a boost SMPS adds complexity but makes most other 
aspects simpler. Art's MC34063 is cheap and versatile. Also 
inefficient compared to some modern alternatives but good enough for 
this task, as he says. A modern White LED series-LED IC would do the 
whole task BUT may want a low side current sense resistor (as do 
several other suggestions) which your scheme avoids. The '34063 has 
high side current sense capability which would suit your application.



        RM 

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist