Everything on model trains, model railroads, model railways, locomotives, model train layouts, scenery, wiring, DCC and more. Enjoy the world's best hobby... model railroading!

Make Your Own Realistic Model Train Buildings

Voltage Amperage Ranges With Dropping Resistors?

This question is from Ray J:

“What voltage and amperage rating would I need for the indicator LED’s on the control panel I am building for my O scale setup? My trains are quite old. I know the switches use a standard voltage, but I heard LED’s need specific voltage / amperage ranges with dropping resistors? Sounds complicated?”

Add your comment and view the comments below.

One Response to Voltage Amperage Ranges With Dropping Resistors?

  • Frank Bsays:

    How to Calculate Resistor Values for LEDs

    Ordinary small 3mm & 5mm coloured LEDs (red, green, yellow,
    orange) run at about 2V, but white and blue LEDs run at about 3.3V.

    When using a supply higher than their nominal voltage, LEDs require a resistor connected in series with them to control the current.
    (Without the resistor, the LED will burn out immediately.)

    LED brightness is determined by the current, normally up to about 30 mA maximum for 5mm LEDs.
    LEDs are polarity sensitive, so they must be the right way round. The long lead is positive.

    To calculate the required resistor for a particular supply voltage:
    LED Voltage + Resistor Voltage = Supply Voltage

    For example: To run one white LED (3.3V) from a 12V supply,

    Resistor voltage will be: 12V ─ 3.3V ═ 8.7V
    Resistor value will be: 8.7V ÷ 30 mA ═ 290 Ohms
    So we would use a 270 Ohm resistor (minimum value).
    (As only certain values are made, close to calculated value is ok.)
    A higher value resistor will reduce the brightness.

    To adjust LEDs to your liking, use the minimum value resistor plus a 1k ohm potentiometer (variable resistor) in series, twiddle to the desired brightness.
    Measure the total resistance, resistor plus potentiometer, with a multimeter, then find a resistor (or two in series) to approximate that value of resistance.

    You can buy very cheap potentiometers on Ebay, so you can set one to about the needed resistance, leave it in the circuit and tweak it slightly as required.

Leave a Reply to Frank BCancel reply

Your email address will not be published. Required fields are marked *

Add a photo or image related to your comment (JPEG only)

Use Tiny Railroad Micro Controllers

Model Train DCC HELP


Submit Your Model Railroading Questions!

Before you submit your model railroading question please add some feedback, answers or comments to other postings on this model train blog. What goes around comes around... so if you can help others in the hobby, someone else may help you.

Important - Please add plenty of supporting details to any question you submit (eg. scale, solutions you have already tried etc.) , as the clearest and best questions usually get the best answers. Also, please check your spelling and punctuation as all questions need to be approved by the blog moderator prior to publication. Approved questions are normally published within a week (if not sooner).

Submit your model train questions here.

Download Your Free Catalog

N Scale Track Plans

Watch Video

Model Train Help Ebook


FREE Tour Inside Club

Take a FREE tour inside the club.

Scenery Techniques Explained

Scenery & Layout Ideas

Model Railroading Blog Archive

Reader Poll

Sorry, there are no polls available at the moment.