# Suitable resistors for LEDS

Hello, I am new to Arduino and i just bought UNO. I want to light a LED can anyone help me out? Provide some links or something...? As I have seen i need a resistor. What exactly must i buy? How much Ohm?

thank you very much!
Billys

220 Ohm is a good value for your first LEDs.

fungus:
220 Ohm is a good value for your first LEDs.

Thank you for the response. Less Ohm will burn the leds? More Ohm is not necesary?

Hello xmarkx,
Welcome to the community. Have a look at it.

V = I*R
(5-0.7) = .020 * R
R = 4.3/0.020
R = 215 Ohms

V= applied voltage
0.7 = approx voltage drop due to diode
I = Current flowing through the circuit (20 mA = 0.020 A is more than enough to lit an LED)
R = The resistance we are looking for

Hope it helps.
Cheers.

LEDs are more in the 2.5-3.5 range, depending on color.

@nightcrawler218

The current produced by the pin is 40mA atleast on UNO. Look at the Summary section here

In this case, wouldnt the resistance value then change to
4.3/0.040 = 107.5 ohms

Yes, The information you gave is true. But the fact is that the maximum current that each pin can provide is UPTO 40 mA. The overall impedance/resistance of a circuit will determine the amount of current to be conducted through the circuit.

You have Vf of the LED too low.

And 40mA is absolute MAX, after which damage to the IO transistors can occur.
Keep it safe at 20mA per the datasheet notes in Section 29.

hudu:
@nightcrawler218

The current produced by the pin is 40mA atleast on UNO. Look at the Summary section here
http://arduino.cc/en/Main/ArduinoBoardUno

In this case, wouldnt the resistance value then change to
4.3/0.040 = 107.5 ohms

No, 40mA is the absolute maximum you should source or sink.
Don't get close to this number, and consider 30mA as a more sensible maximum

nightcrawler218:
Hello xmarkx,
Welcome to the community. Nice to meet you nightcrawler218:
V = I*R
(5-0.7) = .020 * R
R = 4.3/0.020
R = 215 Ohms

V= applied voltage
0.7 = approx voltage drop due to diode
I = Current flowing through the circuit (20 mA = 0.020 A is more than enough to lit an LED)
R = The resistance we are looking for

5 V is the default voltage in arduino UNO?

and 20mA is the default current?
by the word default I mean the voltage and current when i connect a battery or usb cable connected to my computer.
Sorry for asking so much questions maybe out of topic but i dont know many things and i am courious. I tend to create an opensource quadcopter as my master next year so I need to make quite enough examples.

5 V is the default voltage in arduino UNO?

Yes.

and 20mA is the default current?

No. There is no such thing as the default current. The current you can get from 5V will depend on the resistance of what you connect to the 5V.

You made that clear enough! Thank you all for the answers...

1. Yes, The maximum output voltage on any I/O pin on arduino is 5 V.
2. Ummm... "Default current" is little bit confusing. I have learnt from experience that the optimum current to lit an LED at its brightest is around 20 mA, thats why I suggested so. Generally I use 1 KiloOhms resistors for this purpose.
Remember that the current is inversely proportional to the resistance. the current depends on the overall resistance of the circuit.

And never say sorry. To Question is a good quality. Guys, if I don't use any resistor I will burn my LED right? :? Can I try this? Is it safe for the arduino?

You can try it, yes.
It isn't safe for your Arduino, correct.

xmarkx:
Guys, if I don't use any resistor I will burn my LED right? :? Can I try this? Is it safe for the arduino?

If you don't use a resistor it is both the LED and the Arduino that you are stressing. Both will suffer damage although they will probably both continue to work but they will fail sooner than they would have done.

Conclusion, I will use resistor Thank you Is there a way to know whether it will damage the led or the arduino ?

Flip a coin.
Sometimes it will fall on its edge.

@xmarkx
Why will the LED be damaged if you don't use a resistor?
The LEDs are basically diodes. A diode acts like a valve in electronic circuits. When you don't use a resistor, it almost acts like a short circuit (with a minor voltage drop though) with no resistance. Now look at the Ohm's Law.

V = I*R
I = V/R
I = V/0 ......... ( Since R=0 )
I = Infinite

Now, the question is why Infinite current is harmful?
In this phase the power dissipation comes into the scene.
P = V*I
P = infinite
Infinite current will cause a huge power dissipation in form of heat. EXCESSIVE HEAT. That's why the LED will burn out with holy smoke.
I just love the quotation that appear on the cover page of the "MAKE: Electronics" book
"Burn things out, Mess things up - That's how you learn"

@hudu - have a look!
http://www.ruggedcircuits.com/10-ways-to-destroy-an-arduino/
Interesting facts to know.