[sdiy] voltage and LEDs

[Oren Leavitt]

db wrote:
> 
>>
>>
>> You'll want to use a power transistor, maybe a power darlington 
>> transistor for the trigger switch.
>> The 2N5088 won't handle that high of current.
>> Using the 317 as a current source the LED goes to the "adj" pin not 
>> "Vout"
> 
> 
> 
> Ah, silly label error.
> 
> Any suggestion for a suitable transistor?
> 

A 2N3055 or TIP41 ought to work.

> 
>> ..Or you can use the 317 as a voltage regulator, with some small R 
>> between Vout and the LED, and put a small transistor switch in the 
>> resistor divider that sets the Vout. Use that to electronically turn 
>> the 317 and LED on/off.
>> The datasheet has an example "5V Logic Regulator with Electronic 
>> Shutdown" to give you an idea.
>> http://www.national.com/ds/LM/LM117.pdf
> 
> 
> http://yfrog.com/jvimage1jpj
> 
> So this new way would be to have the 317 do the big work, so a small 
> transistor would do the switching?
> 

Yes

> In this '5V Logic Regulator with Electronic Shutdown' circuit, they show 
> trigger is 'TTL.'
> I am using a 2n5088 and it's working from my trigger. No reason not to 
> use that 2n5088, right?
> 

The 2N5088 or any small NPN switching transistor should work fine.

> R1 stays at 240 and I R2 sets the voltage I need for the LED, yes?
> Wouldn't this *shut* the power when it sees v+ at the base of that Q? Or 
> am I still confused?
> 
> 
Yes that configuration will shut off (actually reduce to about 1.2V) the 
LM317 when current is flowing thru the base of the transistor.

You may also want to consider the specialized LED driver ICs that Achim 
brought up, too..
http://para.maxim-ic.com/en/search.mvp?fam=hbled&hs=1
..and lots more.
Have fun!
> 
> 
> dennis.barton:skylab2000[socal]
> 
> 
> 
>