Car LED lights get extremely hot.

[specter333]

Hello, I have very little electronics background and need to know if I am understanding the use of these lights correctly.

I bought these LED panels off ebay,
http://www.ebay.com/itm/262387867187?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

While researching I found more information about them on Amazon, 12v 6 watt.
https://www.amazon.com/dp/B00L11HM62/?tag=cpf0b6-20

They are made to directly replace a car's dome light without any extra limiting. However I found that they were getting extremely hot, too hot to touch. I'm not using them in a car but powering them from a 12v 1.25amp power supply.
http://www.ebay.com/itm/40086784319...49&var=670416496949&ssPageName=STRK:MEBIDX:IT

Is it normal that they would be to hot to touch? I added a 10 ohm resister and that helps the heat quite a bit but looks like I made a mistake.

If I understand correctly, at 12v and 6w they are pulling 1/2 amp each. So I can power only this pair from the power supply and would need resistors rated for more than 6 watts instead of the 1/4 watt resistors from my Arduino kit (they are getting pretty hot themselves). Is this correct?

 

[FRITZHID]

Those are poorly designed, cheap LED modules and almost guaranteed to fail at their "rated" specs. Some fail within an hrs use, others color shift, dim and then fail in a week or two. Overdriven, undercooled = LED failure.

 

[specter333]

Those are poorly designed, cheap LED modules and almost guaranteed to fail at their "rated" specs. Some fail within an hrs use, others color shift, dim and then fail in a week or two. Overdriven, undercooled = LED failure.

Thank you for ignoring my question and complaining about my choice of lights without even offering an alternative.

Can someone in the forum please tell me if I am understanding and doing the math correctly.

 

[FRITZHID]

Your math is fine, buy a proper product that's suited to your NEEDS and you'll be much better off, OR build one yourself.

 

[DIWdiver]

Your math is right but your assumption (which you may not even know you made) is wrong. The current will not remain constant when you change things.

When you add the resistor, you change (decrease) the current quite a bit. In order to calculate, or even make a good estimate of the new current, we'd have to know things about the LED that we just don't know.

Fortunately, there's a simple solution. If you have a voltmeter, just measure the voltage across the resistor. Then divide by 10 (the resistance), and that's your new current. Voltmeters can be had for a few dollars at many different places.

 

[specter333]

Ok, I'll dig out my multi-meter and measure it. Thank you for the genuine help.