# cut the power of a LED light from 12v to 500mA

Discussion in 'LEDs and Optoelectronics' started by tom356, Feb 19, 2013.

1. ### tom356

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Feb 15, 2013
Hi

Is there a resistor I can use to cut the power of a LED light from 12v to 500mA or less..
I have a 12v led and want to cut the power to 500mA or less can some one tell me how to do this?
If so what parts do I need and were can get them
thank you

2. ### Harald KappModeratorModerator

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Nov 17, 2011
You're mixing voltage and current, two very different things.
There is no such thing as a 12V LED. If this unit was designed for direct use on 12V, this is certainly an assembly including one or more LEDs plus some electronics, in the simple case only a resistor.

To reduce the current consumption you can either lower the voltage or add a series resistor between the unit and the 12V source. In order to determine the required resistance we need to know the technical data of the unit (rated voltage, rated current). An exact type number or datasheet would help.
Be aware that reducing the current will also reduce the light output.

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

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Jan 21, 2010
Unless he has a module or a COB array where the sum of the Vf add up to about 12V.

These are not uncommon.

Running them from 12V is problematic due to the very small differential between the Vf and the supply voltage.

4. ### Harald KappModeratorModerator

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Nov 17, 2011
I know, in the meantime rather common as a replacement for 12V halogen bulbs. That's what I meant by
and why I referred to the term unit further on.

5. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Honestly guys, this is a first time for me .. really! I thought there always had to be current limiting (active or passive). When I say current limiting I'm including source internal resistance too. I would think that with absolutely no current limiting the source voltage would have to be like the Rock Of Gibraltar to absolutely never exceed the combined series LEDs' Vf. Wouldn't the slightest change in source voltage cause a massive change in current?

Is this common practice?

Chris

6. ### Harald KappModeratorModerator

10,593
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Nov 17, 2011
It would, without current limiting. As you say. The current limiting is built into these LED modules. Cheap ones just have a bunch of resistors, the better ones use an electronic circuit.

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I'm not sure what I read or thought I read that prompted me to post that. I don't see anything in this thread now? Admittedly, I had consumed a fair amount of suds yesterday but I thought it was sensibly balanced with an equally large Pizza!

Chris

8. ### PStevenson

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Mar 1, 2013
to OP you use ohms law my friend
you have 12 volts and you want only 500mA to be drawn from your voltage source

12/500mA or 12/.5 = 24

so you need a 24 ohm resistor

then you need to make sure that the resistor you use doesn't burn out
so we calculate power..

I(current) x V(voltage = P(watts) in your case .500 x 12 = 6

so you need a 6 watt resistor

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

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Jan 21, 2010
Only if the LED is a short circuit (which it almost certainly isn't)

10. ### PStevenson

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Mar 1, 2013
the LED is an unknown variable - the information given is that he wants to limit current output to 500mA from a 12 volt supply.

you can only answer a question on the information given, not on speculations

so if we use a 24 ohm resistor from a 12 volt supply - no more than 500mA will be drawn.

11. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
The LED isn't an unknown variable. At least it doesn't have to be. Tom can measure the voltage drop across a single LED. It then becomes a Constant. He can also extract this information from LED data sheets. Admittedly Vf increases slightly with an increase in LED current but he only needs a ballpark figure to calculate Rs.

Chris

12. ### PStevenson

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Mar 1, 2013
yes, but I'm the one answering his question and I am doing so with the information present. I'm not trying to argue about it.

all I have is the voltage and the current - for whatever reason he wants to limit it at 500mA from a 12v supply which suggests whatever he is powering needs to be limited at 500mA

personally if it was just one normal LED I'd stick a 470ohm - 1K resistor in there and be done with it.

it's a very strange question to begin with so I don't think it's really worth carrying on discussing it until the OP is a bit more specific

13. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Uh, isn't that what we do here?

Today is Saturday, so I'll be heading out to my watering hole soon. When I return I'll probably be inclined to agree with you.

Chris