LED/Resistor Calc

[Khas Evets]

This is an easy one, but I could use a little help.

In determining the correct current limiting resistor, R = (Vs - Vf)/If. Vs is 9V and Vf is ~1.6V for a red LED, but what should I use for If. Is it the current rating of the LED or the desired current drain?

I'm currently using a 470 ohm resistor in my pedals (this came with my LED's) and it works fine. In searching the forums, I've seen people using up to 10K, which would be too dim. Even 2.4K was a little dim. I'm not using batteries, so it's not an issue for me. I'm just concerned about battery drain, now that I'm building for others.

[ErikMiller]

Go here: http://www.electronics2000.co.uk/

....and download Electronics Assistant, which among many other great functions, has a calculator for LED series resistors.

[petemoore]

470 ohm seems small, the smallest I use is 2k2 [?]..
 I used to trim a value added to 1k [a 10k pot as V/R and 1k fixed series resistance] then trim the pot to the desired LED brightness on chosen PS Voltage, pull the pot, measure, replace resistor [about the value of the pot] where the pot was.

[amz-fx]

I'm currently using a 470 ohm resistor in my pedals (this came with my LED's) and it works fine. In searching the forums, I've seen people using up to 10K, which would be too dim. Even 2.4K was a little dim.

You need you use LEDs with higher millicandela (mcd) ratings...  the higher the mcd the brighter it is at a given current.  A 1000mcd LED will be fairly bright with a 2.2k droppping resistor.

http://www.muzique.com/schem/led.htm

regards, Jack

[Khas Evets]

That makes sense. Mine are about 100 mcd. At 470 ohms, I'm at about 15mA. If I double it to 940 ohms, it's still bright enough at 7.5mA. I guess I'm not sure what mA is acceptable for resonable battery life. Are these LED's useless? I bought 100. :oops:

[Ardric]

Alkaline 9V batteries can usually supply around 600 ma/hour.  At 15 ma of current draw, the LED/resistor combo will drain the battery in about 600/15=40 hours of runtime.

[Transmogrifox]

Ardric-That's a good number to know.  Thanks.  I have been looking for it for a while--obviously not in the right places.

What I would really like to know is the battery drain calculation formula as a function of current, voltage and time--and a list of necessary constants for the 9V alkaline battery.  I have built an envelope filter, rather a massive filter pedal with an envelope follower that seems to drain a pair of 9V batteries (uses 18V supply) in a matter of minutes.  I think I should have just done without the battery snaps.

Oh, and a minor issue:  
battery ratings are in units of Ah, (or mAh), where it's amps (or milliamps) TIMES hours:

(time battery discharge)*(current sourced) = rating (mAh)

...so your calculation was right based on that simplistic model.  We obviously don't want our 9V batteries operating at much less than 6.5V for most pedals we use, so how long does it take to discharge to 6.5V?

Maybe I should get a data logger and do some experiments and perhaps form an empirical formula for the alkaline battery.

[Ardric]

Thanks for the correction!  I was assuming a purely linear relation as you say.

I found via google that Energizer has datasheets for their products, including the 9V.  They have curves for cutoff voltages of 7.2, 6.0 and 4.8V.  Perhaps you can derive something that fits their supplied data.

For really high-draw devices, like 386 amps, it makes more sense to use a bank of AA or C cells.  I guess there's no chance to fit 12 AA's in the pedal, is there?

What happens if one of the 9V's runs down before the other?  Is this a split supply, or are they just in series?  Is the draw fairly equal on both sides?

The pedals I've built so far seem to only draw a few ma.  It would be a shame to drag one of them down with a big LED when they'd last far longer without, so I've been using 1k to 2.2k current limiting resistors.  But I guess the LED won't really matter if the circuit is already hungry.

Another option I've tried is the LM334Z, a 3-pin current regulator in a TO-92 package.  It's programmed with a resistor to a fixed amount of current, then maintains that current over a wide range of voltages.  Used in series with the LED instead of the normal resistor, it prevents the LED from getting dimmer as the battery weakens.  I've often wondered if one of these followed by an RC circuit would make a good PS sag emulator.