LEDs

[tomcat623]

Help me please for some reason my common sense isn't working today. Heres my question, with a nce d13sr decoder and 3mm yeloglo led do I use the 4-9 volt resister or the 10-16 volt resistor that came with the leds?
I'm thinking 10-16 but I'm not sure. Any input would be greatly appreciated.
                            Thanks, Tom

[tomcat623]

Never mind, I think I figured it out, the 10-14 volt resistors, right?

[Nigel]

Tom;

Resitors are measured in Ohms, not Volts.

Volts = Amperes X Ohms

Watts = Volts X Amperes = Volts^2 / Ohms = Amperes^2 X Ohms

Diodes (including LEDs) have approximately zero resistance (close enough for model railroad applications) AND they have a current rating. 

Using the above equations, you can determine the value of the resistor required, using the above equations, the current rating of the LED and the voltage setting on the DCC system(s) you are using.

[rikc9]

Correct.

http://www.members.optusnet.com.au/nswmn/index.htm

http://www.tonystrains.com/technews/install-leds-decoders.htm

You can always cheat and do a search with www.google.com if you get a question like this in the future. Most of the time the answer is out there.
As an example, do a search for led dcc.

rikc9

[tomcat623]

 Thanks go to you all,
Sometimes I speak before I think and don't say what I mean.
I got a ten pack of led's from tony's trains and they came with two resistor types, one for use with 4-9 volts and one for 10-16 volts. I wasn't sure which to use but I figured it out with your help, thanks again!
Tom

P.S.----I've been away from trains for almost 15 years, I'm retrofitting all my engines and rolling stock with decoders and kadee couplers, I installed  decoders and led's in nine engines yesterday (they all work well!) I'm quite pleased.

[Jim Banner]

While resistance (ohms) is one of the characteristics of resistors, you can, for a given application, use volts.  This is equivalent to saying "use resistor A if the source voltage is between 4 and 9 volts or use resistor B if the source voltage is between 10 abd 16 volts."  The idea is to dumb down the selection process by letting the supplier decide on the current range and voltage drop for his product.

For those who might be interested, some of the other characteristics of resistors include:
- power rating in watts
- tolernace in %
- thermal coefficient in %/degree
- voltage rating in volts.  This may be flashover voltage for high resistances working at high voltages or it may be the withstanding voltage of the resistor's coating.

[bevernie]

Wouldn't it be safer, and just as effective, to just use 1/2-watt resistors regardless of track voltage?   ... just a thought...                                                                                  Ernie
                                                                                         Hendersonville, NC

[Nigel]

Wouldn't it be safer, and just as effective, to just use 1/2-watt resistors regardless of track voltage?   ... just a thought...                                                                                  Ernie
                                                                                         Hendersonville, NC

Yes.  Space constraints may drive you to lower Wattage ratings.

[r.cprmier]

What Nigel says is right.  About the only reason that I can think of for a smaller resistor is space consideration, as long as your wattage requirements do not supercede the rating of the resistor.   The resistor components in a semiconductor (chip) are infitesimally small sizewise and commensuate with that, of course, would be the wattage requirements.

Dynamic braking employs a resistor grid; and although I have never checked out the innards of a-say-F-3, the size of these resistors have got to be significant because of the job they are supposed to do (act as a "load" for the traction motors, once they are switched to brake mode-at which time, they act as generators; and the circuit is switched into that grid).

RIch

PS:  For Nigel;
I had wished I had been able to check out the electricals of a New Haven EP-5 (Rectifier motor) before they went to scrap.  Those ignitron circuits must have been something!

[Bojangle]

Before I install any replacement item, (or design a new circuitry) I first check  it out on a  test board  using a fairly precision variable power supply and a resistor substitution box. It is then easier to use test meters    Better to fry on the board than after installation.  Here is an interesting site   http://led.linear1.org/1led.wiz
Bo

[rikc9]

When you get the resistance value, use the below link to find the resistor wattage. I tried to find the wattage calculator at the LED center Site but it must be well hidden.

http://www.opamplabs.com/eirp.htm

rikc9

[Bojangle]

At the LED center, the wattage is given in the answer when you click "Find R" 
Example:

The wizard recommends a 1/2W or greater 1500 ohm resistor. 

Resistors are  cataloged wattage first,  resistance second.

The LED center calculator expresses current in Milliamps, the other site in  Amps. At values of one amp or less, it is customary to express in milliamps. For those less familiar with electricity, 1 amp =1000 milliamps.

According to the site, all leds have similar forward voltage characteristics,  but the current value range depends on whether you are using "indicator" grade (signals, etc) or "illumination" grade. (lighting).  If you want a "flamethrower" on your loco, you need to use an "illuminaire" device to concentrate the light. One of the hottest selling items now is the pocket LED flashlight , you see buckets of them at  checkouts for  around $8.  The "illuminare" consists of 9 concentrators, enough to equip 9 locos.  I just made the mistake of looking directly at them, I still see spots. More info than requested, but it is interesting reading. 

In all cases, follow  the manufacturer's specs.
Bo

[Hunt]

At the LED center, the wattage is given in the answer when you click "Find R" 
Example:

The wizard recommends a 1/2W or greater 1500 ohm resistor. 

Resistors are  cataloged wattage first,  resistance second.

Bo and all,

Be cautious, such do-the-math-for-you calculators/wizards are only useful when they are given accurate information and the user has at least some general knowledge of the basis of the calculation (formula) used by the calculator.

Reason Tom worded his question the way he did is the Yeloglo, 3 mm diameter, LEDs he purchased has supplied with them the following resistors,
  •   270 Ohm for use with 4 - 9 volts
  •   470 Ohm for use with 10 - 16 volts

[Bojangle]

I agree that any calculator must be given the correct values to be of any use, and I am thankful to have a background in math and electronics.  We didn't have calculators when I went to college. 

But for those who may not be math or electronic oriented, the online calcs at least give you a starting recommendation.  From there I would still do a preliminary test on the bench to verify everything.   

I understood Tom's question perfectly, expressed in layman's terms.  I would strongly recommend that anyone who is working with these devices pick up a book on basic electricity and circuitry, and gain at least a general understanding of Ohm's law.
Bo