Have i got my led's wrong ??

[collectors]

Hi, i have made up some simple red/green signal lights using some 5mm led's on 5 volts with both negatives connected to a single 150ohm resistor, leaving 5v going to one or the other positive led leads. On all of them, the green led is perfect, but the red led is noticeably dimer. Any suggestions as i assumed the red & green have the same values ??. Thanks.

 

[PhilP]

The forward voltage of you red LED, will be around 2.2V
For the green, could be as high as 3.4V

Take the 150 ohm resistor out, calculate and fit a resistor in each of the switched legs..

It will also depend on how efficient your LEDs are, we also 'see' the same lumem level of different colours, as different brightness.
You may need to adjust your resistor values, to make the perceived brightness match.

PhilP.

 

[The Shed]

 

[dunnyrail]

Always tempting to use the same resister for different LED’s, but as has been said not the wisest of things. Sheds diagram brill.

 

[collectors]

Thanks guys, mut admit i was getting a reading of 2.2v on the red with a 150 ohm fitted & 2.8 on the green with the same resistor. Both when running separately & across the led legs. What is the best voltage to look for? As this may explain the greens brightness.

 

[Cobalt6700]

When building signals for my layout I had exactly this issue, even with the right value resistors on each LED.

I bought a shed load of different LEDs in each colour to try and get it right. In the end, I found a seller on ebay selling LEDs for LED matrix displays. As you can imagine, if one of the 'pixles' on a display is brighter than the other, your going to notice.

These were the best brightness match I could find, only downside is they are slightly oval - I'm guessing this is to allow them to be grouped tightly together.

I'll update later with a link if you are interested

 

[collectors]

When building signals for my layout I had exactly this issue, even with the right value resistors on each LED.

I bought a shed load of different LEDs in each colour to try and get it right. In the end, I found a seller on ebay selling LEDs for LED matrix displays. As you can imagine, if one of the 'pixles' on a display is brighter than the other, your going to notice.

These were the best brightness match I could find, only downside is they are slightly oval - I'm guessing this is to allow them to be grouped tightly together.

I'll update later with a link if you are interested

Must admit i am not that happy with the led brightness in general & have ordered some 1 watt versions & will see how these fare. At least i can make them dimer, where i cant make the normal led brighter without shortening the life of the led. I m also wanting to see these from 30met away in the daylight.
I Might have to find a night time dimer. Possibly a photocell with an extra resistor that kicks in when the local light level drops to make them less bright in the evening..

 

[PhilP]

There are a number of things you need to consider..
You will never beat Old Sol!
A flat plate behind your 'lamps' will help. - Think traffic-lights.
A cowl over the lamp, as well.
The wavelength (and spread of wavelengths) emitted by your LEDs.
The efficiency of the LEDs. - Lumem value.

Once you start using high-power LEDs, you will need to consider the power rating of your resistors, and the fact that you will have heat to dissipate. - A black-painted lamp housing, in full-sun, containing an illuminated 1W LED, will possibly be running pretty close (exceeding?) it's upper rated operating temperature.

PhilP

 

[Cobalt6700]

1W LED's - crikey. Worth noting that you will need resistors to match, and likely heatsinks. Edit - PhilP makes some good points.

As for a dimmer - have a look at this: dimming led with photoresistor

You would likely need to dive a Transistor / MOSFET to deal with the current draw of a 1W LED - I would imagine around 300mA.

 

[collectors]

1W LED's - crikey. Worth noting that you will need resistors to match, and likely heatsinks. Edit - PhilP makes some good points.

As for a dimmer - have a look at this: dimming led with photoresistor

You would likely need to dive a Transistor / MOSFET to deal with the current draw of a 1W LED - I would imagine around 300mA.

If my calculations are right on the 1 watt? with a FC: 300-350mA & a FV: 2.2-2.4Va 9.1Ω resistor at 5v or a 30Ω at 12v. But will experiment with these to help with the heat. Its all part of the fun.
PS: Thanks for the photocell idea. I like the simple NO:2 version for a tight space.

 

[PhilP]

Calculate the wattage being dissipated in your resistors..
Double it, for the rating of resistor you buy. - They will run cooler, and give you a little peace of mind.

PhilP

 

[Greg Elmassian]

I read through this thread, nowhere do I see anyone telling the OP that LEDs work on current, not voltage (since they are really a low resistance short in one direction)

You need to buy LEDs with specifications, max current, and good ones will give you brightness...

You calculate the dropping resistor based on the specified current, usually 20 milliamperes or so... a PROPER LED resistor calculator also takes into consideration the NOMINAL voltage drop on the LED, since it affects the R=V/I calculation.

LED Lighting Basics

Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers

elmassian.com

If you are getting picky about brighness, then you need to do this right...

Greg