DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

Hi, I just finished this psu and it works like a charm. :D


I just need help figuring out what value of current limiting resistor i need to change to put blue leds instead of red.


I know the resistors i need to change are r8 and r9 (both are 3k3), but
im confused because its between +18 and -18, how can i calculate for blue leds ?

I tried 1k ohm and the leds don't light up! (at least they dont blow up!).
I think i misunderstood the formula. :roll:

Can someone explain what is the right formula? im confused.


Thank you very much for your help.


John



after i will try to calculate for all the colors and post it here to help newbies...

LEDs are polarized. Did you try inverting the LED?

For the resistor value, figure out first what's the current draw of your blue LED.

Compute R = E/I where I is current in Amps, E in volts, R in ohms.

You can fudge the R values if you need to make it dimmer or brighter.

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[quote]Quote:
I tried 1k ohm and the leds don't light up! (at least they dont blow up!).


LEDs are polarized. Did you try inverting the LED? [/quote]



I did not try, but im prettey sure i inserted them according to the tutorial
(the long leg (+) to the bottom hole (the upside is with the lm317 facing top)


Ok, so I think they are 20ma . i did it with 18 volt and it give me 900ohm,
should it work with 1000ohm i already have?
or do i need to do it with 36 volt ? (+18 to -18)?

my blue leds are 20ma 3v.


thank you very much owel


john

Try inverting the LED.



Yeah, but those were for my LEDs.

You substituted parts so just try flipping your LED around and see if it lights up.

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Sucess!!! damn, i flipped the led and it works :oops: ,

Thank you Owel for stressing that point, i guess i should never take for granted led polarity....


So anybody that want to replace the 2 red leds with blue ones, just
put a 1k resistor in the place of r8 and r9.
It's pretty bright, so you might want to try higher resistance.

This works for blue leds 3v/20ma and should for white leds with the same specs.


Thanks again Owel, I'm starting to understand the formula now.


John