The 470 is close the minimum value you're likely to want with an LED. The 1k is a good all around value, especially if you like your LED to not be light sabers waiting to poke out your eyes. I actually use values like 2k or 3k and above quite frequently for what are supposed to be older headlights on steamers.
I'd go for the 1k for all around versatile.
Mike Lehman
Urbana, IL
cedarwoodronI have used 470 ohm resistors in several structure and engine repower projects, but- before I place an order for a bulk purchase of resistors, I thought it might be a good idea to ask which resistor is the most appropriate in general for LEDs. I believe that the "on time" of my builds would be less than 10 hours per month, based on prior experience. I didn't see a very significant lighting difference between the two when alternately used in two diesel projects- that is, there may be a reduced current load with the 1K resistor, but - with the lights out- the illumination appeared to be pretty much the same. Any feedback would be appreciated. Cedarwoodron
Since you don't see a significant difference in the lighting between the two resistor values I would recommend that you use 1K resistors. The lower current will result in longer life for your LEDs.
Joe
More and more, recently, I have been using 1K ohms resistors almost exclusively.
Rich
Alton Junction
What voltage are you driving them with? If it's 12V or better, 1K is what you want. 470 ohm is a good option for putting an LED in a DC loco where the voltage will vary - much higher and the loco has to be running at a pretty good clip before the LED even comes on, much lower and it will be over the current limit at full throttle.
You won;t see much brightness difference - LEDs have a very narrow range of current where the brightness noticeably changes/ IE 1ma, the LED might not light, 3ma, it's as bright as it is at 10ma. That's why the usual method to 'dim' an LED is to actually turn it on and off quickly, more off time in relation to on time, the dimmer it looks.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinker470 ohm is a good option for putting an LED in a DC loco where the voltage will vary - much higher and the loco has to be running at a pretty good clip before the LED even comes on, much lower and it will be over the current limit at full throttle.
That is what I do. Mine comes on just as the loco moves and I am pretty satisfied with the brightness. For structures, I use the 1 K resistors.
Jim
I always use 1K for LED's.
Modelling HO Scale with a focus on the West and Midwest USA
For DCC installs, you'll want the 1K.
I have a good supply of 470 ohm, also a bunch of somethign else - 860 or something. I usually get the Miniatronics Yelo-Glo LEDs, and they come with two different resistors - maybe it's 270 and 470, anyway they come with them and I never use them.
Rather than guess, many years ago I learned to use a multimeter and made the following measurements.
Measured with 12.2 VDC supply using 20 ma max current LED's. In electronics, I rarely go over 75 percent.
Today, some LED's are super bright and require over 1k.
Resistor Current
1000 ohm 9.0ma
750 ohm 12.45ma
680 ohm 13.12ma
510 ohm 16.25ma
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Rich,
I'm confused.
If you have a 12.2 VDC supply and a 1000 ohm resistor, why wouldn't the result be 12.0 ma?
LION buys 1K ohm resistors by the THOUSAND (cheaper that way, and besides, I shall shortly have to buy another thousand).
LION uses 5.1 ohm resistors for track-speed control. LION puts 10 volts of regulated DC power on all tracks, embedded resistors slow the trains down for the stations, and a gap stops them on the mark. A clock-work timer pulls a relay to let the train continue on its route. It takes 20 minutes for a train to make the round trip run from 242nd Street to South Ferry. Trains depart 242nd street every 3 minutes. The signal system keeps them from running into each other.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
The white LEDs I have are almost always 3.5 volt. Take a 12.2V power supply, minus the 3.5V drop on the LED, gives you 8.7V, divided by a 1K resistor is 8.7ma.
So 9ma. Close enough.
The REASON i just say use a 1K resistor is that I do know all the formulas, and did do the calculations. And 1K it is. So when people ask, I cna offer the formulas, but lots of people just want to hook up their LEDs and not worry about electronic theory, so - 1K resistor for a white LED. Done.
rrinker The white LEDs I have are almost always 3.5 volt. Take a 12.2V power supply, minus the 3.5V drop on the LED, gives you 8.7V, divided by a 1K resistor is 8.7ma. So 9ma. Close enough.
The white LEDs I have are almost always 3.5 volt. Take a 12.2V power supply, minus the 3.5V drop on the LED, gives you 8.7V, divided by a 1K resistor is 8.7ma. So 9ma. Close enough.
ahh, you subtract the LED voltage.
Yes, the other complement to Ohms Law that you need for calculating these things - Kirchoff's Laws.
Things in series, the voltage adds, the current is evenly divided. So if the LED drops 3.5 volts, and the power supply is 12V, then the voltage across the resistor is the difference. If the current through the resistor is 9ma, then the current rhough the LED is 9ma.
I would add one point to the resistor question. That is that in some cases you want the LED to draw as little amperage as possible. For example, if you are installing multiple LEDs in a locomotive (headlamps, reverse lamps, ditch lights, number board lights, marker lights, step lights, cab lighting....) you could exceed the maximum output limit of the decoder by using 1K resistors. You can still get lots of light out of an LED with a 4.7K resistor which lowers the amp draw significantly. So, for example, you could use 1K on the headlights/reverse lights/ditch lights which you want to be bright, and 4.7K on the rest and hopefully stay within the maximum output of the decoder. In fact, things like the number boards and marker lights will look more realistic with somewhat lower light output.
I have just done an FM H16-44 where I used two LEDs for each number board to get even illumination. That was in addition to head and reverse lights and marker lights. 14 LEDs in all. If I had used 1K resistors for all, the draw would have exceeded the decoder function's maximum.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
If you need to calculate, this is handy:
http://www.quickar.com/bestledcalc.php
I just say, When in doubt, add more resistance.
richg1998 Rather than guess, many years ago I learned to use a multimeter and made the following measurements. Measured with 12.2 VDC supply using 20 ma max current LED's. In electronics, I rarely go over 75 percent.
What happens to the LED when you use a resistor, but it still results in the current exceeding the ma max of the LED?
For example, if you use a 270 ohm resistor, the resulting current will be 32 ma exceeding the 20 ma max of the LED.
Will the LED instantly burn out or just slowly degrade?
There's a threshold, and it's going to vary from LED to LED, and it's not linear. Overcurrent a little, the LED will work but have a reduced lifespan. Overcurrent too much, and the LED will pop pretty quick. The problem with an LED is that with no current restriction, it will continuously flow more and more until the junction is overwhelmed and destroyed. It's a little bit like light bulbs and voltage - a little over the limit doesn;t burn the bulb out instantly, it just reduces the life. A lot over the limit and it emulates a flashbulb. I've seen LEDs literally blow a small chunk out of the case when they pop. Other times there appears to be no physical damage yet it no longer lights up, becoming a device technically referred to as a DED, dark emitting diode.
Thanks, Randy, for that additional information.
This has turned out to be a pretty informative thread.