Resistors - Choosing the Right One

Model Railroader Electronics and DCC
1

Let me start by saying that I am no electronics expert. There was a time when I didn’t know the difference between a resistor and a diode.

That said, I use resistors extensively on my layout to control LED lighting on signals and control panel lights.

I pretty much use resistors that I purchase in 5-packs from Radio Shack.

All of my signals, search lights and dwarfs, are from Tomar Industries.

All of my control panel LEDs are from Miniatronics.

When I first got into this back in 2004, I was advised to use resistors ranging from 330 ohms to 1500 ohms. The higher the ohms, the dimmer the LED. The lower the ohms, the brighter the LED.

That has been the extent of my knowledge until I recently bought my latest search light signal and found out that the LED was much brighter than previous LEDs with the same value resistor.

So, yesterday, I wound up experimenting with higher value resistors from Radio Shack including 2.2K, 3.3K, 3.9K, 4.7K, and 5.9K ohms.

Previously, I might have thought that a 5.9K ohms resistor would essentially prevent the LED from even lighting. But now I discover that even with a 100K resistor, an LED will light, albeit very dimly.

Now I worry that these LEDs will have a shorter life if the resistance is too low.

Can the experts weigh in here and give us electronics novices a primer on the proper use of resistors?

Rich

2

Rich,

I’m no expert either. But what kills LEDs is too little resistance, not too much. If they get bright enough, it lets out the magic smoke.Lots of resistance actually prolongs their life.

You have discovered an interesting property of LEDs that some people even believe is impossible, namely that you can dim LEDs by adding lots of resistance. This works great with lighting structures, as it results in LEDs that look darn close to bulbs.

With loco headlights, I make the older ones, especially steam, dimmer. Modern diesels I tend to make brighter, but not as bright as you usually see with factory-installed LEDs.

3

Ahem… for the same price (more or less) I can by a box of 1000 resistors from All Electronics.

Before you tell us you do not need that many, I must tell you that I am now down to my last 100. LEDs forever… My platforms are 4’ long, each one has 20 to 50 LEDs on it, those things can eat up resistors in a hurry.

ROAR

4

LOL

I wanted to challenge that statement, but your are correct. You are not LION, or even MONKeying around.

All Electronics prices:

10 for $0.80

100 to 199 $0.045 each
200 to 999 $0.037 each
1000 or more $0.018 each

Still, Radio Shack is only a 5 minute drive away.

Rich

5

The key is the total current flowing through the LED. The spec on the LED is a max. Do not exceed that, and you are fine.

Back to the old favorite white LED for DCC headlight. Typically they are rated at 25ma max. A 1K resistor at HO track voltage results in about 9 ma throught he LED - less than half the maximum. It will last forever.

For a simple circuit of LED and resistor in series, connected to a power source, the current through the resistor (and by Kirchoff’s Laws, the current through the LED) will be the voltage across the resistor divided by the resistor value in ohms. Also by Kirchoff’s Laws, the voltage across the resistor will be the power suppyl voltage minus the voltage drop of the LED, which will be given in the specifications as well.

The white LEDs I use are 3.5 volt. So with a 12V power supply, the voltage across the resistor is 8.5 volts. With a 1K resistor, that’s 8.5/1000 amps, 8.5ma.

Most of the colored LEDs, red, green, yellow, are rated around 2.7-3.1 volts, and as high as 40ma, so despite the lower voltage drop compared to the white ones, you can use a smaller resistor and still be well under the maximum current.

There is a minimum current needed to light the LED - as you put bigger and bigger resistors in series, eventually you’ll hit a point where the current is so low, the LED won;t light up, even dimly. Brightness control via current is very touchy, it’s usually not linear. The more common way to dim LEDs is to drive them via pulse width modulation, the longer the ‘on’ time of the pulse the brighter the LED appears to be.

–Randy

6

Not to hijack this thread, but…

That 5-minute drive adds even more to the cost - gas and wear/tear on your car, and your time.

The trick is to anticipate your needs.

For example, I have ten DS64’s and seven UP5’s on my layout, all with external power. Since both devices need 2.1mm id plugs for that external power, I bought nine of these from All Electronics for $21.15 plus shipping (which since it’s a flat rate can be spread out over other items).

On the other hand, nine of these from Radio Shack would cost me $31.41 plus shipping or gas. Probably a lot more gas, since it’s unlikely that any given Radio Shack would have nine of these in stock - I’d have to go to at least a couple of them, or multiple trips to the same store after they’ve restocked. AND I’d lose the advantage of the pre-attached pigtails the ones from AE have.

So Radio Shack in this instance is at least 50% more expensive for an arguably less user-friendly item, and one for which bulk pricing at AE doesn’t even come into play.

The bottom line is that it pays to anticipate your needs and plan your purchases accordingly. This is especially true with places that offer bulk quantity discounts, or that have flat-rate shipping where you can drive down the shipping cost per item with a larger order.

Even Radio Shack advertises free shipping for orders over $50, so the same principle of anticipating your needs ap

7

Stevert, I don’t disagree with you.

But, in this case, I had a more than adequate supply of resistors on hand with various resistance values.

It was only when I hooked up my most recent search light signal that I realized that I was going to have to purchase a resistor with a higher value.

So, I took that quick trip to Radio Shack.

Prior to that, I never had a need for a resistor with a value higher than 1.5K ohms.

Rich

8

I agree with Randy. If you want to operate an LED properly, you need to operate it within the current rating that it is specified for.

In most circuits the voltage is constant, and depending on the voltage of the LED and its maximum current rating, there is one resistor value that maximizes brightness. see LED circuit for determining the resistor value.

But in model railroads, the LED voltage may vary, and a current regulator is appropriate, which limits the current once the voltage exceeds some value. See A Better Constant Brightness Circuit for LEDs which describes a the use of a small 20 ma regualtor. This of course only works if the max current rating of the LED is 20 ma or less

9

Rich.

When I put the red LEDs in my gravel plant load out tracks for signaling the switcher I ordered plain old 3mm red. What I received was 3mm super bright LEDs. I was shocked at how bright they were. I can’t remember the final value of resistors I used to dim them to around a stop light brightness but it was quite a bit. They are driven by a function only decoder from track power. Lately I have been wiring some SMLEDs for headlights in my brass steamers. It takes 3.3 to 4K to make them look decent and the decoder dimming function work. A couple months ago I wired up a friends diesel loco ¶. He supplied the LED and 1K resistor for the headlight. It turned out to be too bright for the dimming function to work also. 2K seemed to work out good for that LED.

I too buy from the Shack. Its on my way home from work so I don’t have to make a special trip. One word of advice is to check them with your ohm meter before you instal them. One of my LEDs went POP and it turned out the resistor was packaged in the wrong package. I do not know the color codes of resistors and it never dawned on me to check. I now check them before I leave the store.

Pete

10

Pete, that is great advice, and it is something that I will do going forward. It is one thing to burn out an LED, it is quite another if that LED is part of a Tomar Industries double search light. That signal cost me $40, so I cannot afford to burn it out with an inadequate resistor.

When I first installed the signal using 1.5K resistors, the LEDs were so bright that I could not see the black shield that surrrounds the LED. As I stood there, looking down my layout, I noticed that I could clearly see the black shield on those signals with less bright LEDs.

It will now be my practice to check the value of each and every resistor before installing it.

Rich

11

Rich,

I don’t know why I didn’t think of this before, BUT years ago Radio Shack sold a conversion chart, a 41/2 inch long, by 21/2 WIDE device that had wheels on the top with square holes that show a color when you rotated the wheels, below the squares were circles, that displayed a number to correspond with the color,just dialing in the color in the squares,would give you the resistance of the resister or capacitor… faster than always measuring. Also used for inductor and tolerance of same… Like I said it was years ago so I don’t know if it still is available, But it wouldn’t hurt to find out, still have mine to this day… Sorry I can’t show a pic, cam is broke…

Cheers,

Frank

12

I have two of those that I purchased many years ago from Allied Radio Corporation. They are called a RESIST-O-GUIDE and were actually made by International Resistance Company. I believe both of those companies are long gone.

13

Frank,

I use this on line calculator for this purpose.

http://www.csgnetwork.com/resistcolcalc.html

Works real well.

Rich

14

Rich,

That works great… Din’t have home computers back in my day, LOL…

Cheers,

Frank

15

My platforms are 4’ long, each one has 20 to 50 LEDs on it, those things can eat up resistors in a hurry

LION, why not wire your LEDs in series? That would cut down on the number of resistors in use or even eliminate them if the proper number of LEDs are seriesed (is that a word?) together

16

Wiring LEDs in series saves not only on resistors, but the amount of wiring you have to do. This calculator will design your circuit for you no matter how many LEDs you want to use. You can never totally eliminate resistors altogether though.

Six two volt LEDs in series on a 12 volt supply will still require a 1 ohm resistor in the string.

Mark.

17

Frank mentioned the old RS resistor color code translator. Don’t know how many of you have smart phones but I do know that there a few really good IPhone apps available for that purpose The one I use is called “Resistor Code” I am not sure it is available any longer but you can either dial in the colors & get the value or type ina value & it will show the colors

18

Not a very good pic but here is a link to a photo of the RS device mentioned earlier http://stevenjohnson.com/cardboard/images/RS-res.gif

19

Somewhere I have one of those RS color wheel things. But there are numerous little ditties to remember the resistor color code by, unfortunately the only ones I know are not suitable to print here.

–Randy

20

I found this years ago.

http://members.optusnet.com.au/nswmn2/ResColour.htm

It has helped me identify resistors.

Pete