How To Apply LED Formula to Multiple LEDs?

[lgreen]

I'm interested in powering multiple LEDs from a voltage source. I'm aware of the LED formula provided a while ago here. To restate:

the formula is R=(E-Vf)x1000/I where R is the series resistance in Ohms, E is the supply voltage, Vf is the LED's forward voltage (typically 2V), I is the LED current in mA

so for a normal, 5mm LED for example, running of +22V you need 1KOhm

for an ultra bright blue LED, Vf=3.8V, you need 910Ohms, using 1K is fine.

25V would give 1060 use 1.1 or 1.2K
32V would give 1410 use 1.5K

simply use the formula and then the nearest, HIGHER, common value

i.e. 800=820, 910=1K, 1100=1200 etc (@10%tol, less tol will provide more divisions and a closer match - but again, that's totally unnecessary)

My question is how does one apply this to multiple LEDs that are to be connected in parallel? Do you Add all the Vf's, I's etc... or is there some other way? Do you use as many resistors as LEDs or can you use one resistor and power them all?

[Christer]

For parallel LEDS, divide R by the number of LEDs. This this will
nog guarantee equal current sharing, though, unless you match
the LEDs. If you are running them well below max current it
should be no problem. If you insist on equal current sharing,
let each LED have its own resistor.

If supply voltage permits, you may instead connect the LEDs
in series, in which case you just let Vf in the formula be the
sum of Vfs for the LEDs, or in other words, the Vf of one LED
multiplied with the number of LEDs.