Light Emiting Diodes

[GuitarGas]

Hey guys, I'm looking at buying a bulk lot of LED's.
Here is a listing on eBay that I might go for???
http://www.ebay.com.au/itm/100pcs-Mix-3mm-Blue-Red-Yellow-Green-Round-Light-Emitting-LED-Diode-Assorted-Kit-/371247279872?pt=AU_B_I_Electrical_Test_Equipment&hash=item56700fef00
Does it matter what voltage and current they are?
The ones listed are...
Working voltage 1.8-2.2 (V)
Current: 20 (mA)

Cheers

[Frank_NH]

Those look about right.  3mm red leds usually measure around 1.6V forward voltage for me and go up from there through the spectrum of colors to about 2V.  I use the waterclear type for on/off LEDs for my 3PDT switch, but diffuse are good for clipping in circuits.

[GuitarGas]

Thanks Frank, do you use the 3 or 5mm?
So the forward voltage doesn't really matter when using for the on/off indicator because of the resistor in front of it?
Here are some white 5mm with a forward V of 3.2~3.4 would these also be ok? and what size resistor would you put in front?
http://www.ebay.com.au/itm/100Pcs-5mm-White-Ultra-Bright-LED-Light-Lamp-Emitting-Diodes-15000MCD-BY-/221610327882?pt=LH_DefaultDomain_15&hash=item3399013f4a

Cheers

[Heath]

I usually go with about 2k-2.7k for diffused and around 3.9k - 5.1k for waterclear/brights.

A good starting point is 2.2k and 4.7k

[Frank_NH]

I use 5mm clear for on/off indicators and 3mm diffused for clipping (but have used 5mm - the size doesn't make much difference).  You can calculate the resistor value required (to keep the LED from burning out!) knowing the source voltage and LED forward voltage drop and current.  For example, let's assume a forward voltage of 1.6V and typical current around 2 mA  (check the data sheet for your LEDS!).   For a 9V source, and using Ohm law, we have:

R = V/I = (Vsource - Vf)/If = (9 - 1.6)/(2xe-3) = 3.7K ohms

Remember that the indicator LED will drain power from your supply, so using the lowest current while maintaining a reasonable brightness is the way to go (less current = less bright).  Given that fact, and that LEDs vary in forward voltage from 1.6 - 2.0 or more volts, a current limiting resistor (CLR) value of 3.5K - 5K will work fine.  I tend to use 4.7K or 5.1K depending on what I have available, and the light seems bright enough.

Hope this helps.

[GuitarGas]

Thanks Frank,
I'm not totally sure on the maths you mentioned but I will try to get my head around it
I did order the ones listed above, thought I'd give them a go.
Cheers

[Muadzin]

Lately I use 1K5 because I like my indicator LED's to be really bright. Anything that will cut through the lighting mix that light techs seem to love. I've even gone as low as 470R for a while on the account of getting a whole bunch of them with an eBay LED purchase.

[GuitarGas]

Ok, so I bought the white LEDs I linked to above and I just wanted to know what resisters I need?
The led I measured with my ultra accurate $25 multi meter measured between 3.9 and 4, i'm assuming that would be volts?
Using a 9v power supply would I need a 5k resister?
Cheers

[Beaker]

Frank's maths is sound, so anything over 1K resistor will be fine.

It's less a case of "what do I need", and more a case of "what do I want" with LED resistors.

The best way to work out the perfect brightness for you, is to make up a little temporary jig.

Simply connect a battery to a battery snap, and the snap leads to a 10K pot, or trimpot. (legs 2 and 3). Touch the LED leads to the terminals, and turn the pot to change the brightness.

When you find the ideal brightness level, take the LED off and measure the resistance across the pot. Whatever it reads e.g. 4.55K use the nearest standard value - in this case 4.7K.

Honestly I have used everything between 470R to 10k for my 3mm superbright blue LED's, so there is no right or wrong value on this. (Unless you go too low and blow the LED).

[GuitarGas]

Cool, thanks Beaker, i'll give it a go.