Resistor for LED Lights (72W / 24V)

[Nishan.dj]

Hi Friends,

I am having a pair of 72W LED Spot Lights. Here is the spec for those,

Power : 72W
Inout Voltage : 24 or 12V
No of LEDs : 24
Current : 12V / 2.25A OR 24V/1.125A
 

And I would like to install this on my Jeep. It is 24V and having two 90A batteries. So, I need to install a circuit or a resistor to manage the current I hope. I consider 2 options.

1. Buy a voltage step down unit for 24V to 12V (5A). Because sometimes the system goes up to 28V due to charging and discharging. So, this way I can get exact amount of 12V so the voltage will not fluctuate.

2. Add just a resistor as per your suggestions. 

So, Please help me with this.

Thank you.

[Rumesh88]

@Nishan.dj Well according to the specs the light has switch mode regulator built into it to take care of the LED current. All you need is just connect it to either a 12V or a 24V system.

[MV-5]

what is this 72W rating, it is not the consumption (12 x 2.25 = 27W), right ? 

[Rumesh88]

  On 6/27/2017 at 7:07 AM, MV-5 said:

what is this 72W rating, it is not the consumption (12 x 2.25 = 27W), right ? 

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No it is not the consumption. In fact this 72 watt power rating is something like the RMS power to peak power to Peak Musical Power Output (PMPO) ratings used in audio power amplifiers. So the audio amp manufacturers quoted for the same 10W RMS amp and figures of 20W peak or 40W PMPO just to impress their customers. 

If I may explain in a simplified model, let's say I use a 100W incandescent lamp with a beam width of 360 degrees to get a certain level of luminous intensity (measured in candela or cd). Now if I use a perfect reflector to reduce the beam width to 180 degrees the luminous intensity doubles. Then I can use a 50 watt lamp and say I have a light source of 100W equivalent power with a beam width of 180.

[The Stig]

  On 6/27/2017 at 7:07 AM, MV-5 said:

what is this 72W rating, it is not the consumption (12 x 2.25 = 27W), right ? 

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Good point, P=VI

I think most of these Chinese light outputs are exaggerated. 

[Nishan.dj]

  On 6/27/2017 at 8:23 AM, The Stig said:

Good point, P=VI

I think most of these Chinese light outputs are exaggerated. 

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This light has 24 3W LED bulbs. Here is the each 3W LED spec;

Power: 3W
Luminous flux: 240LM
Input voltage: DC 3.2-3.4V

Datasheet is attached. Thanks.

[Rumesh88]

The above specs of individual LEDs shows 3W as an absolute maximum rating at 25C ambient temp. So the actual consumption is always less than that and the figure 72W (3W x 24) in the lamp spec sheet does not make much sense. In fact if you are comparing lighting power of LEDs or LED based lamps you should always compare the luminous flux (lm) rating and the 50% power angle/beam width.

[The Stig]

We were saying that the wattage indicated should be proportionate to the power drawn. Watt = voltage X current (amps) 

For the light to output 72w the current draw at 12v should be 6 amps (assuming there is zero wasted energy). Which is not the case. 

Or are they implying that it produces a similar light to a 72w incandescent light.