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Resistors for LEDs/Bulbs

This section has multiple topics.

1) CALCULATING RESISTANCE FOR A LED
2) 
LED CALCULATION EXAMPLES
3) CALCULATING RESISTANCE FOR A BULB
4) BULB CALCULATION EXAMPLES


CALCULATING RESISTANCE FOR A LED (For Bulb, see below)

UNLIKE A LIGHT BULB, A LED is VOLTAGE POLARITY SENSITIVE.  YOU MUST CONNECT THE LED CORRECTLY OR IT WILL NOT WORK.  You can actually damage the LED.

For more information about LED polarity, go here: LED Polarity

SIMPLE ANSWER

Use 1K (1000 ohm) 1/4W resistor for any LED.  Why?  Unlike bulbs, LED BRIGHTNESS DOES NOT VARY DRAMATICALLY with wide variations in current.  By extension, this also means a wide variations in the track voltage too will not vary brightness much.  1K is also low enough in current it is compatible with any size LED (around 10mA) over the DCC track voltages you will encounter.

COMPLEX ANSWER:

To do this calculation
 you need to know:

1) the track voltage of your layout.  Use 16V for O scale, 14V or HO, and 12V for N.

2) the LED current.   Y
ou cannot go wrong here if you use any current between 1mA and 15 mA.  15mA will offer you a much brighter LED than a 1mA current.   If you want to have the brightest light output possible, you must c
onsult the LED datasheet for your specific LED.
 If you want to learn more about LED current ratings, go here: LED Electrical Ratings

3) the typical LED Voltage drop.  This term also goes by the name of "LED Forward Voltage".  This is the tricky part since most LEDs you buy do not automatically come with published voltage drop rating.  With white & blue LEDs, you typically will have 3.3V voltage drop while any color LED will have about 2.2V drop.  
If you want to learn more about LED voltage drop ratings, go here: LED Electrical Ratings

Simple Equations are:

Resistor Ohm Value:    R(res) = (Vtrack - Vled) / Iled  The ohm value to drop the voltage.
Resistor Watt Value:   W(res) = (Vtrack - Vled) * Iled  The heat the resistor will generate.

It is possible to do a more accurate/complex calculations such as accounting for the decoder voltage drops and the actual track voltage, but the track voltage tolerances will make it a waste of time in the sense you will not see any difference in the LED light output.


LED CALCULATION EXAMPLES:

Here is a on-line calculator for LEDs: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
The calculator will work with one or multiple LEDs.  You will still need the information discussed above to input value into the calculator.

1 LED Example::  White LED rated at 3.3V and 10mA (0.01A) with a HO track voltage of 14V

R(res) = 14V-3.3V / 0.01A = 1070 Ohms....nearest 5% standard ohm value is 1.1K ohms.

The resistor will drop about 11V to give you about 10mA in the LED.

For a list of standard 5% resistor values, go here: Common Resistance Values

W(res) = 14V-3.3V * 0.01A = 0.107W Watts...nearest standard watt value is 0.125W or 1/8 watt.  But 1/4W will be easier to find.

For a list of standard resistor wattage values, go here: Common Resistance Values

(Note: The physical resistor watt rating MUST be equal or greater than the heat value the resistor will generate.)

2 LED Example:

If you have two leds you want to light up for the same function , wire them in series and design for a 6.6V 10mA LED.   However, it is best that they be 100% identical LEDs from the same manufacture to get even lighting between them!  Not all LEDs are created equal but the variation is not to bad.  There are 3 benefits of wiring of the LEDs in series.
1) The reduction of the heat generated in the voltage dropping series resistor.
2) It also keeps the long wire run between the LEDs and the decoder down to two wires.
3) It keeps the resistor solution down to one resistor.


CALCULATING RESISTANCE FOR A BULB
 (For LED, see above)

To do this calculation
 you need to know:

1) the track voltage of your layout.  Use 16V for O scale, 14V or HO, and 12V for N.

2) the bulb voltage.

3) the bulb current rating.  This is the tricky part since most bulbs you buy do not automatically come with published current ratings.  If you had a bulb watt rating, one could calculate the bulb current rating.  But watt rating is also not provided for the same reason as the current.  This leave you to two options:

a) Only buy bulbs with published current ratings.  Easy for Miniatronics buyers who DOES PUBLISH its bulb current ratings and are available at most hobby shops.  See Miniatronics here: http://www.miniatronics.com

b) Measure the bulb current rating.  This can be a bit tricky if you do not know what your doing. This involves using a DVM in DC current measurement mode, a regulated but adjustable voltage source with a voltage readout (or a second DVM).   Using a1.5V battery is OK for testing 1.5V light bulbs in go/no go test but not for accurately measuring the current.  Why?  The DVM in the process of measuring the current will add some series resistance in the circuit giving you a lower current reading than what the bulb will actually draw.  Furthermore this gets really bad if the battery is not FULL!

Simple Equations are:

Resistor Ohm Value:    R(res) = (Vtrack - Vbulb) / Ibulb  The ohm value to drop the voltage.
Resistor Watt Value:   W(res) = (Vtrack - Vbulb) * Ibulb  The heat the resistor will generate.


BULB CALCULATION EXAMPLES:

1 Bulb Example:  .Light bulb
 rated at 1.5V and 15mA (0.015A) with a HO track voltage of 14V

R(res) = 14V-1.5V / 0.015A = 833 Ohms....nearest 5% standard ohm value is 820 ohms.  

For a list of standard 5% resistor values, go here: Common Resistance Values

The resistor will drop 12.5V to give you 1.5V accross the bulb.

W(res) = 14V-1.5V * 0.015A = 0.187W Watts...nearest standard watt value above is 0.25W or 1/4 watt.

For a list of standard resistor wattage values, go here: Common Resistance Values

(Note: The physical resistor watt rating MUST be equal or greater than the heat value the resistor will generate.)

2 Bulb Example:

If you have two 1.5V bulbs you want to light up for the same function , wire them in series and then use the above equation but design for a 3V 15mA bulb!   However, make sure both bulbs are 100% identical bulbs from the same manufacture.  Why?  To get even lighting between them because not all 1.5V bulbs are created equal.  Been there done that.  

There are 3 benefits of wiring of the bulbs in series.
1) The reduction of the heat generated in the voltage dropping series resistor.
2) It also keeps the long wire run between the bulbs and the decoder down to two wires.
3) It keeps the resistor solution down to one resistor.