Resistors

First.... The 5 and 10 mm leds have any value of resistance ....... Second..... Can i make a 550 ohm resistance with 330 ohm and 220 ohm by connecting them in series? ....in my logic i think yes but i don't see it in arduino :smiley:

There is an associated dynamic resistance to a LED.
Look at the graph of the forward voltage drop vs current flow then calculate the value.
Example graphs:

550 ohm resistance with 330 ohm and 220 ohm by connecting them in series

Yes

That is one seriously bizarre graph. Very colourful and imaginative, but negligible connection with the characteristics of real LEDs. Where did you get it?

Of course, you can make 550 ohms with a series combination of 220 and 330. But only out of desperation if those are the only values you have. In practice, you use just go and get a 560 ohm resistor, which is a standard value and there are very few situations where you need something different to a standard value.

Most certainly, for controlling the current though a LED - and you must use a resistor to control the LED current - you choose either 220, 330, 470, 560, 680, (820,) 1k or so on. Why do I bracket 820? because the visible difference between 680 and 1k is the same as between 220 and 330, and 560 is actually relatively uncommon also.

I really like the green line :wink:
Looks like negative resistance.

  1. The resistance of the leads is negligible at that kind of resistance.

  2. Yup - consider investing in a resistor assortment pack though, it comes out to under a penny each after shipping.
    http://www.ebay.com/sch/i.html?&_nkw=resistor+assortment

so there's no difference between the 5 mm and 10 mm leds at resistors particularly......should i connect them with same 330 ohm ...... the current won't come over 20 mA .......so the difference between size of the 5 and 10 mm leds what does this difference lead to ?

so there's no difference between the 5 mm and 10 mm leds at resistors particularly

That is right, the chip inside is the same.

should i connect them with same 330 ohm ...... the current won't come over 20 mA

Yes that is right.

so the difference between size of the 5 and 10 mm leds what does this difference lead to

How you mount it and the viewing angle.

You may find something interesting here:

Oooh, hum, think I misread the original question on 5mm vs 10mm, I saw "leads" not "leds", and thought he was referring to the pin length...

But the result is the same - that number has no bearing on their electrical properties, only their physical size. As crossroads mentioned, the actual LED in various sizes of through-hole LEDs are often no different.

All diodes have the current as a roughly exponential property of voltage, so the curves get very steep
very rapidly, the so called 'knee' of the curve is aptly named.