Electronics Led question

I need to dim a high brightness blue led , to the same level of brightness as some standard leds.

Gonna run them all from 5v , have the resistors worked out for the rest ,but not for this one yet.
Now the standard ones that i have are around about 2V , the blue 1 is 4.5v 20mA. I assume I must get the blue led down to 2v aswell to make it work right.

So I need a 150ohm right , 5-2/0.02 =150 . 2 as I wish to run it on 2 volts not 4.5. Then at that is will take 5-2/150=0.02.

Is that all correct will the led run ok at 2v even though it' meant to be four , are my calculations for what current it will take at 2v correct as i do not want to kill it as it was £1:25


Thanks

 

Nope, nein, no, nuh-uh.

First off, at 2v, a blue LED will not light period. There's a mininum voltage that all diodes need, in the proper polarity of course, in order to conduct. Not sure the exact figure for 3.7v blue LEDs, but I'd guess that it's more than 2.5v.

Second, LEDs do not decrease in brightness in a linear fashion in relation with current, so it's not a simple matter of half brightness = double resistance.

Basically, what you're going to have to do is get a potentiometer with a high enough resistance, probably multi-turn for better accuracy, and adjust it until you find the suitable level of resistance and use that.

Oh, and you do realize your typical red/green/yellow/orange indicator LEDs are diffused, not clear-lens like our LED? That'll affect how you percieve the light as well.

 

Well , where can I get a standard blue one. I been looking everywhere for one and cannot find one in the uk , only one usa site had one and I aint importing a single led.

Pretty sure I have a multiturn variable resistor still about from that knightrider led thing that is on the main page.

Also the leds are gonna go behind the xbox controller button , like on the tutorial on xbox-scene , just don't want the blue one being to bright compared to the rest.

 

Have you tired Maplin, or RS Electronics?

 

Maplin only have the high brightness blue ones on there site no normal ones ,I tried the maplin shop near me as they sometimes have some stuff not on the site , but they suck , did'nt even have any resistors in stock.

Tried http://rswww.com/ which is where I think you are on about and they had none. Tried a local independent shop and they only had the bright one.

 

Bummer.

I don't know any other places for LEDs, although I'm sure there must be more. There's always ebay.

 

tru sanding down the lens of the LED and go with the potensiometer idea... i guess that will be your best bet... they will NEVER get the same brightness anyway...

 

Try with about 1mA forward current (1k2 or 1k5 resistor on 5V), if that doesn't hit the spot use half or double the value. Leds will glow down to 0.2mA so ignore the 20mA on the packet, that's the equivalent of saying a car will do 140mph so you must always go at that speed.

ESR are good for leds, much cheaper than Maplin. They do blues 40-1000mcd, get the Optoelectronics pdf (and the Resistors one while you're at it).

 

Ok thanks for the help , I have it got it sorted. Using a 1650ohm Resistance ,the led is just about glowing and has 3.35V going through it and just about 1mA. then it glows around the same brightness as the others. So I will use a 1500ohm and 100Ohm in series cos you can't get a 1650ohm resistor and I aint got a 1600ohm

 

For future referance, if you look at the datasheet for the LED, there should be a curve showing the luminosity in MCD vrs current.

That way if you just match the MCD of all your LEDs they will be the same brightness.

Often thou, you can't get datasheets for the LEDs.
Gits.

 

And also for future reference, you can get great deals on all sorts of luminescent goodies at Ultra LEDs : Clicky

 

Thanks , nice site . Lots of choice.

 

Yep, if you buy in quantities of about 25+ then you get them dead cheap. I'm about to order some bl00 stuff from there so I'll post about whether they're any good...maybe....