# LED's and resistors.



## Jerry Barnes (Jan 2, 2008)

I know I need to use a resistor with a LED, if I am using several in a building, do I need a resistor on each led, or will one take care of several led's?


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## Stan Cedarleaf (Jan 2, 2008)

Jerry........ I find it just as easy to put a resistor on each LED. That way I don't worry about voltage or current draw. 

I'm sure there will be other thoughts but I find that to be a very easy way to do it. Up to 24 volts, generally a 1K resisitor on most white or warm white LED's works just fine.

I've lighted all my USA Overton passesnger coaches that way. 2 LED's in each of the 4 coaches, all daisychained together using 1 14.8 volt battery pack. Turn one switch on and all the coaches light up. 

That's just the way I did it and it works...


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## Greg Elmassian (Jan 3, 2008)

You need to be concerned about the wattage of the resistor when you start "sharing" it with other LEDs. 

It's an extra calculation, if you don't want to get involved at that level take Stan's suggestion. 

Wattage is Volts times amps, or current squared times resistance. W = V*A = I**2 R 

Remember to only use about half of the rating of the resistor... 

For example, you have one led on a 1,000 ohm resistor and it draws 20 milliamps (0.02 amp) 

so 1,000 * .02 * .02 = 0.4 watts, so a half watt resistor will get hot! (might need a 1 watt resistor) (which can be found at Radio Shack) 

if you now wanted to use 2 LEDs from the same resistor, you would be using 40 milliamps (0.04 amp) 

so 1,000 * .04 * .04 = 1.6 watts... You need about a 3 watt resistor !!! (which you will NOT find at Radio shack) 

Bottom line is you need to work it out to check the wattage, or find an online calculator. 

Here's a simple calculator: *[url]http://led.linear1.org/1led.wiz*[/url] 

Regards, Greg


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## Nicholas Savatgy (Dec 17, 2008)

Ive been using LED drivers from Mouser electronics [To- 92] they cost 44 cents and been running 3 LEDs per driver and they have been working out fine. i plan to replace ALL my passenger car lighting with these. Very small and very powerful







they light at 9 volts and go well above 24 volts.


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## Greg Elmassian (Jan 3, 2008)

The driver Nick is referring to is the CL2 (TO92 is the case size). Nice and easy solution.


I have the data sheet at the bottom of my LED page:

*http://www.elmassian.com/trains-mai...ing-basics* 


Regards, Greg


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## toddalin (Jan 4, 2008)

Posted By Nicholas Savatgy on 04 Feb 2010 10:07 AM 
Ive been using LED drivers from Mouser electronics [To- 92] they cost 44 cents and been running 3 LEDs per driver and they have been working out fine. i plan to replace ALL my passenger car lighting with these. Very small and very powerful







they light at 9 volts and go well above 24 volts. 


Will actually go over 90 volts without harm.


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## Greg Elmassian (Jan 3, 2008)

Over? 

How high have you run them? 

Spec'd for 90... 

Regards, Greg


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## Jerry Barnes (Jan 2, 2008)

Okay, I went with one resisitor per LED. Thanks to all!


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## kormsen (Oct 27, 2009)

jerry, i hope you do not mind, if i will hook in here. 
as i understand it, all agree, that LEDs NEED resistors. 
but hereabouts they sell cheap throw-away lighters with an integrated mini flashlight. 
the lighting device consists of nothing but one LED, two to three mini batteries (like in watches) a short stripe of metal, that connects one leg of the LED with the bottom of the battery pile and a plastic switch, that pushes the other leg against the side of the batteries. 
absolutely no resistor! 
why do they work, if resistors are a must?


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## lownote (Jan 3, 2008)

You don't need a resistor if the voltage is low enough--most 5mm resistors can take 2-3 volts without a resistor in the circuit.


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## kormsen (Oct 27, 2009)

so, if i understand you right, i could hook 55 LEDs in series and plug them into 110 volt without resistors?


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## Stan Cedarleaf (Jan 2, 2008)

Posted By lownote on 05 Feb 2010 04:28 AM 
You don't need a resistor if the voltage is low enough--most 5mm resistors can take 2-3 volts without a resistor in the circuit. 
Note......... I do that all the time with my EOT red blinkers. Just run 2 AA's in battery holder, put a spst on/off switch, glue the batteries under the caboose/car and let 'em run. Done this for 10 years and I'll get a year or 2 out of the batteries.


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## Dennis Cherry (Feb 16, 2008)

Posted By kormsen on 05 Feb 2010 07:13 AM 
so, if i understand you right, i could hook 55 LEDs in series and plug them into 110 volt without resistors? It does not quite work that way.

If you are powering the LED's with filtered DC then you use the DC voltage, BUT if you are powering the LED's with AC then you have to calculate using the Peak Voltage.

LED's are fast turn on/off devices, must faster than incandescence. 


So for AC you take 120 Vac (RMS) and multiple by 1.414 and get 169.68 Volts Peak. Led's only conduct in one polartity direction only. That is why you only use the peak voltage and not the Peak to Peak voltage (2.828).


LED's come in different voltages depending on the color. I am not going to explain this any further as it can get technical.


RED, YELLOW, and ORANGE are somewhere is the 2.4 volt range.

WHITE, GREEN, and BLUE's are about 3.4-3.6 Volts


Normal recommended operating current is around 20 milliamps (0.020A) 


So to put a series string together of RED's it would take 70 LED's 2.2V *70 = 168 Volts. 


For the W/G/B Led's the maximum is 47

Christmas LED's light strings are configured for 70, 50,and 30 light strings in series for the R/Y/O LED's and at least 1 or more resistors in series to limit the current. 


The W/G/B LED's are configured as 35,30, and 25 lights in series. Again series resistors are used. 


Yes some LED's light strings are wired with full wave bridge rectifiers wired into the light strings, this does not increase the peak voltage but does increase the cycle on time from 60 times per second to 120 times per second. Makes them look brighter.

Now there ares C7 & C9 replacement LED Lights, these are being call retro's. They may have from 1-6 LED's with a bias resistor and maybe a bridge rectifier.


Hope that helps you understand how they work


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## noelw (Jan 2, 2008)

Jerry.. Your save as others have said..Put a Resis. on each LED is safe but trying to do the calculations of groups or if larger than 5MM LEDs is not safe.. 
We use 1600 ohms 1/4 watt on our track sig do to the over voltage some non regulated power packs that are out there. but most of the time on our own stuff is 1000 ohms 1/4 watt. on the Red , Yellow & Green a little more voltage. The White, Blue and some others are like a color tv set.. It take more in a package to make up the other colors. So it take more voltage and currernt to excite them. On our Fri. night show some time we will show how to calculate them like Greg suggested and short cuts to play it by ear once you under stand the LED and carbon resistors. One thing on Carbon Resis. is not to place them next to plastic. When shorting out can explode and burn or start a fire. If you look at aa pc board some time that had a burnt out carbon resist you will see the burnt spot. The other thing it can just go open in a flash high votage or current change. Like we said we will get in to it more if some want on our fri. night show sometime do to it get deep to understand and a show and tel work out with a live chat to ans questions than writing a story here. If in douth us the calclations that Greg suggested.
Stan and I are from the old school and of oops that didn't work trial and error . ok better look in more to info on it.. laf.


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## lownote (Jan 3, 2008)

This online calculator will do it for you:

LED resistor Calculator


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## kormsen (Oct 27, 2009)

dennis, 

well, i tried to understand your explanation. but i'm not sure that i succeeded. 
what i did understand is the point about the different nature of 120V AC and DC for LEDs. 
i will rephrase my question. 
i got a couple of powersources for telefones. (output 9V DC) hooking a series of 5 LEDs onto those would work? 
if not, why? (if i have not missunderstood,the resistor evades, that too much voltage(?) flows through the LEDs?)


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## Greg Elmassian (Jan 3, 2008)

Korm: 

you seem insistent on trying to avoid a current limiting resistor. 

What needs to be understood is that a very small change in voltage can make a huge change in current. 

LEDs do NOT work like incandescent light bulbs. 

So, if you are trying to "divvy" up the voltage between the LEDs based on what you THINK the forward voltage is, you are on a course for disaster. 

You have to stop thinking about controlling the voltage and think about controlling the current. You do NOT know the exact voltage drops and/or the exact resistance of the LEDs in order to do this properly. 

Regards, Greg


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## kormsen (Oct 27, 2009)

greg, 

that was clear enough for me to understand. 
and, yes, i tried to avoid resistors. - they are a little problem for me. 
while it is no problem to scavenge LEDs from lighters and broken mashines, i have to travel 300 miles to the next radio shack. 
(apart from the problem, while thinking in german, buying in spanish what has been explained in english...) 

but thanks to you all, for explaining.


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## Greg Elmassian (Jan 3, 2008)

It's a entirely alien concept for most people, because they are used to voltage being the controlling factor. 

Current is different, and harder to measure too. People see a "specification" on the forward voltage drop on an LED and assume it is like a normal bulb, but you can't do it. 

The key concept is that when you deal with devices with very low resistance (LEDs, semiconductors), then a small variation in voltage makes a very large difference in current. 

Consider Ohms law: V=IR .... you can see that if R gets very low, then V will vary like crazy... there's the scientific explanation. 

One of the misconceptions is using a couple of LEDs on a battery and no resistor and no problem... turns out that LEDs will "light" at quite a bit less than their "working" current, leading people to thing they are "voltage driven". 

If you want to get rid of all resistors, and you really want to string a lot of LEDs in series, you will have to "cut and try" the chain of them in series, monitoring the current, and start with too many, and then take them out a bit at a time to get the desired current. 

You CAN do it... but you can't really adequately calculate it EXACTLY beforehand. 

Hope this helps a bit more... (sorry if I sounded gruff before)... 

Regards, Greg


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## armorsmith (Jun 1, 2008)

Jerry, et al, 

For those with memberships, LSOL has a neat article on LED's this week. 

Bob C.


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## kormsen (Oct 27, 2009)

ahhh! 
now we get down to business! 

a chain, that is too long, cutting off till it fits. that's my way to do things. 

when i look at LED flashlights, it seems, if there are too many LEDs for the avayable current, they just shine less bright. right? 

and concerning sounding gruff - what else to expect from a man, who needs a stove outside....... 8-p 
(seems, that i read too many of Nick's posts. i'm adapting) 

cu later on Jerry's


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## Greg Elmassian (Jan 3, 2008)

Normally, the LED flashlights have them in parallel, not series... if in series, one fails, they all fail... not good in a flashlight.. 

Regards, Greg


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## noelw (Jan 2, 2008)

Posted By noelw on 05 Feb 2010 11:53 AM 
Jerry.. Your save as others have said..Put a Resis. on each LED is safe but trying to do the calculations of groups or if larger than 5MM LEDs is not safe.. 
We use 1600 ohms 1/4 watt on our track sig do to the over voltage some non regulated power packs that are out there. but most of the time on our own stuff is 1000 ohms 1/4 watt. on the Red , Yellow & Green a little more voltage. The White, Blue and some others are like a color tv set.. It take more in a package to make up the other colors. So it take more voltage and currernt to excite them. On our Fri. night show some time we will show how to calculate them like Greg suggested and short cuts to play it by ear once you under stand the LED and carbon resistors. One thing on Carbon Resis. is not to place them next to plastic. When shorting out can explode and burn or start a fire. If you look at aa pc board some time that had a burnt out carbon resist you will see the burnt spot. The other thing it can just go open in a flash high votage or current change. Like we said we will get in to it more if some want on our fri. night show sometime do to it get deep to understand and a show and tel work out with a live chat to ans questions than writing a story here. If in douth us the calclations that Greg suggested.
Stan and I are from the old school and of oops that didn't work trial and error . ok better look in more to info on it.. laf. 
Sorry missed that.. the Resis. I use are 1/2 watt not 1/4 watt. Post timed out before I could corect it.


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## Nicholas Savatgy (Dec 17, 2008)

Posted By kormsen on 05 Feb 2010 03:08 PM 
ahhh! 
now we get down to business! 

a chain, that is too long, cutting off till it fits. that's my way to do things. 

when i look at LED flashlights, it seems, if there are too many LEDs for the avayable current, they just shine less bright. right? 

and concerning sounding gruff - what else to expect from a man, who needs a stove outside....... 8-p 
(seems, that i read too many of Nick's posts. i'm adapting) 

cu later on Jerry's 

My heart is broken.


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## Del Tapparo (Jan 4, 2008)

This same topic seems to come up at least once a month. Is the search function broken?


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## astrayelmgod (Jan 2, 2008)

There is one other point that nobody has addressed yet, and that is how reliable is your power source? For batteries, no problem, you know what they will do. But anything that traces back to the wall outlet is at the mercy of the power company. Usually, they do a good job, keeping the voltage between 110 and 127V. But if a nearby customer turns off a large industial motor, or if there is a nearby lightening strike, you can get hundreds of volts. I had some Malibu lights in my back yard, the 12V incandescent kind. Every bulb eventually failed the same way: the filament vaporized. It wasn't broken; it had been painted onto the inside of the bulb. I don't know how much voltage that takes, but it is a lot. 

Yes, that sort of thing will destroy LEDs, too, but using a resistor or constant current source will make the bad guys work harder to finish them off. 

There used to be a guy here who absolutely insisted that LEDs would explode if you hooked them to a battery with no resistors. No, they don't. 

Dennis showed some voltage values for various colors, but it is more complicated than that. Some reds and greens are below 1.8V, while some blues are 5V. 

If you are allergic to adding resistors, you can buy LEDs with integrated resistors. 

You don't have to drive 300 miles to a Radio Shack. DigiKey sells every kind of LED and resistor you can imagine, and lots you can't. www.digikey.com.


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## kormsen (Oct 27, 2009)

astrayelmgod, 

thanks for the explanations. 

You don't have to drive 300 miles to a Radio Shack. DigiKey sells every kind of LED and resistor you can imagine, and lots you can't. www.digikey.com. 
i'm afraid, that is no real option. 
each money transfer to northamerica costs me about 20 bucks. postage for a handfull of electronic thingies about down here abot 10 to 15 bucks. price of the goods itself plus import tax, if i'm unlucky... 
so about or above 50 greenbacks. two days work. 
before i would spend that just to light a couple of houses, i will go back to the old christmaschain bulbs.


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## thumper (Jan 31, 2009)

Kormsen: 

If you want to light structures, or even flood light an area, go to your favorite store that sells solar powered LED landscaping lights. You know, the kind used to light a walkway or even provide a flood light for a statue or a fountain. They are cheap and each one has everything you need. 

I pick the kind with the yellowish light [to give a warm lighting effect]. I disassemble the light, mount the solar cell on the roof of a house [it must face the sun], etc., and solder a jumper to connect the solar cell to the electonics and bulb which I hide in the house. The solar switch on these lights is normally attached directly to the solar cell, so the only wiring to be done is to solder a jumper of the appropriate length so you can put the light where it is needed. At night, the lights turn on automatically, and for about 8 hours, each building has a nice warm glow. 

All of the lights use a rechargeable AA battery [1 or 2 batteries depending on the type of the light], and usually the batteries are NiCds. Li Ion are better, so when the NiCds wear out or develop a memory, simply replace them with a new Li Ion battery. As for resistores, etc., you don't need to mess with them in this situation as the circuit is already set up in the light. 

I like the separate battery powered lights as they avoid running a wire to every location. The total cost for each light is about $3.00 at Kmart or Walmart [no shipping, duty, money orders, etc. to run up the cost]. If you wonder how one would look, just buy one and try it out. 

Good luck, you will like the results. 

Will


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## kormsen (Oct 27, 2009)

thumper, 

thanks. 
i think, i will steal one of those my wife has in the garden. for scientific investigation. 
have to find out, how well they charge indoors from the lamps.


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