Mike said:
Thanks. It's getting clearer.
How did you know to use the 1/2 watt resistor and if my formula changes
(more LEDs), how do I come up with that?
Here's the computation I used: 10 LEDS at ~1.8V per is 18
volts. Your supply is 27 volts. The difference is 9 volts
which must be dropped in the resistor. With a 330 ohm
resistor, current will be over 27 mA to drop 9 V,
( current = 9/330 = .02727 )
and the power dissipated will be > .245 W
( power = 9*.0272 = .24545 )
That's just a bit under a 1/4 watt. A safety margin in heat
dissipation is always advisable, so you go to the next higher
standard wattage, which is 1/2 watt. The formulas:
I = (SupplyV-Total_Vf)/R
where Total_Vf is the forward voltage drop of one LED
times the number of LEDs in the string; R is the resistor
value in ohms, and I is the current in amperes.
P = E*I
where P is the power to be dissipated in the resistor; E
is the voltage across the resistor; and I is the current
determined in the first formula.
If you increase the number of LEDS and do not change the
supply voltage, just use 1/2 as the wattage figure and
you'll be fine on that account. If you change the supply
voltage, you must do the computation.
And when you bumpbed the resistor
from 330 to 470, how did you know that? Is it that they don't make any
other ohm resistor in the middle?
You can get resistors between 330 ohms and 470 ohms. But those
two - 330 and 470 - are the most commonly used standard sizes
in that range, and are available at Radio Shack if that's
where it's convenient for you. Sizes between those values are
not available there.
In a general project, a good target for current through a LED
is usually considered to be 20 mA. The 470 ohm resistor will
give you 19.1 mA, while the 330 ohm will give you 27.27 mA.
470 therefore gets you closer to the target and provides a
larger margin between the operating current and the maximum
current the LED can stand.
At this point, I'm guessing about the 10 LEDs. Is this a trial by error
situation? Setup 10, see how bright it is, add/subtract? I think we would
want the white LEDs to try and emulate the arena lighting.
Determining the resistor size is by formula - but for
figuring out the most pleasing light, I suppose it is
trial and error. If you use white LEDs you need to find
out what their Vf figure is. It will *not* be ~1.8 volts.
If you do not have a data sheet on the white LEDs you
plan to use, then figure the Vf at 3.6 volts per LED.
Ed