LED current limiting resistor

[Randall]

A client wants me to build him a simple 1 input to 6 output amp switching box with the intention of having or possibly two amps switched on at any given time. This should be simple enough using 6 DPDT foot switches with one one side the amp jacks being grounded in one position, and connected to the input jack in the other. The other side of the switch will be for LED indicators. The problem is, I don't know how to pick a good value for the current limiting resistor. Also, if powered by a single 9v battery, will a single LED shine brighter than 2 or 3 LEDs on at the same time? And to take it further, is it even possible to have all 6 shine at once? (Not that I want to, just for explanation.)

Here is the LED I am considering. https://www.cedist.com/products/led-holder-3mm-red-led

[Enzo]

5ma seems enough to me, they don't have to light up like an airport runway, but even at 10ma, they need to be independent, so just make 6 parallel circuits of LED, switch, resistor.

LED resistors are simple to figure. Start with your power - 9v. The data sheet says 2.7v forward drop on the LED. SO in our circuit, to get 2.7v across the LED, we need to drop 6.3v across the resistor. Ohm's LAw tells us that when 10ma flows through a resistor and 6.3v drops, then the resistor has 630 ohms resistance. 680 is a common standard value close to that.

If you had all six lit, you'd draw 60ma from the battery. If you wanted more than one LED to light up together, you could series them and use the same 10ma through both, but you want them all independent, so you can't series them. SO six LEDs, six resistors, six switches.

[Randall]

Enzo, when you say "5 ma seems enough to me", where do you get that? I don't see 5 ma anywhere, so I'm not seeing your point.

Is the 10 mA what it draws, or it's max draw? And concerning voltage, is 2.7v it's turn on voltage, or max forward voltage?

LEDs are not labelled like rectifier diodes, resistors, etc, so is there a way to know how to handle a nondescript LED that I might pull out of my parts supply to use?

[The Dude]

If I were doing this, I would just temporarily wire a battery, pot, and LED up. Adjust the LED for the brightness you want. Measure the pot when done, and use a common resistor close to that value. You can then use those nondescript LED's you have without knowing much about them.

[Enzo]

I get that from experience, the data sheet said the thing was tested at 10ma, a pretty common current for plain old LEDs. In my experience, half that is usually plenty bright enough to see on stage. I did the math at the 10ma though.

It is a diode, it doesn't draw, it just conducts. Once the junction voltage is reached, 2.7v, the thing should light, then the more current, the brighter the light. The voltage across the diode should stay more or less constant. If you got out your 12v supply and stuck the LED across it, the LED would get REAL bright briefly, until it burnt up.

Got a bench supply or a 9v battery? Set up a simple circuit with a pot in series with the LED. Pot wired as variable resistor - ie wiper and one end. Start at max resistance, then dial it down until the LED reaches a nice brightness. Your current meter in series will tell you the current for that level brightness, and you can then measure the resistance your pot is set at to determine your resistance. Then pick a standard value resistor near that amount. Or if you don't want to measure current, just set the pot for desired brightness and then measure the pot resistance. You already know the voltage you started with, so using the voltage drop across the resistance gives the LED current.

Aha: Dude hopped in with the same thing while I was typing.

[Randall]

This makes good sense. But how do you know if you are within the parameters of the LED ratings if you don't know what they are? Is dialing it in to a reasonable brightness close enough? Or are they all pretty close enough to each other to make a judgement call?

[The Dude]

I would go until the LED hits max brightness and then back the pot off a bit. That will tell you that you aren't driving the LED to over current. Back it off even more if you can to save battery life.

[SoulFetish]

This makes good sense. But how do you know if you are within the parameters of the LED ratings if you don't know what they are? Is dialing it in to a reasonable brightness close enough? Or are they all pretty close enough to each other to make a judgement call?

With very few exceptions, the LEDs you'll be dealing with will max out at 15-20mA. This is fine because for the practical level of brightness you can use shouldn't come close to 20mA. In all the stompboxes I've built, I've found that trying to run an LED at full brightness is inefficient and a nuisance. They tend to be super directional (particularly the clear kind I like), and will radiate right in your eyes as you look down to step on it. Enzo, is probably right on the money suggesting 5mA. The lower the better anyways due to the pathetic mA/hr current capacity of 9V batteries. I would try and see how little current you can get away with at an acceptable brightness

[Enzo]

If you get good brightness at 5-10ma, there is no point in cranking it up to 50ma just to see if you can burn out the LED. They are just LEDs, nothing special. That is why I suggested just getting out a pot and starting at the coldest end, you can dial it in until it is bright enough, then stop. "Too much" is REALLY bright.

Try different colors, they tend to have different forward voltages.

[Randall]

Got it. Thanks guys!

[Mick Bailey]

I usually use 1K resistors for LEDS on 9v. 1K caters for the supply being higher than 9v, too. I even got to 4K7 or higher for high-brightness LEDS. I have some that are insanely bright and are way too distracting for pedals unless tamed with a high-value resistor.

One aspect of LEDS is that unless they are a specifically matched set they will have different current-draw characteristics. This means that if they share a common current-limiting resistor, one can hog more current than the others and glow much brighter when several are lit together. Better to have a separate resistor for each LED if you want equal brightness.

[rjb]

One aspect of LEDS is that unless they are a specifically matched set they will have different current-draw characteristics. This means that if they share a common current-limiting resistor, one can hog more current than the others and glow much brighter when several are lit together. Better to have a separate resistor for each LED if you want equal brightness.

Not using LEDs very often, I never noticed how much the brightness can vary for same part number.

Although I find the "kid friendly" prose a bit annoying, this tutorial answers the original question.
https://learn.adafruit.com/all-about...-led-datasheet

This section tells you how bright the LED is in mcd. Since these are general purpose LEDs, the brightness can vary a bit, these LEDs average around 250 mcd, but the manufacturer may sell you LEDs that are as dim as 180mcd. This variation is pretty standard.

And this one kinda sorta explains brightness/intensity specs.
http://www.jameco.com/jameco/worksho...intensity.html

-rb

[olddawg]

Nothing beats breadboard stuff. I decided to build my own outdoor LED garden spots out of those $2 nine LED aluminum Chinese flashlights. I did the calculations on standard specs but had no idea what the real specs of the ultrabright leds were. Everything was run of a standard 12v AC outdoor lighting transformer. I ended up using a tiny diode bridge, a 100uf cap at 16v, and a 150 ohm resistor for each light. I found the resister value using a wire wound pot. They have been running a a years. Cost me maybe $4 apiece. Much better than the $50 to $75 a fixture for the boutique stuff. Smaller and aluminum. I just cut the flashlights in half with a hacksaw. The light is a little bluish but so what? A little off topic but the same in principle and application. Uses a ton less current than the old incandescents and I can run more lights.