# Help with LED's and resistors....

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#### stappin

##### New Member
Hi All in a word HELP!!!

It's been a bloody long time since I dicked around with LED's can anyone tell me how to calculate the resistor I would need to use in the below config. Also do you attach the resistor to the + of the LED. (Yes I know what a numpty question!)

12v car battery to switch to a: 5mm/T13/4, Standard - With Integral Resistor (does this mean it doesn't need a resistor???)

LED specs...

Product Details LED 5MM ULTRA RED Luminous intensity, typ 500 mcd Voltage rating, DC 12 V Diameter, external 5 mm Angle, viewing 60 ° Wavelength, peak 660 nm
Description Luminous intensity, typ 500 mcd Voltage rating, DC 12 V Diameter, external 5 mm Angle, viewing 60 ° Wavelength, peak 660 nm

Cheers

Simon

#### Phasor

##### Member
With Integral Resistor (does this mean it doesn't need a resistor???)

This is correct, you can connect the LED directly to 12Vdc.

#### stappin

##### New Member
So how much current can it take before I have to worry about using a resistor?

Cheers for the quick response

#### Nigel Goodwin

##### Super Moderator
stappin said:
So how much current can it take before I have to worry about using a resistor?

Cheers for the quick response

You don't need to worry, it's specified as working off 12V, as long as you run it off 12V everything will be fine. If you want to use more than 12V, you would then have to add an external resistor as well - you would need to measure the current it already takes off 12V to work out the external resistor using ohms law.

#### B-o-b

##### New Member
LED Resistor

Hi Simon,

Actually, it's rather easy! Personally, I don't worry about all the calculation involved in find'n the right resistor. I'll just put a pot of, say, 5K ohms in series with the LED and slowly decrese the resistance until the LED shows a comfortable brightness. Then I'll measure the resistance with an ohm meter and that's the value of resistor I use. Rather unortodoxed, but it achives the same results, really.

U should also keep in mind, that different LEDs have differing current requirements, along with different brightnesses. If U know how much current an LED needs for moderat operation, then it's just a matter of using ohm's law to figure out the resistor. Also, it doesn't matter what side the resistor is on of the LED. It can be on either side.

BTW, for an ordinary LED and a 12 volt supply, I've used about a 1K ohm resistor and this has always worked.

As was mentioned in, I think it was 1 of the other replies, if U get an LED with a built-in resistor, then don't worry about adding another one... just connect it straight to yer 12-volt supply, and that's it.

Hope this helps.

Cheerz!

Bob

#### captainkirksdog

##### Member
Hi stappin:
FYI:
The formula for calculating the series current-limiting resistor for an LED is as follows:
Supply Voltage (Vs) - LED Voltage (Vled) / LED Current (Iled)

The voltage and current specs for LEDs are found on any LED data sheet. Usually I just use 2 volts for red, yellow, and green, and 3.5 volts for blue. Fifteen to twenty milliamps will give a good, steady light output from most LEDs. So if you are using a twelve-volt supply, your series resistor is 666 ohms. The closest readily available value would be 680 ohms. Cool, huh?

(12-2)/0.015=666.667 rnd> 680

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