Comments on: Red LED Glows Brighter – Why?

By: Stan Arnold

Stan Arnold — Sat, 07 Dec 2019 15:32:40 +0000

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.

By: Bob Rimm

Bob Rimm — Fri, 06 Dec 2019 19:52:30 +0000

About LED’s of all colors. To get the optimum brightness out of an LED you must know the specifications on the specific LED. There are many manufactures of Light Emitting Diodes and the light output can be all over the place for the different sizes and colors from the same manufacturer. If you need the brightness to match, the best circuit would be to use a 555 single timer or a 556 dual timer circuit to match the brightness of a pair. If you go to “DigiKey Electronics” on the internet, you should find a catalog of parts and applications, with diagrams, for the 555/556 timers that will control an led.

Bob R

By: Doug

Doug — Thu, 05 Dec 2019 17:03:07 +0000

aaargh! why didn’t I see this article sooner? I used a single resistor on bi-colour LED’s on my control board, I should have used a resistor for each color from the other lead. Always learning! Great information though – THANKS!

By: Louis van Zyl

Louis van Zyl — Thu, 05 Dec 2019 05:51:57 +0000

When calculating a resistor value I use the Vansoft Model Railroad Calculator tool. I keep the LED current always about 12mA. The problem with the LED being brighter is the mcd value of the LED. The higher this value the brighter it will shine. When buying a LED make sure that this value is below 1000 as above 1000 is generally high bright and not what you want. I have been making a lot of signals and never had the bright red LED problem. See the image of the calculated resistor value and the tool is free from there website.

By: Phil

Phil — Thu, 05 Dec 2019 05:14:34 +0000

In reply to Frank B. Very good information, thanks.

By: Frank B

Frank B — Thu, 05 Dec 2019 01:59:28 +0000

Just need to correct Johan: small red, green, orange and yellow LEDs operate around 2V, white and blue around 3.3V.
I say “around”, because a very small change in voltage across an LED produces a large change in current, and thus in brightness.

Unlike the old incandescent bulbs, LEDs of different colours work at different voltages because they have different atomic structures.

So if you want the same apparent visible brightness, you will need to try out different resistor values to adjust it to your preference.
——————————————————————————————
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 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,

For testing, use the minimum value resistor plus a 1k ohm potentiometer (variable resistor) in series to adjust the brightness of an LED to your liking, 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.

By: Richard E Aubel

Richard E Aubel — Thu, 05 Dec 2019 01:47:05 +0000

Use a higher value resistor on the red LED’s and a lower value on the green and yellow ones. Simple trial and error will allow you to even the intensity for all LED’s.

By: Wes

Wes — Thu, 05 Dec 2019 01:36:03 +0000

In reply to Johan. You might wire a simple diode is series with the Red LED as diodes drop about 1.2 volts. I e used this with several signals effectively. Wes O

By: Dan

Dan — Thu, 05 Dec 2019 00:43:47 +0000

I made a connection error when I was installing led lights on my layout. Instead of connecting them parallel I accidentally connected one of them in series (a simple series/parallel circuit) which made that one different intensity. Check to make sure you didn’t make that same mistake.

By: Johan

Johan — Wed, 04 Dec 2019 23:56:04 +0000

For the same resistor, the red LED will be brighter as the voltage drop across the red LED is around 2.2V and for the green or yellow LED is around 3.1V. The current through the red LED is thus higher for the same resistor.