RCCrawler Forums

BlueMonster · 01-09-2007, 09:54 PM

I bought a pile of LED's with a 3.5 forward voltage rating. I figured, I would run them off a simple 9V battery, so if I wired them in several seperate series of three LED's, I won't need any resistors, correct?

Also, there is a POS and NEG side, right? Which is which, and does it even matter?

TwistedCreations · 01-09-2007, 10:04 PM

umm, couldn't tell ya about the resistors.. I have always ran them and never had a problem.....
http://led.linear1.org/1led.wiz

long side is the postive

JasonInAugusta · 01-09-2007, 10:26 PM

Each LED needs a current limiting resisitor.

Use the calc that TC posted and you'll be fine.

BlueMonster · 01-09-2007, 10:43 PM

Lol, what if I am extremely impatient? Seriously though, I'll see about finding the correct parts. Thanks for the link.

JasonInAugusta · 01-09-2007, 10:50 PM

Quote:

Originally Posted by BlueMonster

Lol, what if I am extremely impatient?

Then you'd better have spare LEDs.

BlueMonster · 01-09-2007, 10:54 PM

Am I missing something though? If I run three 3.7 (avg) volt LED's off a 9 volt battery, I should be perfectly fine. I should be able to run multiple series of three LED's off the same battery and never hurt any of them, at least that makes sense to me.

Is it not that simple?

wyldblu · 01-09-2007, 11:16 PM

no, you cant run them in series, the voltage of the leds is 3.5. 3 3.5 volt lights dosent equal 10.5 volts. you are confusing voltage ratings with resistance. the resistance of an led is very low. you would have to find the resistance of the leds to determine how many to run in series.
it is called ohms law. voltage=amps x ohms (resistance)
did i just make sense?
oh and yes it does matter which way you hook them up. they are diodes they only let current flow 1 way. if you hook them up backwards they will simply not work.
if you supply too high of a voltage backwards it will blow them out though

P47TBolt · 01-10-2007, 09:03 AM

The longer of the 2 connections on the LED should be the positive(+) side.

TwistedCreations · 01-10-2007, 10:04 AM

Quote:

Originally Posted by P47TBolt

The longer of the 2 connections on the LED should be the positive(+) side.

thanks, that was already established in the 2nd post

BlueMonster · 01-10-2007, 07:56 PM

I am an impatient dumbass.

I wired up three LED's, shrink wrap and all, installed them in the truck, plugged in the battery and WOW! They bright, exactly what I wanted. I knew better than all of you!

For like 2 seconds.

Then there was a sizzle and a crackle and they blew out. Crap. I guess this little project needs to wait until I can pick up some resistors...

JasonInAugusta · 01-10-2007, 08:02 PM

P47TBolt · 01-11-2007, 11:18 AM

Next time listen.

one for each led you fubarred.

BlueMonster · 01-11-2007, 05:41 PM

Okay, I picked up some 180 Ohm resistors. Are they directional, and I assume they go on the possitive lead off the LED?

BlueMonster · 01-11-2007, 06:26 PM

Ughhh

I only have ONE spare LED after several attempts to not blow them up!

I must have entered the wrong info into the calc...

BlueMonster · 01-11-2007, 07:10 PM

It says I need a 68 ohm resistor in line with two LED's.

I have a selection of 10,12, 15 and 20 ohm resistors. Can I mix and match them in a series to equal the needed 68 ohms? If I end up with 70 or 75 ohms, it'll just make the light slightly less, right?

I can't find anywhere that says if the resistors need to go in a specific direction or not.

ILoveRCRockCrawlin · 01-11-2007, 07:26 PM

Quote:

Originally Posted by BlueMonster

It says I need a 68 ohm resistor in line with two LED's.

I have a selection of 10,12, 15 and 20 ohm resistors. Can I mix and match them in a series to equal the needed 68 ohms? If I end up with 70 or 75 ohms, it'll just make the light slightly less, right?

I can't find anywhere that says if the resistors need to go in a specific direction or not.

Yes you can mix them up, ex 50 + 20 would be equivelent to a 70ohm. And no, the direction of the resistor does NOT matter.

I love having a mom that works with that stuff everyday.

BlueMonster · 01-11-2007, 07:27 PM

Cool, thank your mom for me!

ILoveRCRockCrawlin · 01-11-2007, 07:29 PM

Quote:

Originally Posted by BlueMonster

Cool, thank your mom for me!

I will.

Rob92k1500 · 01-17-2007, 11:22 AM

Jeremy, let me know if you need a hand with this, I have a mountain of resistors at work and a ton of High Intensity LEDs,

The reason you need the resistors is that LED's are not perfectly matched and they will keep sucking up current as long as it's available, so what you see a lot is 3 resistors (or more) in line (Series) and some are brighter than others, and if they are far enough out of spec from each other, the brighter ones pull more current, and they heat up as they heat up, they pull even more current, finally burning out. (Simplified explanation, but accurate enough).

So each LED needs it's own resistor.

650Ohms should be a good estimate per LED at 9v. I usually run 800-1000 Ohm resistors on LEDs at 12v.

And trust me, I use a lot of LEDs at work.

01-09-2007, 09:54 PM	#1
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Quick LED question I bought a pile of LED's with a 3.5 forward voltage rating. I figured, I would run them off a simple 9V battery, so if I wired them in several seperate series of three LED's, I won't need any resistors, correct? Also, there is a POS and NEG side, right? Which is which, and does it even matter?

01-09-2007, 11:16 PM	#7
wyldblu Newbie Join Date: Nov 2006 Location: Lincoln Posts: 2	no, you cant run them in series, the voltage of the leds is 3.5. 3 3.5 volt lights dosent equal 10.5 volts. you are confusing voltage ratings with resistance. the resistance of an led is very low. you would have to find the resistance of the leds to determine how many to run in series. it is called ohms law. voltage=amps x ohms (resistance) did i just make sense? oh and yes it does matter which way you hook them up. they are diodes they only let current flow 1 way. if you hook them up backwards they will simply not work. if you supply too high of a voltage backwards it will blow them out though Last edited by wyldblu; 01-09-2007 at 11:19 PM.

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01-09-2007, 10:04 PM	#2
TwistedCreations Colt Python/SR9c Join Date: Feb 2004 Location: out in the shop, reloading ammo! Posts: 8,626	umm, couldn't tell ya about the resistors.. I have always ran them and never had a problem..... http://led.linear1.org/1led.wiz long side is the postive

01-09-2007, 10:26 PM	#3
JasonInAugusta 06 Super National Champ Join Date: Jun 2004 Location: Stark Industries Bar and Grill Posts: 11,361	Each LED needs a current limiting resisitor. Use the calc that TC posted and you'll be fine.

01-09-2007, 10:43 PM	#4
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Lol, what if I am extremely impatient? Seriously though, I'll see about finding the correct parts. Thanks for the link.

01-09-2007, 10:54 PM	#6
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Am I missing something though? If I run three 3.7 (avg) volt LED's off a 9 volt battery, I should be perfectly fine. I should be able to run multiple series of three LED's off the same battery and never hurt any of them, at least that makes sense to me. Is it not that simple?

01-10-2007, 09:03 AM	#8
P47TBolt Rock Stacker Join Date: Jun 2006 Location: Harlem Posts: 83	The longer of the 2 connections on the LED should be the positive(+) side.

01-10-2007, 07:56 PM	#10
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	I am an impatient dumbass. I wired up three LED's, shrink wrap and all, installed them in the truck, plugged in the battery and WOW! They bright, exactly what I wanted. I knew better than all of you! For like 2 seconds. Then there was a sizzle and a crackle and they blew out. Crap. I guess this little project needs to wait until I can pick up some resistors...

01-10-2007, 08:02 PM	#11
JasonInAugusta 06 Super National Champ Join Date: Jun 2004 Location: Stark Industries Bar and Grill Posts: 11,361

01-11-2007, 11:18 AM	#12
P47TBolt Rock Stacker Join Date: Jun 2006 Location: Harlem Posts: 83	Next time listen. one for each led you fubarred.

01-11-2007, 05:41 PM	#13
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Okay, I picked up some 180 Ohm resistors. Are they directional, and I assume they go on the possitive lead off the LED?

01-11-2007, 06:26 PM	#14
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Ughhh I only have ONE spare LED after several attempts to not blow them up! I must have entered the wrong info into the calc...

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01-11-2007, 07:10 PM	#15
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	It says I need a 68 ohm resistor in line with two LED's. I have a selection of 10,12, 15 and 20 ohm resistors. Can I mix and match them in a series to equal the needed 68 ohms? If I end up with 70 or 75 ohms, it'll just make the light slightly less, right? I can't find anywhere that says if the resistors need to go in a specific direction or not.

01-11-2007, 07:27 PM	#17
BlueMonster RCC Addict Join Date: Apr 2005 Location: Seattle Posts: 1,437	Cool, thank your mom for me!

01-17-2007, 11:22 AM	#19
Rob92k1500 Quarry Creeper Join Date: Jan 2006 Location: Redmond Posts: 275	Jeremy, let me know if you need a hand with this, I have a mountain of resistors at work and a ton of High Intensity LEDs, The reason you need the resistors is that LED's are not perfectly matched and they will keep sucking up current as long as it's available, so what you see a lot is 3 resistors (or more) in line (Series) and some are brighter than others, and if they are far enough out of spec from each other, the brighter ones pull more current, and they heat up as they heat up, they pull even more current, finally burning out. (Simplified explanation, but accurate enough). So each LED needs it's own resistor. 650Ohms should be a good estimate per LED at 9v. I usually run 800-1000 Ohm resistors on LEDs at 12v. And trust me, I use a lot of LEDs at work.

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