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Subject: Re: Parallel LEDs calculation
Date: Sun, 08 Sep 2002 17:40:51 GMT
NNTP-Posting-Date: Sun, 08 Sep 2002 13:40:51 EDT
> thanks for excellent advises.
> so i'll go with one R for each LED.
> i'll use a voltage regulator to make it 12v first.
> now i have to solder them to a board.
> i can't use solderless breadboards.
Good! The solderless breadboard might not be reliable
in a car due to vibration and heat/cold. FORGET the
regulator. You said you will have a total of 500 LEDs.
Therefore, FORGET 1 R for each LED. Use the scheme
outlined in the thread of 3 LEDs in series with a
resistor. Or 5 LEDs in series with a resistor, which
is likely your best bet overall, if this tail light
will really contain 500 LEDs.
Also, run the LEDs at something below 20 ma for
better reliability. I would use a 220 ohm resistor
in series with 5 LEDs. With the engine off, assuming
a battery voltage of 12V, that will provide about 9 ma
for the LEDS. 500 20 ma leds running at 9ma will provide
PLENTY of tail light - and the car isn't even going.
With the engine running, assume a nominal 13.8
volts at the battery. You'll have 17.2 ma current.
At 14 volts, you'd have 18.1 ma
Regarding the resistor: its power rating needs to
be at least I^2R, and good practice is to double that.
Doubled, it computes to about .15 watt. BUT - this thing
lives in a "hostile" environment - so increase the wattage.
Also, you are going to need 100 resistors, each dissipating
.15 watts, so there will be an accumulation of heat.
I'd use at least 1/2 watt resistors. Better still (reliability
wise), you could use 2 470 ohm 1/2 watt resistors in parallel.
That would give you 235 ohms at 1 watt, and 17 ma when the
car is running, which is better for reliability. Or use 100
and 150 ohm 1/2 watt resistors in series. That would give you
250 ohms at 1 watt, and about 15.2 ma when running. (The exact
values of current in all of the above depend on the battery
voltage.) (I consider 15 ma to be ideal.)
The down side of using 2 resistors is it increases your parts
count and cost.
> "newbie" wrote in message
> > hi,
> > I am connecting 6 red leds in parallel with a resistor in series with all
> > them.
> > the voltage source is 12v.
> > each led needs 20mA with 2v.
> > what should the resistor value be? watts? ohms?
> > ________/\/\/\/\/\__________ ...............
> > | |
> > 12v led ...........other leds.......
> > |_________________ |_____ ...........
> > Thanks