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LED Vs Bulbs

There are several topics here:

1) LED vs BULB.  Which should I use?
2) UNDERSTANDING THE ELECTRICAL DIFFERENCES BETWEEN BULBS AND LED's
3) BRIGHTNESS CONTROL
4) HOW MUCH DOES THE BRIGHTNESS VARY WITH VOLTAGE?
5) CHART OF LIGHT OUTPUT VS TRACK VOLTAGE
6) DO I NEED A REGULATED POWER SUPPLY?
7) 1.5V VOLTAGE REGULATOR OPTIONS


LED vs BULB.  Which should I use?

Do not waist your time with bulbs today.  GO WITH LEDS!!!  Even if you derate the bulb voltage to extend their operating life, they will still burn out sooner or latter.  In other words, the locomotive you just put the new bulbs into will soon become a project again.   LEDs will not burn out.  What would you rather do with your time and money?

There is on exception.  Firebox flicker for a steam locomotive.  The problem here is the limited LED light diffusion/dispersion and lesser so in terms of subjective color.  In terms of diffusion, a bulb is omi-directional in it's light output and this optical property is needed in the tight places to physically locate the bulb to get it to look right.


UNDERSTANDING THE ELECTRICAL DIFFERENCES BETWEEN BULBS AND LED's

BULBS:

Bulbs are voltage based devices which means you MUST limit the voltage across them.  The bulb cannot do that itself.  If you have no control of the voltage, there is no GUARANTEE you will not burn out the Bulb.  Hence Bulbs are rated for a voltage to achieve a certain design life goal and brightness level.  It is assumed the power source has enough current to run the bulb.  Raise the voltage above the rating and, yes your right, the bulb will get brighter but watch your bulb quickly burn out.   Hence we talk about 1.5V, 12V, 14V bulbs which are the designed voltages of the bulb for reasonable service life.  When you run the bulb from a power source who's voltage rating it the same as the bulb's, the current rating is never an issue and hence normally not discussed.  Why?  There is no external resistor involved nor needed. 

LEDS:

LED are current based electronic devices which means you MUST limit the current flowing through them.  The LED cannot do that itself.  If you have no control of the current, there is no GUARANTEE you will not blow up the LED.  Hence LED's are rated in terms of current for a certain design life goal and brightness.  It is assumed the power source has enough voltage to run a given LED.  Raise the current above the LED rating and LED will get brighter but also watch your LED quickly burn up.  Hence we talk about LED currents in the 2mA up to 20mA range which are the common designed currents of LED's for reasonable service life.  When you run the LED from a power source who's maximum continuous current rating it within the LED limits, the power source voltage rating is never an issue and hence normally not discussed.  Why?  There is no external resistor involved nor needed.  

How does this apply to running trains?

With trains, we ALWAYS run the LEDs from a power source (DCC Track Power) who's voltage is greater than the LED's.  Higher voltage will FORCE more current into the LED than what the LED will want.  Hence a resistor is needed.  The same cannot be said for bulbs IF one uses a 12 to 14V bulb.  However in smaller scales like HO, often for both heat and physical size reasons, we replace those bulbs with smaller and lower power versions.  The smallest bulbs that are often used for HO headlights (Grain of Rice size) are most commonly available in 1.5V.  Clearly a 1.5V bulb is a much lower in voltage that what is used on the track.  Hence a resistor is needed.

SUMMARY:

In both cases, Bulb or LED, you use a resistor to deal with the excess voltage in the bulb case or the excess current in the LED case.  In the bulb case, we use the resistor to drop the excess voltage that is above the rating of the bulb.  Hence the term "voltage droping resistor".  In the LED case, we use the resistor to reduce the excess current that is above the rating of the LED.  Hence the term "current limiting resistor".


BRIGHTNESS CONTROL

How bright is bright enough or prototypical?  That is up to you and is subjective.  If you want the headlight to trully work and light up the track, then go for brightness.  If you want something that looks correct at a distance, then a dimmer may be the answer.

REDUCING THE BRIGHTNESS WITH A RESISTOR

In the case of the Bulb, were are talking about the voltage dropping resistor.
In the case of the LED, were are talking about the current limiting resistor.

One can use the series resistor to vary the brightness.  But it should only be done to REDUCE the brightness and not be used to increase it beyond the respective "maximum ratings" of the Bulb or LED.

INCREASING THE BRIGHTNESS 

Compared to a 1.5V bulb, a white LED can put out a lot more light.  In fact they can be too bright.  Hence if the 1.5V bulbs your using are not bright enough, you only real solution is to replace the bulb(s) with LED's


HOW MUCH DOES THE BRIGHTNESS VARY WITH VOLTAGE?

The chart to the left gives you a indication of the relative sensitivity of light output relative to changes in track voltage for both a bulb and a LED.  The LED has a series resistor to limit the current allowing us to do a voltage comparison test.

Looking at the chart, here are some simple results to remember.

Track Voltage Drop Vs Light Output (brightness) loss:

5% drop in voltage   =>  LED = 5%  Loss,  Bulb = 20% Loss in brightness
10% drop in voltage =>  LED = 10% Loss,  Bulb = 35% Loss in brightness

This shows how: 

1) sensitive a Bulb brightness is to small variations in track voltage.  
2) insensitive a LED brightness is to small variations in track voltage. 

SIMPLISTIC BULB VERSUS LED CONCLUSION:  

1) A light bulb lumens (brightness) falls off a factor of 3.5 times faster compared to that of a LED for the same voltage drop in track voltage. 

2) When running from a loosely regulated track voltage and you want the light source to have relative constant brightness, use a LED.

Technical Discussion: Detailed Brightness Comparison:

LED light output level IS NOT very sensitive to current.  It brightness is linear with the current flowing through the LED.  Cut the current in half and you will cut the brightness in half.  That is not true for a Bulb.  It is much more sensitive.  For LED's, I have some LED brightness versus LED current curves here: LED Electrical Ratings

Arbitrary Example:  A 20mA, 1 Lumen LED will see it current vary by 10%.  10% of 20mA = 2mA.  So the LED will run on 18mA.

Equation: Lumens (Low Current) =  (Low Current/Nominal Current) x Nominal Lumens.

Lumen (Low Voltage) = (18mA/20mA) x 1 lumen = 0.9   So your lumen output drop from 1 down to 0.9.  That is a difference or loss of 0.1 lumens light.  0.1 Lumens/1 Lumens = 10%.

Since the current through the LED is a directly proportional to voltage drop across the resistor (Ohms Law), the % changes in track voltage is the same % change in LED current.

LED Conclusion: If the DCC track voltage varies down by 10%, your light out from a bulb will go down by 10%

Bulb light output is IS very sensitive to voltage.  Simplistically, the bulb brightness (measured in Lumens) is very non linear with the voltage applied to the bulb.  The equation of light change versus voltage is given as follows:

Arbitrary Example:  A 1.5V, 1 Lumen bulb will see its voltage vary by 10%.  10% of 1.5V = 150mV.  So the bulb will run at 1.350V.  Bulb are thermalistic devices where the thermal temperature or "heat" level of the filament determines the light output and color temperature.  Stated another way, the lamps are sensitive to amount of watts they are allows to consume.  Unfortunately the resistance of the filament is not constant adding another complex variable.  However there is an equation that gives a good first order approximation of the change in light output as a function of lamp votlage.

Equation:  Lumens (Low Voltage) =  (Low Voltage/Nominal Voltage)^3.51 x Nominal Lumens.

Lumen (Low Voltage) = (1.35V/1.5V)^3.51 x 1 lumen = (0.9)^3.51 = 0.69.   So your lumen output drop from 1 down to 0.69.  That is a difference or loss of 0.31 lumens light.  0.31 Lumens/1 Lumens = 31%

BULB Conclusion: If the DCC track voltage varies down by 10%, your light out from a bulb will go down by more than 30%


DO I NEED A REGULATED POWER SUPPLY?

Simple Answer:  No.  Compared to DC powered layouts, DCC system have a relatively regulated (constant) track voltage which allows us to have constant lighting in the first place.  From a functional point of view, there is no real need to persue any form of voltage regulation for a bulb or LED if there is enough voltage headroom to keep them both operating.  The lights will always be on.

Complex Answer:  It depends on your specific needs or goals.

Among DCC systems, there is still some track voltage variation.   Typically the maximum "no load" track voltage is constant.  But as more engines run, the voltage sags easily 10%.  Some DCC boosters brands do a better job of track voltage regulation under load than others.   LEDS:  Given a LED's relative insensitivity to changes in brightness due to minor variations in track voltage still says there is still no need for regulation.  BULBS: For bulbs it is a bit more subjective depending on your lighting goals.   With a 10% variation in track voltage depending on the what running on it, you can get a +30% variation in bulb light intensity going up and down even if the engine is not moving.   Is this unacceptable to you?  If yes, then yet you can benefit from tighter voltage regulation power source to run the lamps.  


1.5V VOLTAGE REGULATOR OPTIONS

Decoder Based:  Some brands and/or models of decoders offer a onboard 1.5V output for 1.5V bulbs.

Ready To Install and Run:  TCS has a 1.5V voltage regulator module in the shape of a very small decoder.  It is called the VR 1.5.  The VR 1.5 can be used with any DCC decoder if you know where the ground and +12v are located on the decoder.  http://tcsdcc.com/

For more information about locating the ground connection of a decoder, go here: http://www.members.optusnet.com.au/mainnorth/alive.htm
It is the same negative connection that the "Stay Alive" circuit needs to use on a Decoder.

Build your own:

For those decoders do not, Don Fiehmann has written an Bulb article that includeds information on how to build two difference type of 1.5V regulators that is compatible with DCC decoders.  


Here are two regulator circuits shown in that article reproduced here for reference.

1)  This circuit is based on the fact the silicon diodes have about a 0.7V to 0.75V/diode.  It based on the old DC trick of using two series diodes to create a 1.5V supply when wired in series with the motor.  In this case, it is being driven by a Function output.

The value of the current limiting resistor will vary with the current rating of the 1.5V bulbs.  Normally the resistor value is based on the bulb current.  However for this circuit to work, you need to design for a current that is 1.2 times the rated current of the bulb(s) to allow the diodes to clamp the voltage at 1.5V.

Unlike the recommendation of placing the bulbs in series, this circuit only works by placing the bulbs in parallel and adding up the current rating of each bulb to be powered.

Equation:  (Vtrack - 1.5V)/(Ibulbs * 1.2)