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### Topic: Changing the digital ports voltage? (Read 7898 times)previous topic - next topic

#### retrolefty

#15
##### Dec 04, 2012, 04:00 pmLast Edit: Dec 04, 2012, 04:04 pm by retrolefty Reason: 1

Quote
So. 5/.025 = 250.

(dhenry) Ohm's law applies to just linear devices. The above calculation reflects your complete lack of understanding about Ohm's law.

It still can be used here, just not the way you did - the resulting current would be considerably less what what you your calculation suggested.

In this case...  Even if you take into account forward voltage drop of a typical 5 mm LED, you still end up with 220 ohms (Or 180 ohms if you went the other way in standard  values).  If you do *not* understand how Ohms law applies to control the amount of current in a simple 5V LED circuit, those who take your advise should do so with *much* suspicion now.

Ohms Law calculation is about the resistor... which is linear... and used to control the current in that section of the circuit... which includes and LED, which for all practical purposes is a unidirectional conductor.

It is YOU who do not understand. That is clear to me now.

Guys - please stop the flaming... I thought this forum was about helping people interested in using Arduinos in their different projects, not for flaming those who try to help!

@pwillard - I think You too have a reason to review the link posted by Grumpy_Mike. Btw, Your calculation is wrong unless You have different laws of mathematics than the rest of us... 5/0.025 is not equal to 250 - it is equal to 200... And to be honest, if You use a 220 ohm resistor You will end up with a dimly lit led...

@Code - Do not drive the leds directly from an Arduino pin without any current limitation, You may/will end up letting the magic smoke out. Check the specs for Your leds (voltage drop and current needed) and read the link posted by Grumpy_Mike. It is imho a good guide to the correct way to light a standard low power led. Good luck with Your project!

Basically not true about the 'dimly lit led' with a 220 ohm resistor. We have had reports (and I've recreated it on my board) of being able to see a led 'dimly lit' just having the internal pull-up enabled and the pin still in input mode! And that represents about a 40k ohm series current limiting resistor wired to the grounded LED! Today's high efficiency leds will light at a very wide range of currents, there is no need to run them at 20ma just because that is their recommended maximum continuous current rating. The standard arduino boards use a 1K series resistor for the on-board pin 13 led and no one usually complains about the blink sketch being too dim.

So bottom line simple low power leds do need series current limiting resistors to protect the led and the output pin, but anything from 200 to 1K ohms will do the job safely and effectively.

Lefty

#### Grumpy_Mike

#16
##### Dec 04, 2012, 04:02 pm
Quite right, I have a white LED that is too bright to look at directly and that has a 1K resistor in line with it.

#### Henrik67

#17
##### Dec 04, 2012, 04:22 pm
Ok, I stand corrected...

#### GrooveFlotilla

#18
##### Dec 04, 2012, 04:42 pm
Could you invite dhenry to stand next to you?

Not really good for anything, but they bring a smile to your face when pushed down the stairs.

#### Henrik67

#19
##### Dec 04, 2012, 04:59 pm

Could you invite dhenry to stand next to you?

*ignored*

#### retrolefty

#20
##### Dec 04, 2012, 05:03 pm

Could you invite dhenry to stand next to you?

*ignored*

Wisely

Lefty

#### kd7eir

#21
##### Dec 04, 2012, 07:42 pm
Quote

So bottom line simple low power leds do need series current limiting resistors to protect the led and the output pin, but anything from 200 to 1K ohms will do the job safely and effectively.

Lefty

That sounds like the best "keep it simple" advice that I have ever heard in regards to LED's

#### retrolefty

#22
##### Dec 04, 2012, 07:57 pm

Quote

So bottom line simple low power leds do need series current limiting resistors to protect the led and the output pin, but anything from 200 to 1K ohms will do the job safely and effectively.

Lefty

That sounds like the best "keep it simple" advice that I have ever heard in regards to LED's

Yea, but so many of us 'experts' like to snow the newcomers (and impress our peers) to make any subject or question from a beginner to be as confusing and irrelevant as possible. It's what we do as a hobby and our hobbies make us happier in life.

Lefty

#### kd7eir

#23
##### Dec 05, 2012, 12:24 am
Quote

Yea, but so many of us 'experts' like to snow the newcomers (and impress our peers) to make any subject or question from a beginner to be as confusing and irrelevant as possible. It's what we do as a hobby and our hobbies make us happier in life.

Lefty

Well, I guess you could include average junction temperature, operating environment (To include season, prevailing winds, climatic variances, global warming coefficient, and magnetic correction), as well as the heat generated by the proximity of the end user.  If the project is in dairy country, may need to add a correction factor to the global warming coefficient as well.

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