OMG just googling for a decent LED calculator is frustrating.
I have my own. But...
Do you know of an on-line calculator that allows for putting in what you know, and getting out what you don't?
All I've found just now focus on determining the resistor given everything else: supply votage Vcc, forward voltage Vf and desired operating current -> resistor value.
Some also show the resulting current for the nearest standard resistor.
I would like to supply any three of those four (Vcc, Vf, current and series resisitor value) and have the fourth calculated.
Rule of thumb for LED is 20mA. That translates to 50 ohms per volt. Subtract the led forward voltage, Vf, from the supply Vcc, Typical Vf for a red led is 1.6V, so with 5V as Vcc less 1.6V Vf = 3.4V to be dropped by the series resistor R. 50 ohms per volt × 3.4V = 170 ohms.
Now LED's can tolerate a 2:1 currect variation (the 20mA nominal will tolerate 40mA wayyy down under 10mA) so you could use most any R in the range of 100 to 400 ohms with good results. 220 and 360 ohms make good R's for most LED apps at 5V.
Old fashioned, but how about a spreadsheet? Or a pen and a piece of paper? Or, go whole hog. I have an app for my 'droid that does that, and dozens of other simple calcs as well. YMMV...
Well, if a phone app won't do, I guess I could go looking to see if my phone app has a website ancestor. Back shortly...
Nope. Out of luck. It works well, but 'droids only.
I like having it on my phone, 'cause then it's with me everywhere I go, not just when I'm at the PC. It's the only app I've ever paid for.
Because the LED characteristics are a GIVEN! You pick the LED first, this determines both Vf and If. The supply voltage, Vcc, is also a given, dictated by the available power. The only thing left to calculate is the series resistance, Rs. Even then, R's only come cheap if they're a standard value. No point in specifying a high cost +-.01% part when +-50% will work. Recalling 50 ohms per volt dropped is fast, easy, and near enough.
This completely depends what the LED is supposed to do IMHO. If you want to build a LED torch or want to illuminate something, sure 20mA is the way togo if not more is possible.
But if you make an illuminated control panel than I'd go for 1mA, unless the LED is SMD (thus not diffused) I go for half a mA.
Looking in a new LED with 20mA running through it, is eh.. 'unpleasant'. Those led calculators should point this out as well. 'Do you want to illuminate something, create a LED laser or do you want to be able to look at the LED without losing an eye?'
Like I said, you select the LED FIRST! But most T1-3/4 LEDs are nice and bright for general indicators at 10 or 20mA. No matter what the calculator says, the data sheet typical operating specs rule.
