Why use resistors on LEDs?

alvarez · 14th September 2006, 8:50 PM

Iv been adding Leds to most gadgets i own im now pretty handy with a soldering iron, but my wireing is always messy,

Everywhere Ive read says to use one resistor to each positive terminal on each individual LED would it not be neater to put say 2 LEDs to one resistor in series?

Is this acceptable why do we use resistors for each LED and would I still need a resistor if the power source was rated the same as the LED?

Thanx for clearing this up I think it will work but Ive only done basic electrcity in Year 10 so Id feel better if you guys cleared me on this,

Stygian Shane · 14th September 2006, 8:52 PM

A led is a diode. A diode is essentially a one-way wire, but in a LED's case; it emits light at current passes through. Don't use a resistor, and it's effectively a short circuit, which is quite obviously NOT good for whatever's running the led.
(Someone please correct me if I'm wrong!)

dazzawul · 14th September 2006, 9:08 PM

An LED has a certain amount of resistance, when you apply a voltage across it, according to how much resistance it has, it will draw that much current, (you could run a 12 volt device off of a car batt. if you wanted, as long as the voltage is right)

LEDS are meant to run off of 3.4 volts or something like that, when they are fed more than that, they draw more current than they should, so they heat up (which affects their resistance, lowering it, making it pull more current, which makes it heat up more, thermal runaway, and why LEDs burn out :P) thats why you wire a resistor in, to lower the voltage its drawing and keep it from dying..

Buuuuut, I got another question, how the hell do 12v LEDs work? :P
(like in cars, is it a few diodes in series packaged together, or just a normal LED with a resistor built in )

Cheers lads.

dakiller · 14th September 2006, 9:15 PM

Wikipedia sum it up pretty well

Quote:

Because the voltage versus current characteristics of an LED are much like any diode (that is approximately exponential), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.

Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LEDs operating range. Thus the power can be considered to be almost proportional to the current. To try and keep power close to constant across variations in supply and LED characteristics the power supply should be a current source. If high efficiency is not required (e.g. in most indicator applications), an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source is generally used...

...Provided there is sufficient voltage available, multiple LEDs can be connected in series with a single current limiting resistor. Parallel operation is generally problematic. The LEDs have to be of the same type in order to have a similar forward voltage. Even then, variations in the manufacturing process can make the odds of satisfactory operation low. For more information see Nichia Application Note.

To sum up, Variations in the forward voltage bias of LED's will mean that you wont get an ideal current sharing between them

dakiller · 14th September 2006, 9:19 PM

Quote:

Originally Posted by dazzawul

An LED has a certain amount of resistance, when you apply a voltage across it, according to how much resistance it has, it will draw that much current, (you could run a 12 volt device off of a car batt. if you wanted, as long as the voltage is right)

LEDS are meant to run off of 3.4 volts or something like that, when they are fed more than that, they draw more current than they should, so they heat up (which affects their resistance, lowering it, making it pull more current, which makes it heat up more, thermal runaway, and why LEDs burn out :P) thats why you wire a resistor in, to lower the voltage its drawing and keep it from dying..

Just about all of what you wrote is wrong, LED's don't have resistance, they have diode like non-linear I/V characteristics

Quote:

Originally Posted by dazzawul

Buuuuut, I got another question, how the hell do 12v LEDs work? :P
(like in cars, is it a few diodes in series packaged together, or just a normal LED with a resistor built in )

They have a built in resistor

ener · 14th September 2006, 9:40 PM

i though it was to protect the leds from any flucuations in the elctricity passing thru them
(if thats what the wiki said then im sorry but i hate reading quotes i read half of it)

GooSE · 14th September 2006, 9:48 PM

Read what dakiller has posted. Basically it's just to stop over-current. If you leave the resistor out, the LED will draw enough current to destroy itself every time.

To answer the question in the original post regarding using multiple LEDs with a single resistor... It will work as long as the resistor is able to dissipate enough power. To calculate this:

P = V * I, where P is power (Watts), V is voltage drop across the resistor, I is current in Amps.

Common resistors are either 1/4W or 1/2W.

dephilile · 14th September 2006, 9:57 PM

Be very careful using one resistor on multiple LED’s. since LED’s are CURRENT devices, NOT voltage devices slight variations in how they were manufactured gives them slightly different characteristics. What can potentially happen is all the current will flow through one LED. It will burn out. Then the current path will move onto the next LED and burn that out until there are none left. Also note that characteristics of LED's change with temperature.

If you wish to power multiple LED’s with one resistor it’s better to put the LED’s in series and have one resistor at the end of the LED string.

EDIT - if you say for example have a microcontroller and want to attach a number of LED's to different pins you have to have multiple resistors. one for each LED. NEVER RELY ON THE CURRENT LIMITING OF THE MICROCONTROLLER.

Elder · 14th September 2006, 10:29 PM

http://www.overclockers.com.au/wiki/LED

This was started by Goth, unfinished section started by me.

My section shows the configurations you can use. The blank sections were going to be filled with a basic description of practical limitations (supply voltage limiting number of leds in series, supply current limiting number of leds in parallel, failure cases, etc/whatever)

Someone else can jump in and write in my section if you want

I have no motivation to complete it because I'm setting up & managing my works wiki now.

Butcher9_9 · 14th September 2006, 11:01 PM

i got some leds from pc case gear
there was 3 leds in series and no resistor (they where prewired)
will they die ?

dazzawul · 14th September 2006, 11:10 PM

Crap I should pay more attention in physics

(though I blame my metalwork\electronics teacher for not teaching us the right thing in the first place

)

But if theyve made it prewired it should be fine..

Cheers lads.

dephilile · 15th September 2006, 12:04 AM

Quote:

Originally Posted by Butcher9_9

i got some leds from pc case gear
there was 3 leds in series and no resistor (they where prewired)
will they die ?

i'm guessing they are running off 5 volts? hard to say without having access to a datasheet for them. if you run more power than they are designed for through them it will shorten their life.

Butcher9_9 · 15th September 2006, 1:38 PM

i would have said 4 volts but i know shit about electric stuff

3 x 4 = 12

azzachaz · 15th September 2006, 1:55 PM

http://www.dansdata.com/caselight.htm

Dan is the man.

TRG.dOinK · 15th September 2006, 7:36 PM

http://forums.overclockers.com.au/sh...7&page=3&pp=15

I've explained on page 3....

You need a resistor to...
A. Limit current so the led does not blow up/destroy itself.
B. Have a voltage drop across the resistor.

Leds do not run off 12 volts. Each colour is different. Red is around 1.8V to 2V. Blue was around 3.5V from memory, etc etc etc.

Say you have a 12 volt car battery. You connect the red led straight across it. The red led does not like 12 volts across it and will draw too much current. It only wants 2 volts. Connecting a resistor in series with the led, allows the led to put a voltage drop across the resistor. Therefore you want the resistor to have 10v across it, and 2v across the led. Now you need to find out the maximum current the led can draw, usually 20mA. Using V=IR.. R = V/I.. R = 10v/.020A

14th September 2006, 8:50 PM	#1
alvarez Member Join Date: Jun 2006 Location: Geelong 3218 Posts: 2,570	Why use resistors on LEDs? Iv been adding Leds to most gadgets i own im now pretty handy with a soldering iron, but my wireing is always messy, Everywhere Ive read says to use one resistor to each positive terminal on each individual LED would it not be neater to put say 2 LEDs to one resistor in series? Is this acceptable why do we use resistors for each LED and would I still need a resistor if the power source was rated the same as the LED? Thanx for clearing this up I think it will work but Ive only done basic electrcity in Year 10 so Id feel better if you guys cleared me on this,

14th September 2006, 9:08 PM	#3
dazzawul Member Join Date: Apr 2005 Location: the sutho shire (w00t) Posts: 1,449	An LED has a certain amount of resistance, when you apply a voltage across it, according to how much resistance it has, it will draw that much current, (you could run a 12 volt device off of a car batt. if you wanted, as long as the voltage is right) LEDS are meant to run off of 3.4 volts or something like that, when they are fed more than that, they draw more current than they should, so they heat up (which affects their resistance, lowering it, making it pull more current, which makes it heat up more, thermal runaway, and why LEDs burn out :P) thats why you wire a resistor in, to lower the voltage its drawing and keep it from dying.. Buuuuut, I got another question, how the hell do 12v LEDs work? :P (like in cars, is it a few diodes in series packaged together, or just a normal LED with a resistor built in ) Cheers lads. __________________ i7 980x @ 4.0 ghz \| EK Supreme \| Gigabyte GA-X58-UD7 \| 12G Gskill Ripjaws @ 1600 \| WC GTX 480 800/1600/1860 \| 8 TB raid 0 \| Audigy eX platinum \| Corsair AX850

14th September 2006, 9:57 PM	#8
dephilile Member Join Date: Jun 2006 Location: Daejeon - South Korea Posts: 966	Be very careful using one resistor on multiple LED’s. since LED’s are CURRENT devices, NOT voltage devices slight variations in how they were manufactured gives them slightly different characteristics. What can potentially happen is all the current will flow through one LED. It will burn out. Then the current path will move onto the next LED and burn that out until there are none left. Also note that characteristics of LED's change with temperature. If you wish to power multiple LED’s with one resistor it’s better to put the LED’s in series and have one resistor at the end of the LED string. EDIT - if you say for example have a microcontroller and want to attach a number of LED's to different pins you have to have multiple resistors. one for each LED. NEVER RELY ON THE CURRENT LIMITING OF THE MICROCONTROLLER. Last edited by dephilile; 14th September 2006 at 10:06 PM.

14th September 2006, 10:29 PM	#9
Elder Member Join Date: May 2004 Location: Brisbane Posts: 612	http://www.overclockers.com.au/wiki/LED This was started by Goth, unfinished section started by me. My section shows the configurations you can use. The blank sections were going to be filled with a basic description of practical limitations (supply voltage limiting number of leds in series, supply current limiting number of leds in parallel, failure cases, etc/whatever) Someone else can jump in and write in my section if you want I have no motivation to complete it because I'm setting up & managing my works wiki now. Last edited by Elder; 14th September 2006 at 10:35 PM.

14th September 2006, 11:01 PM	#10
Butcher9_9 Member Join Date: Aug 2006 Location: Perth , East Vic Park Posts: 1,676	i got some leds from pc case gear there was 3 leds in series and no resistor (they where prewired) will they die ? __________________ I7 3820 ,X79 UD3, 4X8GB@1600mhz, GTX680, 3X Dell 3008wfp 30" LCD/ Spyder3Express , Adaptec 51645, 256gb Vertex 4 , 61.56TB Internal, 750GB External & 40TB Backup Sever, Z2300, G19 and M950 Acer 1430 11" notebook with 4GB DDR3 and 500GB HDD Sales $3200+, Purchases $500+

14th September 2006, 9:48 PM	#7
GooSE Member Join Date: Jun 2001 Location: Sydney Posts: 6,490	Read what dakiller has posted. Basically it's just to stop over-current. If you leave the resistor out, the LED will draw enough current to destroy itself every time. To answer the question in the original post regarding using multiple LEDs with a single resistor... It will work as long as the resistor is able to dissipate enough power. To calculate this: P = V * I, where P is power (Watts), V is voltage drop across the resistor, I is current in Amps. Common resistors are either 1/4W or 1/2W.

14th September 2006, 11:10 PM	#11
dazzawul Member Join Date: Apr 2005 Location: the sutho shire (w00t) Posts: 1,449	Crap I should pay more attention in physics (though I blame my metalwork\electronics teacher for not teaching us the right thing in the first place ) But if theyve made it prewired it should be fine.. Cheers lads. __________________ i7 980x @ 4.0 ghz \| EK Supreme \| Gigabyte GA-X58-UD7 \| 12G Gskill Ripjaws @ 1600 \| WC GTX 480 800/1600/1860 \| 8 TB raid 0 \| Audigy eX platinum \| Corsair AX850

15th September 2006, 1:38 PM	#13
Butcher9_9 Member Join Date: Aug 2006 Location: Perth , East Vic Park Posts: 1,676	i would have said 4 volts but i know shit about electric stuff 3 x 4 = 12 __________________ I7 3820 ,X79 UD3, 4X8GB@1600mhz, GTX680, 3X Dell 3008wfp 30" LCD/ Spyder3Express , Adaptec 51645, 256gb Vertex 4 , 61.56TB Internal, 750GB External & 40TB Backup Sever, Z2300, G19 and M950 Acer 1430 11" notebook with 4GB DDR3 and 500GB HDD Sales $3200+, Purchases $500+

15th September 2006, 7:36 PM	#15
TRG.dOinK Member Join Date: Jun 2002 Location: Melb East Posts: 2,886	http://forums.overclockers.com.au/sh...7&page=3&pp=15 I've explained on page 3.... You need a resistor to... A. Limit current so the led does not blow up/destroy itself. B. Have a voltage drop across the resistor. Leds do not run off 12 volts. Each colour is different. Red is around 1.8V to 2V. Blue was around 3.5V from memory, etc etc etc. Say you have a 12 volt car battery. You connect the red led straight across it. The red led does not like 12 volts across it and will draw too much current. It only wants 2 volts. Connecting a resistor in series with the led, allows the led to put a voltage drop across the resistor. Therefore you want the resistor to have 10v across it, and 2v across the led. Now you need to find out the maximum current the led can draw, usually 20mA. Using V=IR.. R = V/I.. R = 10v/.020A __________________ ... /\ . _\ /_ Mitsubishi \| Successful Trades: Suicidal, Nerotic, Chiefgc, Mr Chilled, Kooliez, Vortexkrow, Grandarcmaster, AT-AT ./_/ \_\ Motors... \| E8400, NZXT Hush, Corsair TX-750, Gigabyte GA-EP35C-DS3R, 4GB DDR2 Corsair DHX, Asus 8800GTS 512, Auzentech Prelude Xfi, Seagate 7200.11 500GB 32mg cache Last edited by TRG.dOinK; 18th September 2006 at 2:52 PM.