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How To Find Which Resistor(s) You Need For Leds


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#1 EvilWays

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Posted 03 June 2003 - 08:57 AM

I've been seeing a lot of questions regarding which resistor(s) they need for a project using LEDs. Here are two formulas to use to figure out just what you need.

First, you need to know the resistance needed. For this, you need Ohm's Law.

(Vs-Vf)/If=R

where:

Vs = Source Voltage (either 3.3v, 5v, or 12v)
Vf = Forward Voltage (almost always stated on the package)
If = Forward Current (usually 20-30mA, stated as 0.02 to 0.03 in the formula)
R = Resistance in Ohms

For example, a 2.1v, 20mA LED using a 5v power source would figure like this:

(5-2.1)/0.02=R
2.9/0.02=R
R=145 Ohms

Use this number as a rough guide and purchase the resistor(s) that is the next highest value of R (in this case, I believe it'd be 170 ohms).

You also need to know what Watt rated resistor you need.

P=If*(Vs-Vf) <--- Power Law
P = Power in Watts
If = Forward Current from the previous formula
Vs-Vf = what you used in the previous formula also

Continuing with the example:

P=0.02*2.9
P=0.058 watts

If P <= 0.25, then use 1/4 Watt rated
If P > 0.25 but <= 0.5 use 1/2 Watt rated
If P > 0.5 then use 1 Watt rated

Using these numbers will help you purchase the resistor(s) you will need for a project working with LEDs.

*UPDATE*

Here's an update to an old post (for what it's worth)...calculating for parallel and series-wired LEDs.

Parallel-wired LEDs maintain the same Vf, but the If increases for each LED in parallel (e.g., two 20mA LEDs in parallel would use .04 for If), making it more or less IfT=If1+If2+etc.

Series-wired LEDs maintain the same If, but the Vf increases for each LED in series (e.g., two 2.1VDC LEDs in series would use 4.2 for Vf), making it more like VfT=Vf1+Vf2+etc.

Edited by EvilWays, 23 October 2004 - 11:03 PM.


#2 w m z

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Posted 03 June 2003 - 09:15 AM

EvilWays,

Nice post, Nice tits tongue.gif

#3 BEClueless

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Posted 03 June 2003 - 11:18 AM

that's alot of work... this thread should be pinned:
http://forums.xbox-s...,and,calculator

#4 Dankbud

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Posted 03 June 2003 - 04:14 PM

Just use this calculator, no need to do any math. it does it for u biggrin.gif Click Here

#5 EvilWays

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Posted 03 June 2003 - 06:25 PM

Puh-leeze, it's simple math really. Are you telling me that you can't do simple subtraction, multiplication and division?

#6 Dankbud

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Posted 03 June 2003 - 07:09 PM

QUOTE (EvilWays @ Jun 3 2003, 08:25 PM)
Puh-leeze, it's simple math really. Are you telling me that you can't do simple subtraction, multiplication and division?

LOL! Im too much of a stoner to actually want to think about math and shit. the calculator is 10x easier and faster..... im not saying i cant do it, im saying i dont feel like doing it. unsure.gif

#7 EvilWays

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Posted 03 June 2003 - 07:25 PM

That calculator is really doing nothing more than Ohm's Law, it doesn't even determine what watt rated resistor you need (makes a big difference between a working circuit or a small pyro show as resistors pop (tho making cylindrical capacitors pop are better biggrin.gif ) for some circuits).

Edited by EvilWays, 03 June 2003 - 07:28 PM.


#8 jcrack_corn

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Posted 04 June 2003 - 02:45 AM

the diodes have a voltage drop also so you probably dont need resistors when using a higher current LED (~50ma) in a 5v circuit.......(a 3.5v led would need a 30ohm resistor, but the LED itself will drop the voltage enough in that case so that the resistor is overkill)...i've never had one die on me like this in the past 10 years (esp for momentary on, like HDD lights)


#9 chasehbrammer

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Posted 04 June 2003 - 06:27 AM

Ok, I got my lights all working, my case all dremeled and unglued. I have even added a toggle switch to turn the lights on/off. But I had to go through some testing....However, I took out the resister in the process of this testing, and got them to work, but when I feel the diode pins, they are somewhat hot. Is this normal and should I just stay the same? Or do I need to put on a resistor? here are some specs

using 12v harddrive power supply
4 - 5 mm 3.7 volt/20ma

I used the calculator provided as ended up with a 470k-ohm resistor.

What I am asking:
1. Is it normal for the diode pins to be somewhat hot to the touch?
2. If I do put on a resistor, will a 470-ohm do the trick?

#10 TykSak

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Posted 04 June 2003 - 08:55 AM

QUOTE (chasehbrammer @ Jun 4 2003, 08:27 AM)
using 12v harddrive power supply
4 - 5 mm 3.7 volt/20ma

How did you hook them up? series or parallel?

#11 chasehbrammer

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Posted 04 June 2003 - 02:04 PM

I dont know what it is called but I will try to explain it.

It looks kind of like a round daisy loop chain. I am guessing that this means it is a series?

#12 Exobex

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Posted 04 June 2003 - 02:33 PM

Connect them in series. 12V goes to one end of resistor, other end of resistor goes to one end of LED, other end of LED goes to ground/0V.

470Kohm seems a bit high, let's see...

12 volts (supply) - 3.7 volts (volt drop of LED) = 8.3 volts
Current required = 20mA = 0.02 amps

8.3 volts divided by 0.02 amps = 415 ohms

A 470 ohm resistor will run the LED at 17.6mA, that'll be fine.

Colour code (4 band): yellow, violet, brown, whatever
Colour code (5 band): yellow, violet, black, black, whatever
The last band doesn't matter (gold would be 5% tolerance, for example)

FYI:-
4-band colour code is: 1st digit, 2nd digit, no. of zeroes, tolerance
5-band colour code is: 1st digit, 2nd digit, 3rd digit, no. of zeroes, tolerance

Colour code:-
0 black
1 brown
2 red
3 orange
4 yellow
5 green
6 blue
7 violet
8 grey
9 white

Tolerance code is different, you've got 5% gold, 10% silver and various others.
Gold or silver as the multiplier (no. of zeroes) indicates a low value, such as 1.2 ohms or 0.12 ohms.

#13 chasehbrammer

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Posted 05 June 2003 - 03:15 AM

Thanks for the thread on this guys, I successfully got my LED light installed biggrin.gif Once my new jewel comes I will post some pics of it!

#14 enuf

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Posted 07 June 2003 - 03:07 AM

And no questions about anything?? Hmmm.... Sounds like u just hooked up the 12v to the led's.. sure hope u didn't......... if you did tho and it works.. and nothing smoked (heh) ummm your going to screw something up in the long run. Or the short run. It will send to much power to ur led and possibly your xbox. Get a resistor. Your led will last longer and ur xbox won't be a gift to the trashman.

And if i'm wrong... GJ I plan on lighting my box up once I get my modchip in. (I was quite fluent in the newbie talk... I got a gen A modchip for my first chip.. damn I was stupid)

. . .

#15 neolithic

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Posted 17 June 2003 - 08:07 PM

I don't want to make an arse of myself but i'm afraid i'll have to risk it. If i'm replacing the eject button leds, with some 3mm bi-color (or even tri) do i need to add resistors or can i just replace the leds in there with ones i want. Thanks and sorry in advance.




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