# Irrelevant Resistor Value for LED?

Hi, I'm new to electronics and am trying to learn. I have an Arduino Uno R3 and the basic starter kit from Arduino. I am running the Blink sketch, but replacing the 220ohm resistor with higher values such as 1k, 1M, and 10M. I expected that the Red LED would dim as I used higher resistors, but I can not see any noticeable difference. Is this normal? Does the resistor value actually matter? I looked at the datasheet for the RED LED and seeing that it has an internal 12v resistor. Is this the reason why I'm not seeing a difference?

Question #2) I thought that running the program without a resistor would burn out the LED, but it doesn't. Is the internal 12v resistor the cause of this?

Question #3) If the internal resistor resists 12v, and the output of digital pin 8 is only 5v, why is the LED lighting up at all?

Question #4) From my research, I've read that the digital pin could become damaged without a resistor. So, if I use a green LED, which doesn't list an internal resistor, I would probably damage the LED and the digital pin, correct?

Cheers!

I expected that the Red LED would dim as I used higher resistors, but I can not see any noticeable difference. Is this normal?

No. I would not expect a resistor of 1M to make no difference, are you sure it is wired correctly.

Does the resistor value actually matter?

Yes

seeing that it has an internal 12v resistor.

Sorry there is no such thing. Resistors are measured in ohms and not volts. However that is a very poorly worded data sheet. It does indeed say 12V resistor. What it means is a resistor to limit the current to the maximum permitted if you feed it with 12V. While not unheard of such LEDs are not very common.

Question #2) I thought that running the program without a resistor would burn out the LED, but it doesn't. Is the internal 12v resistor the cause of this?

Yes.

Question #3) If the internal resistor resists 12v, and the output of digital pin 8 is only 5v, why is the LED lighting up at all?

Because you only have to have a voltage in excess of the forward voltage drop to turn on an LED, about 3V.

Question #4) From my research, I've read that the digital pin could become damaged without a resistor. So, if I use a green LED, which doesn't list an internal resistor, I would probably damage the LED and the digital pin, correct?

Yes correct.

sinjinmanta:
Hi, I'm new to electronics and am trying to learn. I have an Arduino Uno R3 and the basic starter kit from Arduino. I am running the Blink sketch, but replacing the 220ohm resistor with higher values such as 1k, 1M, and 10M. I expected that the Red LED would dim as I used higher resistors, but I can not see any noticeable difference. Is this normal?

No.

sinjinmanta:
Does the resistor value actually matter?

Yes.

sinjinmanta:
I looked at the datasheet for the RED LED and seeing that it has an internal 12v resistor. Is this the reason why I'm not seeing a difference?

No, you should still see a difference with a 1M resistor.

sinjinmanta:
Question #2) I thought that running the program without a resistor would burn out the LED, but it doesn't. Is the internal 12v resistor the cause of this?

Yes. You can't 'remove' a resistor if it's inside the LED.

sinjinmanta:
Question #3) If the internal resistor resists 12v, and the output of digital pin 8 is only 5v, why is the LED lighting up at all?

LEDs aren't binary on/off. Some current will still go through (although it will be less than if you apply 12V.

sinjinmanta:
Question #4) From my research, I've read that the digital pin could become damaged without a resistor. So, if I use a green LED, which doesn't list an internal resistor, I would probably damage the LED and the digital pin, correct?

Yes.

It probably won't be instant death, but it's doing damage to the chip/LED every second it's connected without a resistor.

How are you connecting the components?

Maybe breadboards don't work the way you think they do...

If you have a multimeter, measure the voltage across the LED and the resistor you added.

Thank you both for your timely help!

Now that I know the red LED that Arduino ships is an outlier, I have switched to using a green LED and I can clearly see the difference between 220ohm, 560ohm, and 1kohm resistors.

Follow up question: I have the resistor placed in line between the digital pin and the anode (+, positive, long pin) side of the LED. I've done some reading on this and, aside from the risk of a short circuit, it seems as though it doesn't matter wether the resistor is between the digital pin and the LED (OR) between the LED and the ground. The only logic that I can surmise from the resistor working on both sides of the LED is that the "flow" of electricity somehow goes both ways simultaneously from - to + and from + to - ... So, my question is: Does electricity "flow" both directions? (OR) Why does it not matter where the resistor is placed?

My current setup: digital pin 8 > wire > resistor > anode LED , cathode LED > wire > Ground Pin

Thanks again!

Does electricity "flow" both directions?

Well yes and no but mainly no.
Electricity is negative charge carriers flowing in one direction and positive charge carriers flowing in the other but that is the wrong way to think about it when you are doing electronic engineering. Stick to the notion of current flowing from + to -.

Why does it not matter where the resistor is placed?

Because in a series circuit the same current flows through each component. It is a circuit, the route that the current takes, that determines the overall resistance and hence the current.
Many beginners think the current flows into the resistor and gets restricted some how. This is not the case. If you have a resistor with one end connected to a voltage the other end of the resistor has the same voltage on it if it is not connected to anything else. This is true no matter what the size of the resistor or what size is the voltage. It is only when you have a path for the current to flow ROUND do you see things like voltage drops across components, but the current is the same for each component.

Think of electricity as little balls.

When two components are in a line, the number of balls going through one component has to be the same as the other component. It can't possibly be different (where would the extra balls go to/come from?).

So it doesn't matter which way you connect them.

This is getting deep pretty quickly. Thanks for educating me. Grumpy_Mike, you nailed it with my beginner level interpretation of what a resistor does. I'm glad to know now that I was wrong. Unfortunately, I'm still struggling with understanding why the resistor is able to reduce the overall current, but not from one side of itself to the other.

fungus, in your analogy, there are still the same number of balls, with or without the resistors. Since an LED has a direction and a limit, how does a resistor after the LED restrict the flow of balls to the LED in the correct direction for it to light up?

Are there any good authoritative resources available on this topic that you could recommend? I've read a few things and watched a few others and they all seem to either contradict or provide unclear explanations. I'm not sure how to know what to believe.

sinjinmanta:
fungus, in your analogy, there are still the same number of balls, how does a resistor after the LED restrict the flow of balls to the LED

Balls can't be created or destroyed.

You can't have more balls coming out of the LED than went in.

If you restrict the number of balls coming out, the number of balls going in is also restricted.

Find an elementary book which deals with electric circuit theory.

Your red led requires about 2 volts and 10 mA to operate.

If you put 10 mA though a 300 ohm resistor, the voltage between the ends of the resistor will be 3 V.

If you connect the digital output pin to the led and then to the resistor, and then to ground, 10 mA will flow in the circuit. The voltage drop across the led will be 2V, from 5V down to 3V, and the voltage drop across the resistor will be 3V, from 3V down to 0 volts.

If you connect them in the other order, the outcome will be the same. The only difference is, if you measure the voltage potential between the wire connecting the resistor to the led, in the first case that voltage will be 3V, and in the second case, it will be 2V.