Welcome, Guest. Please login or register.
Did you miss your activation email?
August 06, 2020, 04:15:35 AM
Home Help Search Login Register
News: Please read the Forum Code of Conduct   >>Click Here <<
+  Bachmann Message Board
|-+  Discussion Boards
| |-+  General Discussion
| | |-+  resistor in series with light bulbs in DCC?
« previous next »
Pages: [1] Print
Author Topic: resistor in series with light bulbs in DCC?  (Read 3479 times)

View Profile
« on: June 21, 2009, 03:02:56 PM »

Hi guys,
I had one more question. I read somewhere that I need a resistor in series with the present light bulbs of my analog loc when I change a loc from analog to DCC. I cannot find anymore where I read this.
Is this true? and what value of resistor do I need?
Yampa Bob


View Profile
« Reply #1 on: June 21, 2009, 03:23:30 PM »

It depends on the age and circuitry of your locomotives. Older "DC only" locomotives usually have 12 volt grain of wheat bulbs that don't require current limiting resistors. If the lights are directional (headlight or taillight that are lit in the direction of travel) there is a small circuit board (light board) with 2 diodes.

If the locomotives are "DCC ready" with a socket to plug in a decoder, then two conditions might exist: 1. The bulbs are 12 volt, again no resistor required, or 2. The lights are LEDs, in which case current limiting resistors have already been installed on the printed circuit board  by the factory.

Some modelers prefer (or find it necessary) to remove the board and "hard wire" the decoder. In this case if the lights are LEDs, then resistors are required. Most LEDs require a minimum 560 ohm resistor in series to limit the current to 20 milliamps (.02 amp)

This page has lots of information about lighting and LEDs:

More information with helpful pictures. At the bottom are links to other pages of electronic information:

A close visual examination of your lights will determine if they are GOW (grain of wheat incandescent) or LED (Light emitting diode), refer to pictures on the above page. Suggest adding above links to your favorites folder for reference.

I might also mention that a handy item for electrical testing and troubleshooting is an inexpensive digital multimeter. Here is one at a great price, every modeler should have at least one.

TIP FOR ALL. Here's a handy way to test (out of circuit) LEDs with the digital multimeter. Set the dial to "Diode", symbol looks like a small black arrow). Touch probes to the LED leads, observing proper polarity, and LED should glow brightly. The diode position provides 3 volts dc from the internal battery. I always connect the probes both ways in case I have made a mistake in polarity.

Easy formula to determine resistor value:  Supply voltage (12 volt typical) minus LED voltage (3 volt typical) divided by .020 (LED maximum current typical) equals resistor value.

12 - 3 = 9 / .02 = 450.....standard resistor value is 470, suggest 560 for safety.

« Last Edit: June 21, 2009, 04:56:00 PM by Yampa Bob » Logged

I know what I wrote, I don't need a quote
Rule Number One: It's Our Railroad.  Rule Number Two: Refer to Rule Number One.
Jim Banner

Enjoying electric model railroading since 1950.

View Profile WWW
« Reply #2 on: June 21, 2009, 05:49:13 PM »

Bob has given you most of the needs for resistors with lights but there is one more you need to know about.  Decoders have a maximum current and maximum peak current rating for their function (lighting) outputs, just like they do for their motor outputs.  The maximum current is usually specified for individual functions and again for all functions combined.  It applies to the average operating current when the functions are turned on.  The maximum peak current is usually specified for individual outputs and is the current that must not be exceeded at any time.

With LED lighting, the peak current is about the same as the average current.  However, with regular incandescent lights, the current at start up is higher, much higher, than the average current.  This higher current is known as inrush current.  Typically for small bulbs like grain of wheat or grain of rice bulbs, inrush current is about ten times higher than average current.  This means that a bulb rated at 50 milliamps will draw a peak inrush current at the moment you turn it on of about 500 milliamps.

Now lets suppose your decoder is rated at 100 milliamps maximum per function, 500 milliamps peak per function.  (Not all of these ratings are always given but maximum peak current is usually about 5 times maximum current if they don't tell you otherwise.)  that 50 milliamp bulb is just fine, it draws less than 100 milliamps on the average and just on the maximum 500 milliamps at start up.

But suppose your local hobby shop is out of 50 milliamp 12 volt grain of wheat bulbs and all they have in stock are some 75 milliamp ones.  The kid behind the counter may try to tell you that grain of wheat bulbs are grain of wheat bulbs and you can use any one you want.  Don't believe him.  The manager may tell you that his 75 milliamp bulbs are well within the 100 milliamp rating of the decoder.  Don't believe him either.  Because you now know that the 75 milliamp grain of wheat bulb they are trying to sell you is going to draw an inrush current of about 750 milliamps at start up, and that is over, at least 50% over, the decoder's maximum current rating.

You have three choices at this point:

- You could try to get some 50 milliamp bulbs.

- You could take a chance.  At 50% overload on the peak current, the decoder will probably survive and you will be happy.  It may survive the inrush current 10, 100 or even 1000 times and you will continue to be happy.  But damage from intermittent overload is cumulative.  And the day will come when the decoder fails, and you will no longer be happy.

- You could avoid the problem by adding a resistor in series with the incandescent bulb.  A 24 ohm resistor would limit the inrush current to 500 milliamps no matter how much current the bulb tried to draw.  And by the time the bulb has warmed up and its current has settled down, the voltage loss will be less than 2 volts at 75 milliamps, so you bulb will still be nice and bright.  Checking the specifications on an actual decoder, the popular Digitrax DH123 has 125 milliamp function outputs and Digitrax recommends using a 22 ohm, 1/4 watt resistor in series with incandescent bulbs drawing more than 80 milliamps.

If you have made it this far, you now know why you sometimes have to use a resistor with an incandescent light bulb, even if the bulb seems to be within the limits of the deocder and even works, at least initially.  And it will not be you crying the blues because your lights used to work just fine but now suddenly they won't work at all, even after you replaced the bulbs.

One last comment - all too often, you go to the hobby shop to buy some bulbs but none of them are marked with current rating and none of the people so anxious to sell them to you has any idea.  So how is a guy to apply his new found knowledge?  Simple - use a meter to measure the current yourself.  Then you will know the answers, and in fact, will know even more than the guys at the hobby shop know about the subject.  Bob has reccommended a suitable and affordable meter above, or you may find one locally for a reasonable price and save on shipping.


Growing older is mandatory but growing up is optional.
Yampa Bob


View Profile
« Reply #3 on: June 21, 2009, 07:59:27 PM »

Thanks Jim
An easy way to illustrate "inrush current" is to consider an ordinary 100 watt incandescent light bulb.  If we divide 100 watt by typical 110 household voltage, we get .9 amp current draw, However that is the current draw for a warm (actually hot) bulb when lit.

I measured the cold resistance of a 100 watt bulb, about 10 ohms. 110 volts divided by 10 ohms equals 11 amps of current, but only for a fraction of a second until the filament heats up. That innocent appearing bulb can suddenly become a 5000 watt "detonator" at peak inrush current.

Now you know why a light bulb usually blows when you first flip the light switch, with a "pop" that will scare the you know what out of you. I've seen some trip a 20 amp breaker, and actually explode, which is why you should never use open bulbs in a potentially explosive environment such as a lighted garage. Ever walk into a closed garage and smell gasoline vapors from a lawn mower, or from gas cans not tightly sealed?  We keep all mowers and gas cans in an unlighted storage shed.

I know some modelers may say: Multimeter? I don't need no "steenking" multimeter!  That's fine, just plan on stocking up on decoders and bulbs because sooner or later Mr. Murphy will pay you a visit.   Cheesy

FWIW, "Murphy's Law" consists of two parts.
1.  Anything that can happen will happen.
2.  Anything that can be screwed up by someone will be screwed up.   Cool
« Last Edit: June 21, 2009, 08:45:35 PM by Yampa Bob » Logged

I know what I wrote, I don't need a quote
Rule Number One: It's Our Railroad.  Rule Number Two: Refer to Rule Number One.
Stephen D. Richards

View Profile
« Reply #4 on: June 21, 2009, 08:48:54 PM »

All,  FYI, in case you're interested, Kalmbach has a book out DCC for modelers or something of that nature.  I'm not at home and can't remember the name.  It has a tester for registering resistance of the various light sources, if you would like to build one.  I am in the process on building one from their suggestions but have expanded mine for 1/2 watt resisters.  Essentially you would connect a decoder of choice to the tester and a light bulb/LED of choice and run through the resisters to see which one is right for your decoder.  Really a neat gadget and fairly easy to build for the average modeler with a little bit of electronic knowledge.  Just in case anyone is interested.  Stephen
Stephen D. Richards

View Profile
« Reply #5 on: June 21, 2009, 08:54:08 PM »

OK, I looked it up.  The book is titled "DCC Projects and Applications"    Stephen
Yampa Bob


View Profile
« Reply #6 on: June 21, 2009, 09:22:54 PM »


Thanks for the reminder.

I have that book (#12407 by Mike Polsgrove) great projects and information. A "must have" for every DCC modeler.  I also have the following companion books:   Kalmbach #12412 Basic Wiring for Model Railroaders by Rick Selby, #12207 Easy Model Railroad Wiring by Andy Sperandeo, and #12417 The DCC Guide by Don Fiehmann.

The four books have everything "you wanted to know but were afraid to ask". 

For those who prefer "ready made", I have been suggesting this handy resistor substitution box, has many uses. Grab a bag of miniature crocodile clip jumper wires from Radio Shack for test connections.
« Last Edit: June 21, 2009, 09:42:21 PM by Yampa Bob » Logged

I know what I wrote, I don't need a quote
Rule Number One: It's Our Railroad.  Rule Number Two: Refer to Rule Number One.
Joe Satnik

View Profile WWW
« Reply #7 on: June 21, 2009, 10:49:01 PM »

Build a lamp resistance tester
Model Railroader, February 2002 page 88
Match headlight bulbs and resistors to DCC decoders

If your loco is too heavy to lift, you'd better be able to ride in, on or behind it.
Stephen D. Richards

View Profile
« Reply #8 on: June 22, 2009, 09:08:22 PM »

Bob, I have those other books as well!  Darn, I did the same basic thing with my resister sub box as the link you posted shows!  Thought I was being inventive!  lol  However, I did put a four post spring loaded connect strip on mine for easy connectins with the light bulbs/LEDs.  The alligator clips keep jumping out of my hands!  lol     Stephen
Pages: [1] Print 
« previous next »
Jump to:  
Powered by SMF | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!