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Resistor for LED question

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Resistor for LED question
Posted by York1 on Thursday, August 15, 2019 12:13 PM

Please!  I have no electronic knowledge.  I cannot make heads or tails of some posted easy answers to questions, so give it to me in Kindergarten (better yet, preschool)  terms.

 

I have a lighting network that uses 12 volts.  I have some LEDs that are listed at 3 volts.

I know I need to buy some resistors.

I checked some sites, and it's like a foreign language in buying the correct resistors.

I need a simple answer ... what resistor should I look for?

Saints Fan John

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Posted by UNCLEBUTCH on Thursday, August 15, 2019 12:19 PM

York1
like a foreign language in buying the correct resistors.

 I hear ya, thats why I went with simple 12V. works great

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Posted by BATMAN on Thursday, August 15, 2019 12:31 PM

John, from one preschooler to another.

http://led.linear1.org/led.wiz

Brent

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Posted by wvg_ca on Thursday, August 15, 2019 12:40 PM

well, a calculator would say around 500 ohms, but that for full brightness [roughly 20 ma], but did you want it a little dimmer ??   in my world i start at 1k [or 1000] ohms and go up from there, because full brighness is simply too bright for me ..

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Posted by rrinker on Thursday, August 15, 2019 12:50 PM

(12V supply - 3V LED rating)/Desired current in amps

10ma is .010 amps

12-18 volt supply, a resistor rated for 1/8 watt is enough. 

The current given in the LED spec is a MAXIMUM. Pretty much never run at the maximum. Half or less will still be plenty bright.

That's how you figure it out.

                                        --Randy

 


Modeling the Reading Railroad in the 1950's

 

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Posted by BATMAN on Thursday, August 15, 2019 12:59 PM

I ordered a bunch of these from China for about $2.00 each, they work well to adjust the brightness. Got them in ten days, that was fast it usually takes three weeks for China orders to get here.

On my mockup.

Brent

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Posted by MisterBeasley on Thursday, August 15, 2019 1:41 PM

1K.  More for dimmer, less for brighter.  I use 1/4 watt.  Those are rules of thumb, but they work for my locomotives.

I do have a panel LED I use for a porch light.  I needed 10K for that at 12 volts.  It is a very bright LED.

It takes an iron man to play with a toy iron horse. 

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Posted by York1 on Thursday, August 15, 2019 1:48 PM

Thanks, everyone, for your help.  I appreciate your assistance.

Saints Fan John

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Posted by richg1998 on Thursday, August 15, 2019 2:07 PM

Since you have your answer, this might be some help also.

I started many years ago by putting a 5k pot in series at max resistence with a amp meter and LED and twelve volt power supply and lowering the resistance. Real easy. I learned some years ago, maybe 1972 , that LED's are current devices. Mostly 20 ma max.

I did a bunch of numbers. As I recall, I had a ten turn pot so I stayed away from going too low. I was working for NASA at the time so I had all kinds of "stuff" at hand. Intro to TTL also and five volt logic. We had time to play in off hours.

Nine ma is common for me with 1K, 1/8 watt resistor. 1/4 watt is more common.

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

Ok, some of this was edit.

Rich

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Posted by rrinker on Thursday, August 15, 2019 6:08 PM

 I would put a 470 ohm or 560 ohm resistor in series with that potentiometer so in case you turn it all the wy down, you still have the minimum resistence to keep the LED within its current limit. Just measure across from the fixed resistor to the pot to get the total resistence needed.

                                          --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by RR_Mel on Thursday, August 15, 2019 6:26 PM

Yea, do the Randy thing with the safety resistor in series with the pot.  If you listen carefully you can hear the LEDs pop, no smoke just a tiny pop.
 
 
Mel
 
 
My Model Railroad   
 
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Posted by rrinker on Thursday, August 15, 2019 6:30 PM

 Don't listen too closely, at least not without safety glasses - I've seen little chunks of the LED case get blown off when they pop, you end up with an LED with a divot in it - that little piece shot off somewhere!

 Just put an LED directly across a 9V battery - there will be a flashbulb effect, followed by the creation of a DED - Dark Emitting Diode. One time only, like old flash bulbs, before electronic flashes.

                                        --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by BroadwayLion on Friday, August 16, 2019 12:10 PM

A 3 volt LED lights up with three volts, not two volts. It will also light up on 12, 16, or 24 volts.

The Limiting factor for an LED is the amount of CURRENT that it uses. Current is measured in amperes.

 

I'll not bother you about the amps... a 1000 ohm resistor will protect your LEDs at any voltage. and yes, a 3 volt source still needs a resistor. You can play with the math, but LIONS do not do this. Just use the 1000 ohm resistors and forget about it.

 

BUT EACH LED needs its own resistor.

The SHORT lead on your LED is negative, and the longer lead is positive. Current will not flow in the other direction, and thus will not light up. MOST do not respond well to this, although there are ways to do this... But you did not want an essay.

 

You can see in this photo that each LED has its own resistor.

 

ROAR

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Posted by BATMAN on Friday, August 16, 2019 1:39 PM

I use a resistor on each light as well. After experimenting and not being happy with the results it was the way to go. I use the little flat Digi-key resistors, you can see them along the beam where the light attaches to the beam. After being painted you cannot see them.

Brent

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Posted by gmpullman on Friday, August 16, 2019 2:03 PM

I find it helpful to have a "decade box" as a handy tool to zero-in on resistance values.

https://tinyurl.com/y23hpuvd

Work your way down to the closest Ω range then fine tune the brightness you desire and see what the value is that works best.

You can make your own using a range of resistors close to what we use for model LEDs but the one I linked to has a wide range and is simple to use. 

Cheers, Ed

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Posted by khier on Thursday, August 22, 2019 11:23 AM

500 ohm or to 1k.

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Posted by swisstrain on Friday, August 23, 2019 8:56 AM

Like you, it is difficult for me to make heads and tails of electronics.

 

I know I will get frowned upon, and will get an earful about this,but instead of trying to calculate resistor values and then experiment with higher or lower ones, use a current limiter instead.  I know they are more expensive (costs you about 70 cents per circuit), but there is no math and fiddling with resistors involved.

Also, I profited greatly from a clinic that I attended about a year and a half ago presented by the owner of microlumina (https:/www.microlumina.com).  He also has a series of really great articles in PDF form posted on his website called light bites that are written to pre-school level, and which helped me enormeously to put together my first few lighting circuits (https://microlumina.com/p/lightbites).

I am not associated with microlumina.

Good luck with your project.

Urs

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Posted by York1 on Friday, August 23, 2019 9:14 AM

Thanks again, everyone.  I'll try to let you know how it goes.

Saints Fan John

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Posted by wjstix on Friday, August 23, 2019 4:28 PM

For future purchases, Evan Designs makes a variety of LEDs that come with a bridge rectifier, capacitor, and resistor already installed. They will work on any power source up to 14V, AC or DC without needing any modifications or 'extras'.

https://www.modeltrainsoftware.com/products/universal-solid-leds-for-transformers

 

Stix
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Posted by rrinker on Sunday, August 25, 2019 8:13 PM

 Those things (current limiters, and pre-wired LEDs) are nice, on the surface - but in the case of the current limiter, it runs the LEDs at maximum current, 20ma, or close to it. What if that's too bright? LEDs work on current, not voltage, so turning down the voltage makes no difference, until you turn it down too far and the LED turns off. The advantage of the current limiter is that it doesn;t care what the voltage is, up to whatever the maximum rating it, the LED will always get 20ma, with a 5V power supply or a 15V power supply. But 20ma may be too much.

 The Evans LEDs with the rectifier and resistor already built in - same sort of deal, the resistor value is a compromise to woork across the wide voltage range - the LED may be too dim at the lower end of the voltage arange and too bright at the upper limit. The advantage of these is they work on AC oor DC (because of the bridge rectifier) so you can hook them to the AC or DC terminals of a power supply - just don;t exceed the maximum voltage because what that actually does is exceed the maximum CURRENT of the LED.

 I showed the math previous - it's just simple math, it's not hard to calculate and get the right resistor every time. Doesn;t need advanced calculus or anythign to figure it out, all you need to knwo whare the things you better know before hooking anything up - voltage of the LED, current of the LED, and voltage of your power supply. All givens, nothing to figure out there. 

                               --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by Mister Mikado on Wednesday, September 04, 2019 12:23 PM

I stick with 1000 ohms/eight watt strung to a yellow LED, just the right brightness for a yellowish loco headlight.  I replaced the blue-white LED or incandescent bulb in several locos with this arrangement.  It looks convincing and is automatically directional with near-constant brightness.  It can be wired right to the existing headlight wires even if they come off a circuit board, just make sure the polarity is right for forward operation in a steam loco, or whatever is forward in a two-ended diesel or reverse in a tender headlight. -Rob

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