# 12v LED resistors

Discussion in 'LEDs and Optoelectronics' started by ajstars, May 14, 2015.

1. ### ajstars

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May 10, 2014
I have a number of 6 - 12v DC micro leds which I want to fit to a vehicle, but I am concerned they will blow when the voltage tops 12v (vehicle will be running at around 14v). I was wondering if I can reduce the voltage by around 3v using a resistor. It does not matter that the leds would not be as bright. I have been told that this is not possible - any ideas or suggestions? If it can be done what resistor would I need?
Thanks

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Get to know the current these LEDs draw at 12V (datasheet or measure it). You can drop any voltage by adding a series resistor. Ohm's law tells you which value you will need: R = V/I where V is the voltage drp (e.g. 3V) and I is the current.
Yull also have to observe the power rating of the resistor P=V*I.

Have a look in our ressources section for more information about driving LEDs.

3. ### ajstars

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May 10, 2014
Thanks for the info. So I need to test the current draw on the led. I can't find the specs in the data sheets.
My multimeter should therefore have the red cable in the dc port or the ma port?
Then where does the dial need to be set?
My Avatar is my multimeter!
Thanks

Last edited: May 14, 2015
4. ### poor mystic

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Apr 8, 2011
It's a good avatar but a bit low-resolution for my poor eyes.
Looks like the red lead goes in the left-hand hole to me... is that marked 10A?

5. ### BobK

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Jan 5, 2010
Yes, try the 10A range first. Then if is below 1A, use the mA range to get a better reading.

Bob

6. ### ajstars

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May 10, 2014
Ok thanks, yes 10A on the left. Which setting does the dial need to go on? I have ohm, V-..., V~, A-... and a few others all with various values!
Sorry, even the electronics for dummies struggles with me!
Thanks

7. ### ajstars

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May 10, 2014
So I put the multimeter in series first on the 10a range, no reading but the led lit up - then I tried on the mA range and tried all the settings in the V-... range and the best I could get was a reading of 14 but the led did not light up.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
If the LED didn't light up on the mA range it probably means you've blown the fuse on that range.

9. ### ajstars

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May 10, 2014
Yup, fuse is blown. Is that because I put it on the wrong dial setting? Still not sure which one to use - 200m, 2000m, 20, 200, 600?
Will grab a new fuse in the mean time.

10. ### BobK

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Jan 5, 2010
The range does not matter, the fuse will probably blow somewhere above 2A. That was the reason for doing the test on the 10A range first.

BTW: I blow that fuse on my multimeter regularly, usually be forgetting it is on current range and trying to measure a voltage.

Bob

11. ### ajstars

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May 10, 2014
I think it was probably blown before. Just need to get some new ones and will try again.

12. ### ajstars

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May 10, 2014
Ok, new fuse in and I have selected the A-... 20m setting with the test lead in the V ohm/mA port. I get a reading of 18.58 and the LED lights up. Have I done this correctly? If so please can someone advise what resistor I will need please? All I know is the LED will work between 9 and 12V DC. I want to use it in a car that charges at around 14V so was thinking of reducing voltage by around 3v at the LED. It matters not if is is not so bright. Thank you

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13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Ok, so let's call that 20mA.

If you were powering this from 12V then you need 100ohms to drop 2V.

More generally you would need v/0.02 ohms to drop v volts. This works out as 50 ohms per volt.

14. ### ajstars

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May 10, 2014
Ok thank you, I shall go and visit Maplins!