LED dropping resistor value for 12v supply?

[Brad Libby]

I'm having a brain cramp. Does any one know the typical value for a
resistor in series with a LED operatiog from 13.8 volts.

Brad

[Charles A Davis]

OK, let's do the math.

13.8v available - 2v to iluminate the LED = 11.8v
Normal/average/expected current draw .020 A

Using Ohms law voltage (in volts) divided by current (in amps) =
resistance in ohms.

11.8 / .02 = 590 ohms

Now if I haven't dropped a decimal, or made some other stupid mistake,
your answer is, of the 'standard' values easily available, 560 ohms
seems to be the most available. This will result in a hair more than
.020A current, but still within tollerance.

Chuck D:

--
-----------------------------------------------------------
He, who will not reason, is a bigot; William Drumond,
he, who cannot, is a fool; Scottish writer
and he, who dares not, is a slave. (1585-1649)
While he that does, is a free man! Joseph P. 1955-
-----------------------------------------------------------
Chuck Davis / Sutherlin Industries FAX # (804) 799-0940
1973 Reeves Mill Road E-Mail -- c...@sutherlin-va.com
Sutherlin, Virginia 24594-3819 Voice # (804) 799-5803

[Gene]

I agree with the answer given by Charles BUT check the LED
specs. Some have different voltages and/or current draws
than the 2 volt, 20ma example given. The math is the same
(just do the substitutions if necessary (probably wont be).
BTW, this math example is one all of us need to keep handy
as well as variations for LEDs in series and parallel.
Gene

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[Charles A Davis]

Jack Mason wrote:
>
> No decimal points dropped, but I think an 'l' fell out of illuminate
> and landed in tolerance. :-)
> Jack

Ah Ha!!!!!

Another Nit Picker exposed.

[Jack Mason]

[Gndlfstram]

>Some have different voltages and/or current draws
>than the 2 volt, 20ma example given. The math is the same
>(just do the substitutions if necessary (probably wont be).

Will be using that formula myself as I am putting 3 doides in parallel {red and
white at 2v and a blue at five} Remember to multiply current draw by voltage
drop across the resistor to get the right power size resistor {and ALWAYS use
the next higher rating}

Jack "The trolley nut" Priller
Knowledge is of no use until it is shared

[Bruce H. Stull]

Hi, Brad - et al:

Au contraire!

You didn't tell us what COLOR!

For blue, use 330-560 ohms,

white, use 68-150,

red, use 240-330,

yellow, use 150-240,

and for green, use 110-150.

And yes, when in doubt, always use the higher value, but
note you may vary the hue within these values. This is
important because your "green" may look a little "yellow"
or a little "orange", etc.

Set them to what YOU like.

FWIW, our factory testing layout has operated these LED's
for some eight years (or as long as they have been
available) without problems.

Bruce

Scale Shops

[Colin Fitzgibbon]

Brad Libby <libb...@bangornews.infi.net> wrote in article
<37F7C6F3...@bangornews.infi.net>...

> I'm having a brain cramp. Does any one know the typical value for a
> resistor in series with a LED operatiog from 13.8 volts.
>
> Brad
>
>

My father says that the typical value would be between 470 ohms and 1000
ohms, depending upon the brightness. Wattage rating is 1/4 watt.

Regards
Colin Fitzgibbon

[Gene]

> Will be using that formula myself as I am putting 3 doides in parallel

> Jack "The trolley nut"

Jack:
If all three LEDs have the same specs, recommend you try hooking them
in series before you solder this circuit up. Why? Your note sounds
like you want all three to glow at the same time. Although you can
accomplish this with either series or parallel circuits, the current is
split among the branches in a parallel arrangement. I have found that,
with the wide variation in LED quality - even from the same pack - you
approach the same brightness from each LED in a series arrangement.
This is because each LED gets the same current. To use the formula
mentioned earlier for 3 LEDs in series, add the three spec voltages (6
volts?) and use the common current (0.02 Amp?) once.
Gene

Sent via Deja.com http://www.deja.com/
Before you buy.

[Jon Miller]

Start at 1K and work down. For my white LED's I use a 1K resistor.
True it only draws 10ma but it will still project a light across the room.
In most circumstances it is not necessary to run the LEDs at maximum
current. Also what Bruce says about the yellow and green LEDs is correct
depending on the quality of the LED. Good ones, read expensive, will be
more correct to their color.

[Mark F]

Resistor values will vary depending on the supply voltage. I assume
your table is based on 12vdc?

Mark

[Colin Fitzgibbon]

My father read your post and said that the resistor would be between 470
and 1000 ohms, depending upon the brightness, and would use 1/4 watt power.

[Bruce H. Stull]

that's what he said -

Bruce

[Gndlfstram]

Your note sounds
>like you want all three to glow at the same time

I swear I was sober when I wrote that last post [I! misspelled diode!] Actually
what I am doing is using 6v power source to theplus side of the 3 LED's and the
open-collector outputs of a random flasher circuit. Using fiber-optics from the
LED's for scale size lights around outer edge of an amusement park ride. Why
red, white and blue, you ask; because it will be a kiddy plane ride
commemorating Lindburg's 1927 flight to Paris!

jack "The trolley nut" Priller

[TOM]

Actually he said:

"I'm having a brain cramp. Does any one know the typical value for a
resistor in series with a LED operatiog from 13.8 volts."

But, who's counting??? :>))

<><><> TOM <><><>
-----------------

[Solarscout]

R = Volts / Current
Most LED's use about 15 to 30 ma of current so the resistor should be 920 to
470 ohms. Start with the larger values and work down to get greater
brightness. Use 1/2 watt resisters to be safe.

David, K&TRR Engineer.

[Gene]

In article <19991006011743...@ng-fh1.aol.com>,

and...when the LED stops glowing, you went too far (trash the LED). Of
course, if you are using the new 2ma. types, the LED will only glow
once - VERY quickly (trash the LED). It's always better to rely on the
formula and the component specs rather than color or what someone else
says might work.

[jrl...@jps.net]

First, I'm not out to be critical of the info provided by anyone thus
far. I just want to clarify the actual specifics behind the
calculation of resistor values for use with LED's.

The applied use of LED's is purely a current issue. Although, the
operating voltage is a necessary factor to calculate the resitance
value required to limit the current to the LED and to determine the
power (Watts) rating of that resistor. All LED's have a typical
current rating for a given luminosity(light output). This data is
usually supplied by the LED munufacturer. Ohm's Law is used to apply
the operating voltage(E) to the LED current requirement(I) to resolve
the limiting resitance(R) and power(P) as:

R=E / I in Ohms,
P=E * I in Watts.

So, 12/.020 (20 mA) = 600 ohms and 12*.020 = .24 (~1/4 W). LED's
range widely in their current draw depending on their specific's. You
can easily extract more light from an LED by reducing the resitance
(Ohms), but at the cost of power and life. The element of power as it
relates to the same voltage and a different resistance is explorered
as:
E / R = I , 12 / 560 = 0.02142857142857 Amps,
I * E = P, .02143 * 12 = 0.2571428571429 Watts.

One could get vastly more intense and derive the releative photon
emmision (or luminosity or irradiance) changes, as well, if so
desired, but I would highly recommend obtaining a mfr's data sheet,
first. You're not likley to find that at your local hobby shop.
Anyway, you can quickly see how the resistance changes things. The
same is true with changes in voltage. One can easily run over the
power rating of the resistor or the typical current of the LED.
Under- and Over-rating the components use directly effects its life.

This is much more difficult to actually calculate without some physics
background and information from the LED manufacturer you probably
won't find that datasheet I mentioned earlier. But, engineer's have
some pretty basic rules of thumb to extend the life of components in
use. One major contributor to a components life is heat. More heat,
less life. Less heat, more life. If one were to over-rate the
resistors to say, 1/2 watt the life would typically increase and the
operating temperature would decrease. As with the LED, if one were to
de-rate the applied current to any LED to say .015 Amps, this also
would directly play to the extended life of the part; at the cost of
emmitted light, of course. The closer one operates the components to
the rated limits the shorter their life is expected to be;
respectively

So, in short (ha ha), if one picks their LEDs and resistors carefully
they can construct very suitable lighting sources, that are also power
efficient and have resaonably good life expectancy. Now, considering
the scale nature of the hobby, does anyone know if the die-hard
prototypers have applied any serious emphesis to scaling the
wavelength of light, from these very light sources, in an effort to
maintain a truely, unviolated scale realism? Not to mention that
particle physics really gets in the way here... Just a thought (or
two).

Hope this all helps a bit.

-Jeff