LED Question - DOM?

DaveB · 29-07-2008, 13:55

Getting an LED tonight to wire in as a shiftlight that outputs from my Apexi. The one I'mlooking to buy (from maplins) has a brightness of min 8 and max 20.

Is that 'bright' ? lol

Dom · 29-07-2008, 16:42

Depends what the 8 and 20 are

If its mcd then its not that bright
If its CD then its pretty bright

Viewing angle is quite often more important than the brightness to be honest with you. If its a very narrow angle LED (10 degrees or less) then you're going to have to be in the right position to view it.

MSG · 29-07-2008, 17:01

yup I cd (Candle) is equal to 1 million micro candles mcd

hence 8 mcd is goiung to be ok if used for just indication purpose in a shadow or indoors but may not appear lit in bright sunshine environment, whereas if Maplin are selling very high brightness LEDs with their light output measuerd in CD, they are used for lighting purpose and not for indicating some status. There are very few or extremely rare if LED's which come in power levels measuring in CDs. The brightest one that I havfe come across was a 50,000mcd. and it was a white LED for making LED torches. This sort of LED projects a blinding narrow beam of bright white light usually around 8 to 10 degrees viewing angle.

DaveB · 30-07-2008, 07:37

Light output min.@ 12V: 12mcd
Light output typ.@ 12V: 30mcd
Viewing Angle: 60deg

Would that be OK for a shift light? Should I get 3 or something

What's 'bright' then?

DaveB · 30-07-2008, 07:39

oooh, they do these ones:

Light output min. @ 20mA: 9000mcd
Light output typ. @ 20mA: 12000mcd

I'm guessing that'd be OK? LOL What size resistor would I need so 12v didn't kill it

cambridge · 30-07-2008, 08:03

12-2 (ish) = 10V

V=IR
R=V/i
R = 10V/20ma
R = 10/0.020
% = 500 ohms... or there abouts

Dom · 30-07-2008, 08:05

Need to know the forward voltage but if you look at www.picbits.co.uk/forums under the tech articles I've got a guide on calculating LED resistor values.

cambridge · 30-07-2008, 08:19

I went with 2v as thats what I always use and sees me well

DaveB · 30-07-2008, 08:34

Forward voltage is max 4v and 100ma

So:

13v(ish) - 4v/0.1 = 0.325

is that ohms? lol

cambridge · 30-07-2008, 11:43

325 by the looks of it... link to spec

DaveB · 30-07-2008, 11:57

http://www.maplin.co.uk/module.aspx?...20752&doy=30m7

Thanks

Dom · 30-07-2008, 11:59

I wouldnt go by the max ratings - go by the normal ratings.

I.e. if Vf=2.8v (Vf max = 4v) then always go for the typical value

Vf @ 4v and If @ 100mA is very high and may only be specified for pulse rating.

Dom · 30-07-2008, 12:02

Looking at those specs I'd say go for a Vf of around 3 volts for a 4v max device and 20ma max.

14v - 3v = 11v
11v / 0.02 = 550 ohms

Give it a go with the nearest resistor you can get to 550 ohms and see how it looks.

DaveB · 30-07-2008, 12:03

ok thanks. I hate going to maplin to buy stuff. Too many geeks

lol