# Why a 220 ohms resistor is needed on chapter 5.2(Getting Started with Arduino)

Hi,
I am new to Electric.
I have read chapter 5 and I think the circuit diagram may like this I attached below.

What I have thought:
i. current circuit voltage is 5, Resistor is 220 ohms. So I = U/R = 5v/220ohm = 0.0227A = 22.7mA.
ii. The minimum conduction current of a LED is about 10mA. And that is a big current could made it bright enough.
iii. So the entire circuit current is about 32.7mA

The question is:

1. why a 220 ohms resistor is needed in this circuit?
2. how did you make sure that this circuit will generate a nearly 33mA current?
3. As I said this is a current circuit. if led broke, it will be a open circuit. and a led is also a load unit, the circuit shall not short. Why a resistor is needed in the circuit？

1. Currents do not add up in a circuit loop. Voltages do. Learn Kirchhoff’s and Ohm’s laws, they are absolutley elemental. Don’t touch wires until you understood them.
2. LEDs do not limit the current properly by themselves. There is not 10mA “minimum conduction current”. It will light up with <1mA, and it will pass 200mA until it and your Arduino pin go up in smoke.

A red LED may have a forward current of 1.8V at 18mA. But other than an Ohmic resistor, it will not not have a forward resistance of 5V at 50mA. Instead, it will pass 100mA at 2.5V and at 5V, the current goes through the roof. In addition, these values are temperature dependent, so even if you initially provide a voltage that gives you a proper current, when the LED heats up, that may change (usually to the worse).
So if you have 5V, and you need 20mA at the LED, and you know that roughly corresponds to a forward current of 2V, you add a resistors that drops 3V at 20mA, i.e. 150Ohm. Be a little more save, 220Ohm. There’s your number.

Hello,

Clearly your both having problems with basic electronics.

Have a look at this video and if you still have questions then ask them.

Led current limiting resistor

ElCaron, ohms law U=IxR or R=U/I
3/0.02=150ohm

Hope this helps,

Mike

ElCaron:
Learn Kirchhoff's and Ohm's laws, they are absolutely elemental.
Don't touch wires until you understood them.

I do like that

Yours,
TonyWilk

Mikeb1970:
Clearly your both having problems with basic electronics.

And what makes you think that, person I had to explain what a snubber circuit is a while ago? (Where you also concerned someone due to a lack of your own understanding ...)

Mikeb1970:
ElCaron, ohms law U=IxR or R=U/I
3/0.02=150ohm

And how does that differ from what I wrote?
"you add a resistors that drops 3V at 20mA, i.e. 150Ohm"

ElCaron,

I owe you an apology, I am taking medication for poly neurological pathy and this effects my eyeside in a very bad way. I thought you wrote 1500 and 2200 because of the Ohm behind it. I have bought a new laptop with a larger screen for my problem, but I was using my phone. This is no.excuse towards you and I hope you accept my sincerest apologies.

Mike

Sure, apology accepted. You get the Not-being-GM award for this week

I have no clue what you mean with GM. Remember, I am from Europe, Belgium.