uthenob
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Do I *need* a resistor for an LED?

Wed Aug 21, 2013 1:34 am

Do I *need* to use a resistor to power an led (as shown https://projects.drogon.net/wp-content/ ... ample1.jpg)?
Thanks.

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solar3000
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 1:43 am

Yes you do.
It may not burn the red LED right away; it may not burn the raspberry pi right away.
But slowly it will damage the pi.
Antikythera

uthenob
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 1:54 am

solar3000 wrote:Yes you do.
It may not burn the red LED right away; it may not burn the raspberry pi right away.
But slowly it will damage the pi.
Okay, what ohm resistor do I need (or does it matter)?

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ddxfish
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 2:09 am

It varies per LED, but yes u need one.

a 330 ohm 1/4 watt would work on most if you are looking for an "indicator light"

http://led.linear1.org/led.wiz
Will help you determine which resistor is used to drive your LED at FULL POWER, so use one a little larger in value in my opinion.
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klricks
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 3:11 am

You need to obtain a manufactures data sheet to get the info you need to calculate the resistor value. You also need to know what voltage you will be using. The RPi GPIO pins are 3.3V.
When you have the data, current and forward voltage, and supply voltage, then you can plug the values into one of the numerous web site calculators which will tell your what resistor you need. This one will show you the nearest standard resistor size:
http://www.superbrightleds.com/led_info.php

Also some LED's have resistors built in. If so they will be rated for a specific voltage and must be used at that voltage. Usually 3.3V, 5V or 12V.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

klricks
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 3:20 am

ddxfish wrote:It varies per LED, but yes u need one.

a 330 ohm 1/4 watt would work on most if you are looking for an "indicator light"

http://led.linear1.org/led.wiz
Will help you determine which resistor is used to drive your LED at FULL POWER, so use one a little larger in value in my opinion.
330 Ohm would probably be used with an LED on a 5V circuit.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

W. H. Heydt
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 4:33 am

Just to add a little background on *why* you need a resistor...

A LED is a forward biased diode. That means that--electrically--it looks like a short circuit from a fixed voltage drop across the PN junction (around 0.7V, depending on the color of the LED). So...no resistor, lots of current. Blown LED and/or blown power source.

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sav25
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 12:51 pm

Option 2 would be to buy a 5V rated LED. Maplins sell them. I have one connected directly to the 5V and GND GPIO pins on my RaspBMC case, so that I can see if I have left it on (as my case hides the on-board LEDs).

No resistors. Works fine.
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joan
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 12:57 pm

sav25 wrote:Option 2 would be to buy a 5V rated LED. Maplins sell them. I have one connected directly to the 5V and GND GPIO pins on my RaspBMC case, so that I can see if I have left it on (as my case hides the on-board LEDs).

No resistors. Works fine.
They do have a resistor. You pay extra for having the resistor built-in.

Maplin http://www.maplin.co.uk/5v-3mm-leds-35737
Two 3mm diameter LEDs from the 5V series in a T-1 package. These LEDs have a 5V integral current limiting resistor, which means no external current limiter is required with a 5V supply. Both types have a diffused lens and a wide viewing angle of 60°.

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pluggy
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 1:30 pm

If you're not too woried by having the LED as bright as absolutely possible, a resistor an order of magnitude greater will work. I often use 1k resistors for red/green/orange indicator LEDS on 3,3v and 5v. Way too high in theory, but putting less current through them isn't going to hurt and if it lights up, what the hell....

Putting more than their maximum current isn;t recommended which is what happens if you don't use one at all.
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

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jbeale
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 2:06 pm

0.7 V is typical for a non-light-emitting silicon diode. The lowest Vf I've seen for a LED is about 1.2 V (infrared wavelength). Visible red LEDs are around 1.8 V, and blue or white can go above 3 V. Table showing forward voltage drop ranges for different colored LEDs:
http://en.wikipedia.org/wiki/Light-emit ... _materials

W. H. Heydt
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 4:25 pm

jbeale wrote:0.7 V is typical for a non-light-emitting silicon diode. The lowest Vf I've seen for a LED is about 1.2 V (infrared wavelength). Visible red LEDs are around 1.8 V, and blue or white can go above 3 V. Table showing forward voltage drop ranges for different colored LEDs:
http://en.wikipedia.org/wiki/Light-emit ... _materials
Yup. I was just giving an example and the 0.7V is the normal drop across a Si PN junction. The principle is the point though...no resistor and you've created a kind of short circuit.

boyoh
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Re: Do I *need* a resistor for an LED?

Wed Aug 21, 2013 10:22 pm

This will help you to use LED's And Ohms Law
Attachments
led Circuit.jpg 1.jpg
led Circuit.jpg 1.jpg (61.34 KiB) Viewed 11945 times
BoyOh ( Selby, North Yorkshire.UK)
Some Times Right Some Times Wrong

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