# The Ohm's Law for a LED is Rated at 12V/5A.

#### biferi

Joined Apr 14, 2017
313
I was about to See if the LED I have works before I Build My Project.
The LED is Rated 12 Volts at 5. Amps.

I have an 18 Volt Battery so I was going to use a Resistor so I can See if My LED works.
Am I Right I will use Ohm's Law like this to Find the Resistor I need?

18 Divided By 5. = the Resistor I need?

#### Papabravo

Joined Feb 24, 2006
18,992
I was about to See if the LED I have works before I Build My Project.
The LED is Rated 12 Volts at 5. Amps.

I have an 18 Volt Battery so I was going to use a Resistor so I can See if My LED works.
Am I Right I will use Ohm's Law like this to Find the Resistor I need?

18 Divided By 5. = the Resistor I need?
Not quite. If the LED takes 5 Amperes at 12V, then you want to know what resistor will drop the voltage to 12V at 5 amperes.
That would be (18-12)/5 = 1.2 Ω
Now the problem you have will be that the resistor is going to dissipate 5 x 5 x 1.2 = 30 Watts. This is not an insignificant amount of power and resistors that can do it are not cheap. You will be much better off in the long run if you just acquire the 12 V power supply that you need.

• Ya’akov

#### Ya’akov

Joined Jan 27, 2019
6,279
Getting 5A of 12V from 18V is not going to be trivial. I buck converter would probably be the sanest way.

5A may not sound like a lot but it is. Your battery, which I assume is a tool battery, might not be very happy delivering it for very long either.

#### crutschow

Joined Mar 14, 2008
30,455
That is a really high powered LED.
Are you sure that current value is correct?

• Ya’akov

#### MrChips

Joined Oct 2, 2009
26,791

#### Ya’akov

Joined Jan 27, 2019
6,279
Maybe the LED current is 5mA?
Could be 5W, 12V 5W LED replacements for 12V 50W halogen lamps are common.

#### biferi

Joined Apr 14, 2017
313
I am so sorry.

If I have an LED I would Deduct the LED Voltage from the Power Source Voltage to get the Voltage Drop.
Then I would Divide the Voltage Drop By the LED Current.
This will give Me the Resistor I need.

But if the LED was in Parallel with the another LED I would do the same Formula for Both LEDs Right?

#### Audioguru again

Joined Oct 21, 2019
4,958
Please post a link to the sales sheet or a datasheet for your 12V, 5A LED so we can see how many LEDs are in a string of LEDs and how many parallel strings of LEDs it has.
Maybe it already has resistors in it?

#### biferi

Joined Apr 14, 2017
313
I am ok with my LED Strip now.

But am I Right on if I have 2. LEDs in Parallel I First Deduct the LED Voltage from the Power Source Voltage to get the Voltage Drop.
Then I Divide the Voltage Drop By the LED Current to get the Resistor I need.

And I do this for Both LEDs in Parallel ?

#### Papabravo

Joined Feb 24, 2006
18,992

#### MrChips

Joined Oct 2, 2009
26,791
Putting two LEDs in parallel is problematic.
Show us the datasheet of both LEDs first then we may be able to provide an answer.

#### Audioguru again

Joined Oct 21, 2019
4,958
Nobody makes a 12V LED. An LED is about 2.0V for a red one or is 3.3V for a blue one. Your "LED" must have a few LEDs in series to be called a 12V LED.

LEDs have a range of voltages, then two LEDs with the same color and same part number in series will have one very bright LED and the other might be very dim or not produce any light unless you test many LEDs and parallel two or more that have exactly the same forward voltage.

Your LED might look like and be built like this one where all the LEDs have matched voltages:

#### biferi

Joined Apr 14, 2017
313
I have a 16 Foot LED Light Strip.
It is Rated 12 VDC at 15 Amps.

I wanted to Connect it to a 12 Volt Battery I have just to See if it works.
And it does work but now I just need to know about Project LEDs?

Say I have an LED that is Rated 3. Volts at 8. mA and I want to Find the Resisor I need for it.

I Deduct the LED Voltage from my Power Source Voltage this give Me my Voltage Drop.
I then Divide the Voltage Drop By the LED Current this will give Me the Resistor I need.

And if I have LEDs in Parallel I would do the same for each LED?

Do I have this Right?

#### wayneh

Joined Sep 9, 2010
17,172
And if I have LEDs in Parallel I would do the same for each LED?

Do I have this Right?
Yes, each LED "string" (in your case just a single LED) is treated independently. This assumption only fails when the current capacity of the power supply is a limiting factor. As long as it has plenty of reserve capacity, one string will not affect another.

Note that if you have enough voltage, it's more efficient to put multiple LEDs in a series string and use a single resistor to limit the current, which will be identical through all of them.