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Topic: Changing the digital ports voltage? (Read 2 times) previous topic - next topic

Grumpy_Mike

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It still can be used here, just not the way you did

OK so how would use it in this case and how would the result be different?

pwillard

#11
Dec 04, 2012, 01:39 am Last Edit: Dec 04, 2012, 01:57 am by pwillard Reason: 1
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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.

Henrik67


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!


Groove

IMO flaming a user who gives poor, misleading or plain simple willy-waving advice is entirely justified.
I wish dhenry would shut up and go elsewhere.
Per Arduino ad Astra

Grumpy_Mike

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

It is the problem is that dhenry has some strange ideas about lighting up LEDs and throws in all sorts of irrelevant curve balls that the net result is that a beginner is confused. He says things and will not back them up, or talks about fourth and fifth order effects of something as if it was a first order effect. For example he is always banging on about the 40R output impedance of an arduino pin and ignoring the fact that this is only for small currents and I have measured pulses up to 250mA from a pin which puts the impedance down to at the most 20R. Anyway I think the result is that he has scared away another newcomer in a effort to make himself look smarter than he is. It is a shame.

It is always been my belief that electronics can be simple. The trick is to simply things as much as possible but not to such an extent as to be wrong. That is what I try to do, I don't always succeed.

On this topic I normally say:-
With an LED you always need something to limit the current, the simplest of which is a resistor.
Now that is not like saying:-
You always need a resistor.
But it avoids all the complication of mentioning constant current supplies and the rest when all the beginner needs to be told is that you can't just stick an LED across a digital output.

Thanks for the comments on my links.  :)

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