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HelicopterOff Topics › Help with resistor for leds
08-02-2011 11:48 PM  6 years agoPost 1
scott011422

rrApprentice

Sycamore, illinois

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Hey all, I'm trying to find a resistor do drive some leds at 12volts. But I'm getting different values from different sources.

Specifications:
Size (mm) : 5mm
Voltage : 3.0-3.4v
Lens Color : Water Clear
Reverse Current (uA) : <=30
Life Rating : 100,000 Hours
Viewing Angle : 180 Degrees
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 24mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

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08-03-2011 12:01 AM  6 years agoPost 2
Peter Wales

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Orlando Fl

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12v-3v = 9v
I=0.024A
V=IR R=V/I
R=9/0.024 =375ohms
Preferred value 390 ohms
Watts = VxI
W = 9 x .024 = 0.216w Go with 0.5w resistor

Peter Wales
http://scalehelicopters.org

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08-03-2011 02:21 AM  6 years agoPost 3
scott011422

rrApprentice

Sycamore, illinois

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So I'm stuck here. I found 470 ohm 1/2w. Which is what the cals say. But everyone is selling 470 hom 1/4 for led??

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08-03-2011 05:46 AM  6 years agoPost 4
dkshema

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Cedar Rapids, IA

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24 ma is the maximum continuous forward current rating of the LED.

Why would you design the darn thing to operate at its limit? Prudent design practices would have you derate the device so that it's NOT operating at its maximum rated current.

You might want to consider running less current to insure long life of your LED. Less power dissipated in the device, less power dissipated in the resistor, the resistor watt rating would be less, and the physical size of the resistor would also decrease. Battery life would go up at the same time (run time per charge).

Of course less current through the device would mean less intensity, but how much do you need for your purpose?

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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08-03-2011 11:45 AM  6 years agoPost 5
Peter Wales

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Orlando Fl

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That's the first time I have heard anyone propose anything on this forum is run at less than its maximum rating

Working it backwards, if you use 470 ohms
9/470 = 19mA which is 20% less current. You would have to look at the brightness curve to see what would happen to the intensity, but it would still work.

9x0.019 is 0.17W so a 1/4 watt resistor would be big enough

Peter Wales
http://scalehelicopters.org

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08-03-2011 03:02 PM  6 years agoPost 6
Pistol Pete

rrProfessor

Seffner, FL

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Just sharing a link...

http://ledcalculator.net/

~~Enjoying the hobby one flight at a time~~

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08-03-2011 09:16 PM  6 years agoPost 7
AirWolfRC

rrProfessor

42½ N, 83½ W

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Yep, 470 ohms 1/4 watt for one led on 12V

or two led's and a 270 ohm 1/4 watt resistor in series.

. . . one thing your spec doesn't call out is the output lumens (brightness).

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08-03-2011 09:50 PM  6 years agoPost 8
scott011422

rrApprentice

Sycamore, illinois

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I ordered the 470 x 1/4.

What difference do the lumens make?

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08-03-2011 10:08 PM  6 years agoPost 9
AirWolfRC

rrProfessor

42½ N, 83½ W

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You can get a resistor like that at Radio Shack.

Lumens is the amount of light output. Not all led's are created equal.

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08-03-2011 10:47 PM  6 years agoPost 10
scott011422

rrApprentice

Sycamore, illinois

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Yes, But what do they have to do about sizing my resistor? Yes, I can get them from radioshack, but I bought 500. I don't even want to know what that would cost through Radio Shack.

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08-03-2011 10:57 PM  6 years agoPost 11
Ironhide

rrVeteran

dexter mi us

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4 leds in series No resistor

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08-03-2011 11:14 PM  6 years agoPost 12
scott011422

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Sycamore, illinois

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True, But some of my placings are 1's 2's and 3's

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08-03-2011 11:27 PM  6 years agoPost 13
AirWolfRC

rrProfessor

42½ N, 83½ W

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Yes, But what do they have to do about sizing my resistor?
Lumens ? . . . nothing

I have about 4,000 470 ohm 1/4 watt resistors in excess inventory

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08-04-2011 01:55 AM  6 years agoPost 14
dkshema

rrMaster

Cedar Rapids, IA

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BTW -- if you're using LEDs of different colors, note that the forward voltage (that would be the voltage drop across the LED when it is forward-biased, conducting current, and being lit up) varies drastically based upon the color of the LED. That means you'd need to use a different resistor value to limit the current into each device. One resistor value will not necessarily work for all different color LEDs.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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08-04-2011 02:34 AM  6 years agoPost 15
daggit

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Claremont, MN

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Just wondering if this is an automotive application. If so does this affect the equation?

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08-04-2011 02:19 PM  6 years agoPost 16
scott011422

rrApprentice

Sycamore, illinois

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It is automotive. Figured with all the electronics guys and the night flight guys, This would be a good place to ask. No, It shouldn't matter. voltage is voltage, caar, bick, heli, moon, japan. 12v is 12v. Well, actually 13 volts with the car running..........

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08-04-2011 03:48 PM  6 years agoPost 17
AirWolfRC

rrProfessor

42½ N, 83½ W

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Yes, it matters. With the car at rest, engine off, the voltage can be as low as 11.5V and running it can be as high as 14.5V.

That's a 37% difference in current based on a 3.5V LED which has a pretty constant forward break-over voltage, just like the reverse break-over of a zener diode.

The ideal running voltage is 13.8 and your milage will vary.

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08-15-2011 06:07 PM  6 years agoPost 18
scott011422

rrApprentice

Sycamore, illinois

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Any problem with running 1/2 w resistors? I ordered some 560's for some 3mm red leds, And ordered 1/2w by mistake

They supplied 470x 1/4, but I don't think thats going to work with 2v leds.........Don't know why they would have givin me those.....

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08-15-2011 06:37 PM  6 years agoPost 19
GyroFreak

rrProfessor

Orlando Florida ...28N 81W

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1/4 watt is fine, remember the rating (1/4) is worst case and your application isn't man rated. 1/2 watt is fine also, it just is physically larger.

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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