LEDs and resistors

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wHITEYLT1

Member - Posts: 5

Joined: March 06, 2004

Location: United States

Posted: November 08, 2004 at 7:25 PM / IP Logged

I'm trying to finish a little project for my car and I am in need of some help. I want to wire up a few LEDs for an electric water pump and electric exhaust cut out and I know I need resistors but the info in the catalog might as well be Greek.

The LEDs are to be the T1 3/4 but there are hundred of choices in the Digi-Key catalog. My first question is what do all of the abbriviations stand for...like Vf, If, Io, etc...?

My second question is what resistors do I need for this application? I took a resistor off that is being used in a similar application and the bands seem to be yellow, red, red, gold which if I'm correct idicates it's a 4.2k %5. Is that correct for what I am needing? If I want the LEDs extra bright or dim what can be done? Example, I have a blue LED that is MUCH brighter than the amber and red LEDs. What can I do if it has a yellow, red, red, gold resistor to reduce the intense light?

Fianlly, what type of resistor do I need? I see there are several different types like Metal Oxide, Metal Film Fixed, Carbon Film, etc..., which would be best for me?

THANKS in advance!

Teken

Gold - Posts: 1,492

Joined: August 04, 2002

Location: Aruba

Posted: November 08, 2004 at 7:52 PM / IP Logged

For the amber & red LED's, a 470 ohm resistor will be fine. Generally speaking, they consume 20 mA but without knowing exactly it is still a safe resistor value to use for a margin of safety.

The ultra bright blue LED, typically consume aprox 40 mA, so a value of 240 - 280 ohms will be fine.

Remember you are taking in to account both 12 volts at rest, and running voltage of 13.4 to 13.8 volts.

Hence the broad value's.

A standard 1/4 watt resistor will be fine.

Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

wHITEYLT1

Member - Posts: 5

Joined: March 06, 2004

Location: United States

Posted: November 09, 2004 at 3:10 PM / IP Logged

Teken wrote:

For the amber & red LED's, a 470 ohm resistor will be fine. Generally speaking, they consume 20 mA but without knowing exactly it is still a safe resistor value to use for a margin of safety.

The ultra bright blue LED, typically consume aprox 40 mA, so a value of 240 - 280 ohms will be fine.

Remember you are taking in to account both 12 volts at rest, and running voltage of 13.4 to 13.8 volts.

Hence the broad value's.

A standard 1/4 watt resistor will be fine.

I am actually going to be using red, green, amber, blue and orange. The blue and orange seem to be noticably brighter than the rest...does the 240-280 apply to the blue and the orange? What would happen if I bought all of the same type resistors for all of the lights? If it can be done, which ones should I use?

When you say standard 1/4 watt resistor, which one of the above listed would be the "standard"?

Thanks!

Teken

Gold - Posts: 1,492

Joined: August 04, 2002

Location: Aruba

Posted: November 09, 2004 at 3:35 PM / IP Logged

The differnce in using a smaller value resistor is that more current will be allowed to flow to the LED. Hence it will be brighter, but also it will make it last less within the margin of safety specified by the maker(s).

If you decide to use the 470 ohm value, the LED's will be dimmer, again, this ensures many more hours of service.

You may purchase any resistor you like, what ever one is the cheapest for you to buy. When I refer to the phrase standard, this means what is used the most in the 12 volt areana.

That being carbon film resistors, the other ones which you listed are standard issue in almost any other commercial & industrial applications.

ie. radial mylar, dipped tantalum, polyester, etc.

Please bare in mind if you wish to have all the LED's to be the same brightness, then you will have to play around with the values to ensure all are consistently the same brightness.

ie. higher / lower values for the yellow / orange etc.

Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

bmw5002

Member - Posts: 14

Joined: November 09, 2004

Posted: November 09, 2004 at 6:13 PM / IP Logged

Does it matter that most LEDs have a voltage rating of like 5 volts? Is it really just the current that matters? So like a 5v 40mA LED with the proper resistor all hooked directly to the battery will be fine?

bmw5002

Member - Posts: 14

Joined: November 09, 2004

Posted: November 09, 2004 at 6:27 PM / IP Logged

Actually I think I just kinda figured it out. Tell me if my theory is correct. I know voltage, current and resistance are all directly related. So basically I can think of the flow of electricity as just electricity, disreguarding voltage and current. Some things need less electricity, like LEDs, and some need more, like motors. Voltage, current, and resistance are the units used to mathematically describe this flow of electricity. When a manufacturer rate their LED at 4.3v and 60mA, they are not explicity saying, "You can't give it more than 4.3v!". Since voltage, current and resistance all vary according to each other. Heres the point: Whether its say 4.3v and 60mA, or 12v and 167mA the flow of electricity is still the same! So your LED is ok no matter what voltage, it just has to have the right current flow, and it will always be the same amount of electricity flow.

Teken, tell me I'm right. I probably sound like a total idiot but the whole concept of ohms law and how it relates to the flow of electricity has been a mental roadblock for me for the longest time. So do I have the right concept?

Mad Scientists

Silver - Posts: 380

Joined: February 07, 2004

Location: United States

Posted: November 10, 2004 at 8:35 PM / IP Logged

Ohm's law consists of three values.. Volts, Amps and resistance. If two of the values are known, you can calculate the third.

Volts = Amps x Resistance

Amps = Volts / Resistance

Resistance = Volts / Amps

In your example of an LED 4.3v and 60mA..

60mA is the maximum current the LED can handle.. any more (continuous) and it'll 'burn out'. Without naming names, there is a law that states that current will be the same throughout the circuit (there's more to it, but stick with that for now).

So, if there is 60mA going through the LED, there will be 60mA going through the resistor you've wired in series with the LED to protect the LED.

Take the voltage number related to the LED (known as Vf). In your case it's 4.3v. Subtract this voltage from the supply voltage... let's use 12v (battery voltage).

12v - 4.3v = 7.7v

The LED will 'drop' 4.3v across it's terminals.. you have to figure out the resistance value of the resistor. Known values are a 7.7v drop, and a current of 60mA. Divide 7.7v by .060 amps to find resistance value.

7.7 volts divided by .060 amps equals 128.33 ohms resistance.

I checked my resistor selection box, but all the labels have fallen off the little drawers.. I think that 130 ohms is a standard value, which is what I would use.

To vary the brightness you could use a variable resistor.. sorta like a dimmer switch..

Post the Digikey parts numbers and we can help you understand what things mean..

Vf = foward voltage

If = foward current

There's more.. but it's time for Jimmy to go to bed now.. post your questions and we can help.

Moderator: is there a FAQ for wiring up LEDs? Is there an interest in one?

Regards,

Jim

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LEDs and resistors

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