resistors for LEDs question

[RY2K]

i have previosly played around with a couple of LEDs from radio shack and the bulbs are 3.7volts min and 4.5 volts max and i did the equation that Illusion had on his website, and he recomended 470-Ohm resistors and i got the ½ watt ones and used them and they came out fine i went to radio shack yesterday and i saw that they had 470-Ohm ¼ watt ones so i picked up both so my question is do i use the ¼ watt ones or the ½ watt ones Thank you, RY2K here is a picture, it's hard to read so i drew on the pic i want to do this the correct way, and not mess up my car so i apreciate any input

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[Bl@ck]

wattage is a measurement of power. the wattage rating on a resistor is basically how many watts/milliwats it can safely dissipate as heat. i'd go with the 1/2 watt ones. better to be safe than sorry.

[luti]

Whats the equation you used to figure this out?

[JERZY-STYLZ]

I usually use 220 ohm.....but da jus me.

If anything is to get screwed up by using the wrong resistor, it would be the LED itself and nothing else....feel free to correct me if im wrong

[Bl@ck]

Originally posted by luti
Whats the equation you used to figure this out?

Ohm's law E=I/R

[RY2K]

Originally posted by /^Blackmagik^\
wattage is a measurement of power. the wattage rating on a resistor is basically how many watts/milliwats it can safely dissipate as heat. i'd go with the 1/2 watt ones. better to be safe than sorry.

ok so i'll use the 1/2 watt, i didn't know about the heat

Originally posted by JERZY-STYLZ
I usually use 220 ohm.....but da jus me.

what is the voltage of the LED you use the 220 ohm resistor with, and does it make it brighter

If anything is to get screwed up by using the wrong resistor, it would be the LED itself and nothing else....feel free to correct me if im wrong

thats good to know cause my friend got me worried, i hope you're correct

Originally posted by /^Blackmagik^\
Ohm's law E=I/R

E = I*R so R = E/I



check out the web page it's explained really well
http://www.igd4u.com/modules.php?op=...=article&sid=2

[Bl@ck]

Originally posted by RY2K
E = I*R so R = E/I
check out the web page it's explained really well
http://www.igd4u.com/modules.php?op=...=article&sid=2

i knew that.... it's been a while since i used it though

here's the extended equation to calculate power in a circuit.



using W=E^2/R you'll see that the 1/4 watt resistors won't cut it. and a burning resistor don't smell good at all.

[RY2K]

Originally posted by /^Blackmagik^\
i knew that.... it's been a while since i used it though

here's the extended equation to calculate power in a circuit.



using W=E^2/R you'll see that the 1/4 watt resistors won't cut it. and a burning resistor don't smell good at all.

thanks for the equation picture

my car smells like cucumber melon, and a burnt resistor would not be very

[Bl@ck]

Originally posted by RY2K
thanks for the equation picture

my car smells like cucumber melon, and a burnt resistor would not be very

yeah, that ish smells like ass... i used to burn one every morning in electronics class in high school just to be an asshole

[digitech]

I go with the 220 ohms with the 3.5v. It's the best combo 4 leds and resistors.

[nhatduongchi]

Originally posted by digitech
I go with the 220 ohms with the 3.5v. It's the best combo 4 leds and resistors.

Depend on the rating of your LED and how bright that you want it to be. For 20mA 3.5V LED then 470 Ohms is ok.

[MrJasey]

uh i am too lazy to do the calculations but use Ohms law V=IR and use the power one P=VI where v is voltage, I is current and r is resistance....also u might have to do simple algebraic manipulations but it should just take a few seconds to figure it out

[PHiZ]

Originally posted by MrJasey
uh i am too lazy to do the calculations but use Ohms law V=IR and use the power one P=VI where v is voltage, I is current and r is resistance....also u might have to do simple algebraic manipulations but it should just take a few seconds to figure it out

Yup, this guy's on to something.

we already covered Ohm's law of

E=IR

but we left out his law for power:

P=IE

So reading your pic of the LEDs it says that can handle 3.7v at 20mA

3.7v * .020A == .074W
5v * .020A== .1 W

1/4 watt == .25w, so a quarter watt will more than cover you.

Now where are you going to get your (max 4.5v) from? Since your car runs a 12v system?

If you decide to just run the LEDs off 12v:

E / R == I
12v / 470ohm == .025A == 25mA

Which appears to be 5 mA over the LEDs rated limit. You can generally get away with overvolting stuff, it's amperage (current) which will cause problems.

So I guess if you wanted to use 12 volts, without exceeding the reccomend current rating, you'd do something like this:

E/R==I => E/I==R

E / I == R
12v / .020A == 600ohms

Nobody makes a 600 ohm resistor. So looking at the equation:

E/R==I

You don't want "I" (current) to go up. So you want your denominator (R) to go be larger. (denominator gets bigger, product gets smaller).

Your resistor choices are:

560 - 600 = -40 ohms from target
680 - 600 = +80 ohms from target.

I lied, I just took 2 1.2kohm resistors (1200ohm) resistors, hooked them up in parallel, and that was exactly 600 ohms.

I took some measurments:

600ohms at 12 v was drawing .015Amps of current
470ohms at 12v was drawing .020Amps of current.

theoretically
12v at 600ohms shold draw .020 amps
12v at 470ohms should draw .025 amps

However, all resistors have 10% margin of error (gold band), or 5% margin of error (silver band).

So the LED manufacturers, I'm sure accounted for that using standard industry design practices, when rating their LED (at .020amps).



Here we see the LED running off of 12v, on the left through a 470ohm load, on the right through a 600ohm load.

What does that all mean? You can use a single 470ohm 1/4 resistor, no problemo. 1/2 watt will not give you any extra protection.

Feel free to ask any question (except "why did you make this post so long" )

-PHiZ

[WideBodyEK]

so yea after you guys lost me with all that math stuff,...what are you guys using LED lights for??? and post pics of what your using them for,..thanks