Powering an LED

[3rdof3]

Hello,


I'm currently building a couple things that use switching and am curious about powering the LEDs for the switching. I've built a tube OD pedal based on Doug's design and would like to add an LED for the "ON" state.  The tubes use 12VDC for the heaters, which is the lowest voltage in the pedal, and I'd like to know if powering the LED using a 470R current limiting resistor will work or just fry the LED?


Thanks!


Will

[eleventeen]

The math: Your typical LED about the size of a BB usually want about 1.2 volts to light up. Different colors are different volts. But let's ignore that for now. They want 10-20 mils so let's go 15 (you would want it a bit on the bright side)


So you start with 12 volts (I am going to assume you mean you mean 12.0 volts and not 12.6---not that critical)


You wish to drop 10.8 volts @ 15 mils. Ergo: 10.8 / .015 = 720. 470 would be on the bright side, running 23 mils through the LED. At the upper end but not flame-out. Probably get a little warm. I would edge your current limit resistor up a little higher, 560, 620, 680.

[PRR]

With modern LEDs on dim-lit stages, pedal builders are using 4.7K and 10K with 9V supply. We don't have to push the 20mA limit like we did in the 1980s. (Summer noontime park gigs may want full juice.)

Taking 1.6V-3V for the LED, that is near 1mA.

Since 12V is bigger than 9V, you could round-up about 33%. Or just figure that relative to hungry tubes, 1mA versus 2mA is not important.

11teen says 3/4 K, I say 5+K. Anywhere in that range will work. What I would do is see if I had too-many of some red-stripe resistor. Maybe the 1.2K drawer is over-flowing. Then that is the "best" value to start with.

BUT: if your "12V" is _AC_, the LED only lights half the time. Whatever you figured for DC, cut the resistor in half to get back to a "nominal starting point".

(Ideally you would also consider the effect of reverse voltage on the 1-way LED. But I assure you that if the forward current is less than max, the reverse breakdown is harmless.)

In any case: just tack the LED resistor at first, gig with it, before you make a final choice. There is too much variation among LEDs (and stages, and player-needs) to pre-compute any "perfect" value.

[3rdof3]

Thanks Gents!  I'm going to start with a 1k5 and go from there.