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Topic: Resistor for LED (Read 359 times) previous topic - next topic

adwsystems

What size resistor to you need for a LED with Vf=3.0 if the power supply is 3.0V? None?

Just checking because I have NEVER run into this before.

DVDdoug

#1
Apr 18, 2019, 08:13 pm Last Edit: Apr 18, 2019, 08:15 pm by DVDdoug
Quote
What size resistor to you need for a LED with Vf=3.0 if the power supply is 3.0V? None?
There's no good solution.  Maybe "assume" 1V or 1/2V "worst case" across the resistor.  i.e. 1V @ 20mA would be 50 Ohms.    That should keep the LED and whatever is driving it safe but you'll get unknown/unpredictable brightness from the LED.

It's a bad idea to run an LED with no current limiting.   

wvmarle

Unless your power supply has current limiting, you need a resistor. Consider getting a higher supply voltage, makes picking the resistor a lot easier.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

DrAzzy

The Vf spec is approximate, generally.

Id use 100 ~ 220 ohm, and adjust to get the desired brightness.

You *always* need a resistor.
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thegoodhen

You don't always need a resistor. If your power supply is below the Vf of the diode, you are technically fine. For instance, you can run a 3.3V led off of 2AA cells and you are safe.

Similarly, if the power supply is a weak battery, such as a coin cell, its internal resistance is enough. This is often exploited in Chinese flashlights.


But if the quoted Vf is exactly the same, some resistor is a better practice than none. 100-220 ohm seems like a lot, I'd go for 10ohms.

MarkT

#5
Apr 21, 2019, 03:41 pm Last Edit: Apr 21, 2019, 03:44 pm by MarkT
What size resistor to you need for a LED with Vf=3.0 if the power supply is 3.0V? None?

Just checking because I have NEVER run into this before.
You can't run a 3V LED from a 3V supply.
LEDs need their current controlled, other by a constant current circuit (which needs some voltage
headroom) or by using a series current limiting resistor (which needs some voltage headroom)

As an LED heats up the forward voltage drops, which can lead to thermal runaway if voltage-driven.


BTW its worth measuring the Vf for your LED, I've seen a lot of LED datasheets that are simply
wrong.  For instance I've some nominally 3.2V blue LEDs that run at under 3.0V in reality,
so can be powered from 3.3V with a small series resistor.

Also if you are prepared to run an LED at lower current than the nominal amount, the forward
voltage is less - often you want to do this because they are so bright these days!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Grumpy_Mike

#6
Apr 21, 2019, 03:55 pm Last Edit: Apr 21, 2019, 03:55 pm by Grumpy_Mike
Quote
You don't always need a resistor.
True but you always need some form of current control. A resistor is the simplest but you can have a boost constant current supply if you want.
What you can not have is nothing.

Just add up all the posts of the people who say you need a resistor and compare it against your post count. Tell you anything?

larryd

You could add a boost cct. to get a supply voltage up to 5v then drive the LEDs with a transistor and a series resistor.

However, use a LED with 2Vf.





No technical PMs.
The last thing you did is where you should start looking.

JohnRob

Quote
What size resistor to you need for a LED with Vf=3.0 if the power supply is 3.0V? None?
All the above recommendations apply, but to answer your question, NO you need a resistor.

The reason; Vf is only an approximate (which was stated before) but the critical requirement is current.


Lets say your LED is actually Vf = 3.0 volts when the current is 10 ma.  So you apply 3V and the LED draws 10 ma.  Now the LED heats up some causing the LED to Vf to decreases to 2.9V now the current is more than 10 ma by what could be a  lot.


Or, lets assume another LED with a stated Vf of 3 volts is really 3.1V, you will get very little current and a dim LED.

Nodes:

1)  The brightness of an LED is proportional to current, not voltage.
2) The ratings provided are "typical" or "nominal".   The actual specification will be somewhat different.
3) I know of no way to operate an LED with a Voltage at or close to its Vf rating.  The other folks proposed suggestions to what you might need.  But to be sure we would have to know more about your 3V source.






Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

thegoodhen

#9
Apr 21, 2019, 08:23 pm Last Edit: Apr 21, 2019, 08:26 pm by thegoodhen
Just add up all the posts of the people who say you need a resistor and compare it against your post count. Tell you anything?
Why so grumpy?

I don't see how my post count is relevant to my experience, if that's what you were intending to suggest.

If you have a bench power supply, set it to 3.3V and connect it to a 3.3V LED, it will be absolutely fine and you won't need any resistor. The supply is still a voltage source, as opposed to a constant current source. The steepness of the (exponential) voltampere characteristic of the LED, together with the manufacturing tolerances of the LEDs, the tolerances of the power supplies, temperature dependencies and more makes it less then advisable to use LEDs without resistors. But in some cases, such as the cases I have listed in the post you were reacting to, you can get away with it. As I have also mentioned, it is not usually viewed as the best practice (and I wouldn't do it in my product).

Grumpy_Mike

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I don't see how my post count is relevant to my experience, if that's what you were intending to suggest.
Well seeing as you are disagreeing with everyone else you must be very naive.

Quote
If you have a bench power supply, set it to 3.3V and connect it to a 3.3V LED, it will be absolutely fine and you won't need any resistor.
Rubbish, please read the replies from the grown ups again and see why this is not OK.

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The supply is still a voltage source, as opposed to a constant current source.
You should seriously learn some electronics.

Quote
But in some cases, such as the cases I have listed in the post you were reacting to, you can get away with it.
That is not transferable knowledge, you have far too many caveats on that. If you know all that information and know it well enough to use then you would not be asking such a question on a beginners forum.

Your answer is of no value at all, and simply reduces the reputation of this forum.

Quote
Why so grumpy?
Suffering fools gladly has never been my strong point.

MorganS

No, it won't be absolutely fine. Not in the sense of "works reliably for years under all environmental conditions."

You can drive your car with your left foot resting on the brake or clutch and it will appear to "work fine" until it doesn't.
"The problem is in the code you didn't post."

thegoodhen

#12
Apr 21, 2019, 08:44 pm Last Edit: Apr 21, 2019, 08:46 pm by thegoodhen
Well seeing as you are disagreeing with everyone else you must be very naive.
Rubbish, please read the replies from the grown ups again and see why this is not OK.
(...)
First off, I wasn't asking the question.
I AM NOT the original poster.
You should seriously learn some electronics.
I do have a bachelors in electronic engineering, getting a masters degree this year and I work as a freelance hardware/software developer. So I am working on "learning some electronics" .

Second off, you can connect and LED to a bench power supply and set the voltage ; then it acts as a VOLTAGE source. It's in a CONSTANT VOLTAGE mode.

The current going through the LED is going to be determined by the voltampere characteristic of the LED. Yes, the characteristic is very steep and if you move the voltage just a bit, the current is going to change dramatically, because it's an exponential. But it can work.

Similarly, if you have a 3V coincell, you can connect it directly to the LED and YOU WILL BE FINE. This is because the internal resistance of the LED works in your favor.

But in MOST cases you need a resistor. But in EDGE cases, you don't.

Never was I suggesting that using an LED without resistor is in general a good idea, but in some cases it can be done UNDER VERY SPECIFIC CIRCUMSTANCES. What don't you understand?

Grumpy_Mike

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I do have a bachelors in electronic engineering, getting a masters degree this year
Wow it shows you how much educational have dropped over the years. I used to lecture at a University for 21 years before I left because of dropping standards.

Quote
First off, I wasn't asking the question.
I AM NOT the original poster.
Yes I know that, your contribution was meant to be an answer which is why it has attracted such criticism. We try and provide reliable information on this forum. This forum is for beginners and your advice is not suitable for beginners.

thegoodhen

This forum is for beginners and your advice is not suitable for beginners.
That is a very fair point that I can accept. You are probably right about that. I just didn't really like you putting words in my mouth and then insulting me for supposedly thinking something that I didn't in the first place.

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