# Need assistance to understand Power and Ohm's Law

Hello I'm looking at a light bar on a vehicle and the specifications say it draws 7.5A at 9-32V and the light bar says it has 36 x 5 Watt LED lights which would make a total of 180 Watts that this device uses is this correct?. My confusion is how can it draw a constant 7.5A at each voltage between 9-32V? If the voltage increase how does affect energy used over time if the current is still flowing to the device at the same speed? and if is powered by 32V at 7.5A this would be a total of 240A which is too much for the device. Would the internal resistance be changing to keep a constant current of 7.5A?

Thank you

The light bar you described seems like it could be using an internal regulator to actually power the lights. It's possible that this has a maximum output power rating regardless of what you provide to the light bar. This regulator will take the 9-32V input and convert it to potentially a 12 / 24V rail that is actually powering the LEDS.

If you could share a link to what LED bar was used that might help us explain what's happening.

Just to clear up your maths and question about Ohm's law, with the provided information you have said that the Light Bar states Max current of 7.5A and an operational voltage range from 9-32V.

The image below shows the equations you should be using to calculate this, to calculate power with the values you have you should do I X V = P. If you want to work out the max current we should do P / V = I. Knowing this we can Knowing this we can calculate all possible combinations, as shown below, Assuming the 7.5A limit is used across all voltages you will be current limited at any voltage below 24V meaning the maximum power output would drop as shown.

From my calculations you can see that the Current rating provided lines up perfectly with the 24V @ 7.5A = 180W so it's likely the numbers you are reading are referenced to a 24V supply. This is also quite a common supply rail for LED's so that seems to make sense.

You're assuming that the current is always 7.5A. Manufacturers often quote the maximum current draw. If the light bar contains a switch mode power supply, then the current draw will decrease if you increase the voltage, keeping the wattage approximately constant.

Just because the light bar contains 36 x 5W LEDS, it doesn't mean that each LED is actually being driven at 5W. Running an LED at less than its rated power makes it easier to keep it cool. It's also a very good way to fool customers into thinking they are going to get a light that's a lot brighter than it really is.

It might have a power supply inside it to drive the LEDs. So there is a good chance the 7.5A is rated nominal value for rated nominal supply, and it will vary according to the applied voltage.

Or it might have a constant current driver so it wastes extra power as heat.

But 180 watts at 7.5A is exactly 24V so they might have rated the consumption at typical 24V usage.