resistors for LEDs......help

importeater38 · #1 04-29-2005, 06:49 PM

ok, i know nothing about electrical. What kind of resistor do i need to get if i want to use 5 volt LEDs? do i just get a 7 or 8 volt resistor? That would drop the supply voltage from 12.6 or whatever to 5ish? Im not sure what i need to look for on the packaging or product discription.

Second, how do i wire it properly? Is it just simply 12.6v lead wire, to resistor, to positive LED wire? most LEDs have a short and long prong--which is the positive? I could be going about this all wrong...please help.

CBFryman · #2 04-29-2005, 09:34 PM

hmmmm....ask the radio shack guys, they know what they are doing....

as for the LED's...i bleive that hte short end is the cathode and the long the anode. the other way to tell is the flat side is the cathode... i think....damn it... thinking back to electronics class...ya my mind is gone right now.

The12volt.com
tells all about voltage drop and LED's and all
that or nsgm will come in here calling me a dubmass and then telling you the right stuff...

Diceman83 · #3 04-29-2005, 09:45 PM

You are. Resistors have a value of resistance measured in ohms. Read HERE as well as below.

A good way to think about electricity is in terms of water pipes. A resistor is analogous to a tightening in the pipe (like a kink or a valve) so there is less pressure on the other side. The "water pressure" is voltage, and the about of "water" passing through a given circuit is the amperage (measured in amps).

All LEDs work only one way, and it's usually the longer lead that is the positive one. LEDs also have a max current rating. See number 35 (the one below 34 if you can't see the numbers) on the site I linked to above. Enter 14.4 volts as the supply voltage, the working voltage the LED spec sheet lists, the max amperage of the LED, and the number of LED's you're going to have in series (connected end to end). If you're going to have all the positive leads connected (this wiring them in parallel), just enter 1. It'll tell you how many ohms the resistor should be. If you give the LED's too much voltage, too much current will flow through the LED, and it will burn out. Trust me, I've done it. A 9 volt battery is too much for an LED.

If you have any more questions (AFTER READING ABOUT EVERYTHING ON THAT WEBSITE) just ask!

PaulD · #4 04-29-2005, 09:49 PM

why not just use 12V LED's ? Resistors are measured in ohms not volts. What color are the leds ?

Diceman83 · #5 04-29-2005, 09:52 PM

I think all LEDs (the part that emits light, anyways) out there are rated for 2.5 to 4 volts. The ones that you can use with 12 volt or whatever have resistors built into the bulb or the leads.

CBFryman · #6 04-29-2005, 09:57 PM

so i was right....the cathode is the shorter lead...friggen awsome....

CBFryman · #7 04-29-2005, 09:59 PM

good link but that calulator assumes you are wireing them in series....

importeater38 · #8 04-30-2005, 01:05 AM

Quote:

Originally Posted by PaulD

why not just use 12V LED's ? Resistors are measured in ohms not volts. What color are the leds ?

the LEDs i have are blue with a clear lens. They are 5 volt, 30mA.

that website is excellent, but when i type in 5 volt and 30mA into the calculator for one LED, it says i only need a 233 ohm resistor? That seems very low when the give example is 680 ohms and they even recomend a 1000 ohm resistor for the example.

Does the 5 volt, 30ma really equate to such a low ohm resistor?

importeater38 · #9 04-30-2005, 01:26 AM

ok, another thing thats confusing me....do i need to be concerned with the correct wattage of the resistor?

sr20de4evr · #10 04-30-2005, 01:47 AM

yes that value is right

Since it's a 5V LED, then you want to drop the other 7V across the resistor (12V total, 12-7=5). Since you want 30mA to flow through the LED, the resistor value should be equal to 7/.03=233ohm. With a 233ohm resistor you'll get 30mA flowing through both the resistor and the LED, and you'll get a 7V drop across the resistor which will give you a 5V drop across the LED.

The power that's burned up in the resistor will be equal to .03*7=.21 watts. The smallest normal resistors available are .25 watt, which will work fine, so no you don't have to worry about the power rating of the resistor since even the smallest ones will work fine.

Diceman83 · #11 04-30-2005, 02:49 PM

uh, you have to drop 9.4 volts (14.4 - 9.4 = 5), because the alternator voltage (~14.4 volts) is the voltage everything runs at in the car when it's on. It usually means adding around 100 ohms to the resistor value.

sr20de4evr · #12 04-30-2005, 03:04 PM

true

I'm not sure what the tolerance is on the max LED current rating. Chances are it would be fine but you could increase the resistor value if you wanted to be on the safe side.

CBFryman · #13 04-30-2005, 04:27 PM

or just calulate for 13.5, which is right around the middle between a fully charged batteries voltage and the maximum voltage most cars altinator's regulator will supply.

Diceman83 · #14 04-30-2005, 09:56 PM

The only problem with doing that is that sending more than the rated amperage through the LED reduceses the lifetime of the LED dramatically. I'm sending like 4.5 volts to two of my LEDs, and while they're really bright, I'm expecting them to die any day now. They're only on when my running lights/headlights are on, but it's still going to wear them out.

CBFryman · #15 05-01-2005, 07:19 AM

then do it for 15v and you whould never excceed voltage.

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