LED resistor value?

[6axlepwr]

In N-Scale, what is the typical resistor value you use for an LED light?

I have in my stash of surface mount resistors the following.

- 510 ohm 1/2 watt
- 1k ohm 1/4 watt
- 1K ohm 1/2 watt
- 1.5k ohm 1/2 watt
- 2k ohm 1/8 watt

Brian

[C855B]

Hi, Brian!

All the old advice for 12VDC points at 680Ω, but with modern LEDs that's awfully bright. My go-to these days is 1KΩ, wattage not of concern. I'm sure @peteski will jump in here with a thought or two.

[ednadolski]

Depends on the current that you want, 5-10 mA typically.   Best bet is to do a few quick tests with a DMM and your voltage source of choice (ie, the decoder that you plan to use).  Start with higher resistances and decrease until you get the brightness that you want.

Ed

[Sokramiketes]

I like picking through the Ngineering site for their suggestions and calculations, as each LED has an optimal current.  Then, each color of LED has its own needs, to balance values if you're doing things like signals.

If you are just wiring a headlight, and the typical warm white SMD LED's, then go with 1K resistors and don't worry about any other calculations.  You'll always be safe. 

[peteski]

White LEDs (which are used as headlights) are very efficient, and even at 5mA might appear too bright. It all depends on the installation.  If the LED itself is visible in the headlight opening, then it might look more realistic at 1mA or even less.  If it is behind a light "pipe" it might need more "juice".

The recommended operating current for small SMD LEDs (like 0603 or smaller) is usually 5mA (with 10mA max). Of course using them at lower current is ok too.  With so many LEDs out there from various manufacturers, their efficiency varies from brand to brand.  It is possible for brand "A" LED at 5mA appear to have same brightness as brand "B" LED at 10mA

The level of brightness is also a personal preference. There is no single formula that works for all.  The 1k ohm resistor is just a suggestion.  It is guaranteed not to burn up the LED.  With a white LED that has Vf of 3V (typical value) and the "blue" lead on the decoder being 12V (again another typical value) 1k resistor will limit the current to (12-3)/1000=9mA.  Power dissipation at the resistor will be R x I²=P  or 1000 x 0.000081=0.081W, so 1/8W resistor or larger is safe to use.

[mmagliaro]

It radically depends on the LED.  Some of them are "super brights" and are blindingly bright with 5 mA going through them.

You really have to hook one up and see how it looks to you.  That's the only way to know.
1k is a good starting point, because you know you won't blow it up.  I wouldn't go lower than 470 ohm.  But beyond that, anything from 470 to 10k is fair game.

I've been  using 2.2k for all LEDs for a few years now, and 4.7k on some of them, because they all seem too bright at 1k,
whether for building lights, headlights, and even indicators on my circuit boards.  In fact, the little SMD LEDs that are on the Arduino circuit boards I've been using lately have 1k ohm SMD resistors in series with them, and they are so bright that it hurts to look directly at them. (and no, nothing weird is wrong with my eyes... LOL!)

[Dwight in Toronto]

I recently posted a topic about having installed two 0402 red led’s in series as marker lamps on a Kato N scale E8 (as per Rick Brodzinski’s tutorial here on the RW).  Although they looked terrific, the 1.8K smd resistor left them much too bright.  Adding a second 1.8K didn’t have a noticeable effect. 

Expert feedback suggested that one might expect to start seeing some dimming at around 8K, but that tens of thousands of ohms, and perhaps even in excess of 100,000K could be called for.

I settled for using LokProgrammer software to sufficiently dim the led’s, and even then had to diminish the slider bar down to the very first notch.  I would have preferred to have accomplished the dimming with a single resistor, but just wanted to button up the project and get the loco back on the rails.

[jdcolombo]

Like Max, I've switched from 1K to 2.2K for my dropping resistors.  I use a 1/4-watt surface mount resistor; much smaller form factor than a traditional round carbon resistor, although also more difficult to solder in-line.  Typically, I use a small piece of 1/32"-thick single-sided circuit board to mount the resistors.  I cut a piece of board 3mm wide by 6mm long, scribe a gap in the copper in the middle of this sliver of board, tin it, then solder the resistor across the gap.  The result is compact enough to mount nearly anywhere, and if need be I can shorten the length with a pair of flush wire cutters.

John C.

[mmagliaro]

Yes, for SMD LEDs especially, you'll want more than 1k.  Those buggers can be VERY bright at only 1 mA of current.

[peteski]

Like John, due to their compactness, I also usually use SMD resistors.  I also most often mount them on little PC boards I etch myself.


But if needed, a SMD resistor can be "spliced" inot decoders wire like this.  Heat shring tubing provided insulation.


As for resistors, luckily electronics is another hobby of mine.  I have every standard resistor value (leaded and SMD resistors of multiple wattage) from 1 ohm to 10 mega ohms. 

[wm3798]

This is a topic I run into frequently.
I picked up this set at the recommendation of the Mighty DKS,

It boasts 1000 resistors with 25 values.
Each pouch has a label that defines the properties.  What I'm challenged by is understanding what I'm looking at.
From what I'm seeing here, I should be looking at the Ohm value, which on this pouch would be 3.3



I'm assuming that 3.3 ohm is exactly that, and not a secret code for 3.3 THOUSAND ohm, since other labels state 1M, 5.6k, etc.  I should be looking for between 1k and 2.2 k to run a SMD white on 12v, is that correct?  And is 1M 1 thousand?  or should I look for a label that reads 1k?

I'm also assuming that the lower the resistance value in ohms, the more current gets sent to the LED... lower + brighter, more likely to blow out. correct?

Please illuminate me so I can move forward with illuminating a couple of projects properly!
Lee

[peteski]

Lee, yes, that is correct if that will result in a brightness that looks good for you. 1k @ 12V will result in around 9mA going passing through the LED.  1.2k will pass  less than 9mA.  Should be safe for small SMD LEDs You likely don't run your trains at full throttle (12V) - or do you?    What I'm saying is that if the LED appears too bright on 9mA (at 12V), when you run at partial throttle, the brightness might be just right.

One more thing:  since you run DC and power the LED directly from the track, you need to add a reverse voltage protection. White LEDs are very sensitive to reverse voltage. You either need to use another white LED wired in parallel reverse then in series with the resistor, or a series connected protection diode.  That way the LED will not smoke when running in reverse or from motor's BEMF spikes when the loco momentarily loses contact with the track.  The reverse parallel double LED method is best when you have models which have front and rear headlights (like diesels). 

I don't  have a diagram handy - maybe I'll post one later.

[mmagliaro]

1M is 1 megohm (1 million)
1k is 1k (1000) ohm
And yes, that one you showed that just said 3.3 means it is only 3.3 ohms.
Be just a little cautious on those bargain parts assortments.  They are probably okay for something
like resistors, because resistors are so cheap to make that they are most likely the real deal,
 but I'd still check the part with an ohmmeter before you actually use it, just to make sure the
resistor is what it says it is and that the resistors didn't get put in the wrong-marked bag.
As long as they are within, like 10%, they are more than fine for something like an LED.
(If they were something like electrolytic capacitors, I'd be saying "no way!")

[peteski]

1M is 1 megohm (1 million)
1k is 1k (1000) ohm

Exactly.  I totally spaced out in my post and failed to explain that.
Actually those k, M, G numeric prefixed are the same for any units.  Most people are familiar with computer memory or disk space sizes.  Kilo (k), mega, (M), or giga (G) are multipliers most of us are familiar with.  Kilogram is thousand grams, or thousand ohms, or thousand bytes, and so on.
Mega byte is million bytes, mega ohm is million ohms.   

(If they were something like electrolytic capacitors, I'd be saying "no way!")

LOL, actually most aluminum electrolytic caps have rather broad tolerances (like -10% +50%). Tantalums have tighter tolerances.

[wm3798]

I still have not clue what you just said, but I believe you!

I'll dig through my box of stuff here and see if I can't isolate the 1k bag, then start doing some testing with a 9v battery.
@peteski a diagram of that 2 way arrangement would be most helpful.  With SMDs I have, I think that would allow me to put back up lamps on my old Trix B6s...  They have a lens and a housing on the tender, so it' shouldn't be much of a stretch to get light in there.

I assume the rig you will share provides for directional lighting?  or do both stay lit in order to protect each other in the reversing process?
Lee