Looking at recent thread posts it seems that the resistors being picked for LEDs are 1000 ohm.
I have a package of Miniatronics 3mm yeloglo white LEDs that has a label on it stating that "for best results" use 270r (I assume "r" means ohms) resistor for 4-9 volts or 470r resistor for 10-16 volts. The package includes the resistors.
There is no information on the package relating to the LED current rating.
So, my questions would be:
1. Is there any reason to use the 1000 ohm resistor instead of the 470 ohm resistor specified on the package?
2. Where did the 1000 ohm number come from in the first place? Is that just a value that seems to work when there is no other information available?
Thanks for your answers.
The 1000 ohm resistor number comes from, as you suggested, "works in most cases" point of view. Since your manufacturer quotes a specific size resistor to work with their LED's at specific voltages, use the one they call for.
LED's are usually 20ma maximum current. I use 1k and I measure 9.5 ma with 12 volts DC applied. I have one loco with a 560 ohm resistor that allows 15ma. With 560 ohm, there is a little increase in brightness. I would suspect the 470 ohm resistor would allow close to 20ma. I will have to try using a 470 ohm resistor to see what kind of current the LED gets.
I suspect the 1k resistor is to remain in the safe area. Some DCC systems can run as high as 19volts.
Rich
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.
richg1998I suspect the 1k resistor is to remain in the safe area. Some DCC systems can run as high as 19volts.
Some LED's wont work with that high of resistance. You have to experiment sometimes depending on the LED. Always go with the manufacturers suggested value if in doubt. It also depends on what voltage is set on the DCC decoder. Some don't need resistors because they are already included on the PC board like NCE does. I know for a fact that the P2K loco's require a specific voltage for their incandescant bulbs or they don't work. I think its 330 ohms. Higher than that it doesn't work, and lower than that the light blows.
My questions regarding resistor values were driven by the pending arrival of a QSI Revolution-A decoder. Looking at the on-line manual a little ahead of time, I saw that the decoder has some on-board chip resistors that have a value of 220 ohms. That was why I was concerned by the 1K ohm value that is thrown around.
However, looking at the circuit board schematic I see that the board lighting outputs have a value of 5 volts. I guess it makes sense that with half the expected voltage I should expect to need only half the resistor value given on the LED package.
Not all LED's are the same. You'll find that different colors have different voltages. They will typically drop that specific voltage and vary on resistance. A higher current rated LED will be more forgiving with the resistor value. Also as they heat up they tend to draw more current. It's best to lean towards the highest value resistor that works well to get the most life out of them.
Springfield PA
The main thing with LED's is current, usually 20ma max for LED's we use in mrr. Don't worry about voltage, worry about current. Adjusting the resistor for 20ma max. Voltage will take care of itself.
If I want a slightly brighter LED, I adjust the resistor value while monitoring the LED current so I do not go over 20ma. I have had to do that with some LED's for signals as one color can be a little brighter than another color with the same value resistor.
With light bulbs you are concerned with voltage, not current, though current ratings are given so you can make sure your power pack can supply the correct amount of current.
For a 5 volt supply, 270 ohms will be just fine. It is a standard value resistor.
If you are going to do much of this, I strongly suggest getting an inexpensive multimeter from Harbor Freight for about $5.00. I have three of them.
Neither resistors cupplied with the Yeloglo LEDs is correct for DCC decoders. Pretty much every 'normal' white LED (leaving out the ones that are super high power and meant as flashlight bulb replacements, etc) has a 20-25ma maximum current rating. And a 3.2-3.5 volt voltage drop. Rememebr the rating is a MAXIMUM. Exceed this and you shortent he life of the LED. Exceed it by a lot and the LED literally goes 'pop'. Using Ohm's law with a 12V output from a decoder, you need to drop 8.5 volts (12 minus 3.5) at 20ma. Ohm's law says Voltage = current (in amps) times resistance (in ohms). Everyoen back to Algebra class, solve for the unknown, which is Resistance. You get R=V/I. (R is reistance, duh. V is volts (often written as E for 'electromotive force'), I is current (intensity)) Plug in the numbers. R = 8.5/.020 (20ma = 0.020A) or 425 ohms. But that runs the LED at the MAXIMUM current. Take you loco to a friend's house where the track voltage on his DCC system is higher than yours, and you'll exceed the LED limit. Plus - they are BRIGHT. Half the rated current or less is PLENTY bright enough to illuminate the track ahead. Back to the forumual: V=IR. Using a 1K resistor, and the same 8.5 volt drop, we have I=V/R or I = .0085A, 8.5ma. A bit under half the LED's rating, and in every loco I've ever put them in (every one I own), PLENTY bright.
Plus most decoders haev a limit per function output. So if you have fancy lighting setups with multiple LEDs and run them all at 20ma, you probably can;t have more than 2 or possibly 3 on a single function output. At 8.5ma per LED, you can easily hook 4 or more to a single function output.
--Randy
P.S. anyone need some 270ohm or 470ohm resistors? I have a ton from the ones Miniatronics supplies that I never use.
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I totally agree with Randy. Why run an LED with the minimum of resistance? Miniatronics will be glad to sell more to you later. They are not in business to sell you LEDs to last a life time. But you can extend its life by limiting the current and voltage being past through it. Ohm out every component you use also. I picked up an old dusty pack of 1K 1/4 watt resistors locally once and blew out a couple sunny white LEDs. They ended up being bad resistors. I tested 5 and found some at 120 ohms to 500 ohms none were 1k or even near it. A 10cent resistor blew out a 3 dollar LED.
Like previous posters have said. Even though our decoders have regulated outputs for lights there is still variations. My Power Cab put 13.5 volts on the rails, The clubs Lenz system puts 16 volts to the rails, My friends Digitrax system puts almost 19 volts to the rails according to the owner. Most modern decoders will regulate the voltage to 12 to 16 volts. Some older and lower cost decoders will let track voltage to the lights.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
rrinkerP.S. anyone need some 270ohm or 470ohm resistors? I have a ton from the ones Miniatronics supplies that I never use
I could probably use some. How many/how much. If this is against forum rules pm me for further discussion.
You could just go to radio shack. About 20 cents a piece.
rrinkerUsing Ohm's law with a 12V output from a decoder, you need to drop 8.5 volts (12 minus 3.5) at 20ma. Ohm's law says Voltage = current (in amps) times resistance (in ohms). Everyoen back to Algebra class, solve for the unknown, which is Resistance. You get R=V/I. (R is reistance, duh. V is volts (often written as E for 'electromotive force'), I is current (intensity)) Plug in the numbers. R = 8.5/.020 (20ma = 0.020A) or 425 ohms.
Okay, to use your example: If the decoder light output voltage is 5 volts as shown on the decoder circuit schematic, and using your voltage drop across the LED of the same 3.5 volts, and the current is the same 0.020 amp, solving for resistance the equation is R=1.5/0.020 or R equals 75 ohm.
Therefore the board mounted resistor of 230 ohms should be more than adequate.
Correct, or am I missing something?
maxman rrinkerUsing Ohm's law with a 12V output from a decoder, you need to drop 8.5 volts (12 minus 3.5) at 20ma. Ohm's law says Voltage = current (in amps) times resistance (in ohms). Everyoen back to Algebra class, solve for the unknown, which is Resistance. You get R=V/I. (R is reistance, duh. V is volts (often written as E for 'electromotive force'), I is current (intensity)) Plug in the numbers. R = 8.5/.020 (20ma = 0.020A) or 425 ohms. Okay, to use your example: If the decoder light output voltage is 5 volts as shown on the decoder circuit schematic, and using your voltage drop across the LED of the same 3.5 volts, and the current is the same 0.020 amp, solving for resistance the equation is R=1.5/0.020 or R equals 75 ohm. Therefore the board mounted resistor of 230 ohms should be more than adequate. Correct, or am I missing something?
WHat decoder are youusing that has a 5V function output? But, assumign it is, with a 230 ohm resistor the LED would get 6.5ma, which should be enough to light it and still be bright enough.
Actually, Miniatronics probably is in the business of selling you LEDs that will last a lifetime. If not, they're not producing good LEDs, and people won't buy them. If you use a proper resistor, your LEDs will likely outlive you.
I would start with the 1K resistor. If you like the result, fine. If you want the light to be a bit brighter, try a slightly smaller resistor. I wouldn't go below 470, though. That's how you would convert your Light Emitting Diode (LED) to a Darkness Emitting Dioded (DED).
It takes an iron man to play with a toy iron horse.
locoi1sa I totally agree with Randy. Why run an LED with the minimum of resistance? Miniatronics will be glad to sell more to you later. They are not in business to sell you LEDs to last a life time. But you can extend its life by limiting the current and voltage being past through it. Ohm out every component you use also. I picked up an old dusty pack of 1K 1/4 watt resistors locally once and blew out a couple sunny white LEDs. They ended up being bad resistors. I tested 5 and found some at 120 ohms to 500 ohms none were 1k or even near it. A 10cent resistor blew out a 3 dollar LED. Like previous posters have said. Even though our decoders have regulated outputs for lights there is still variations. My Power Cab put 13.5 volts on the rails, The clubs Lenz system puts 16 volts to the rails, My friends Digitrax system puts almost 19 volts to the rails according to the owner. Most modern decoders will regulate the voltage to 12 to 16 volts. Some older and lower cost decoders will let track voltage to the lights. Pete
Tell him to stop using the G scale setting Using a standard meter only gets an approximation, but if you measure Rail A to ground and then Rail B to ground (should be the same unless address 00 is sleected with some speed), and add them you get a pretty close approximation. It shouldn;t be 19 volts unless the scale switch ont he front is set to the O/G setting, or the adjustment inside the booster has been messed with.
HamltnblueYou could just go to radio shack. About 20 cents a piece.
Yeah that would be great if I was in the US. There are no Radio Shack's left in Canada and Radio Shack USA will not ship outside the US.
In any event 20 cents each it too much, IMO.
Since the fellow mentioned having a surplus I thought he was being serious and wanted to unload them for a price.
Echoing what others have said, I have used the 470ohm with success but feel its too bright. I now use 1k ohm and feel it works but is very slightly dimmer. If you don't have a source for 1k ohm resistors, you could put the 270 and 470 in series to boost the resistance.
rrinkerWHat decoder are youusing that has a 5V function output?
The decoder I'm waiting for is the QSI Revolution-A. There is a board schematic on page 7 of the operating manual, http://www.qsisolutions.com/pdf/q_RevoA_Manual.pdf. I believe that the +5 I see on the board in the line to the LEDs. (plus I called QSI and the person I spoke to said that the output to the lights was 5 volts).
OK, those do have 5V regulated outputs. There are some decoders that have current regulators built-in so you can use LEDs with no resistors (but be careful - some have 2 sets of headlight conenctions, one that's regulated and one that's not - others have a regulator with an option to bypass, usually by soldering across a couple of pads on the board. Many with regulated headlight outputs do NOT have the regulators on additional function wires, just white and yellow). Best bet in any case is go by the manufacturer's recommendation and specifications.
JSperan Hamltnblue You could just go to radio shack. About 20 cents a piece. Yeah that would be great if I was in the US. There are no Radio Shack's left in Canada and Radio Shack USA will not ship outside the US. In any event 20 cents each it too much, IMO. Since the fellow mentioned having a surplus I thought he was being serious and wanted to unload them for a price.
Hamltnblue You could just go to radio shack. About 20 cents a piece.
Well I do, although if you check ebay sellers and there HAS to be some electronics supplier in Canada equivalent to say Mouser in the US - resistors from such folks are more like 1 cent each if you order a few dozen. I think my parts box has 15 each of 470 and 270, by the time I finish the rest of my locos I'll probably have double that.