## Can't understand resistors power limit

 Hey guys, I'm begging with electronics and I can't understand how to select the power of a resistor. My first project is a LED tree, with 16 branches and one white 5mm LED on the end of each. I'm using an USB charger so DC 5,25 V. I've connected one 150 ohms 1/4W resistor on the positive lead of each LED. Also I've connected one switch with one 'state' direct connected with the LEDs and the other 'state' passing through another 150 ohms 1/4W resistor. This resistor is meant to dim the light of the LEDs, so I can alternate between 'full' brightness and dimmed. Everything works, the current of the circuit é about 200 mA. My questions is about the resistor that I'm using to dim the light. I haven't notice any heat, but I want to understand how to calculate the power that go through the resistor and if a 1/4W resistor is enough! Thanks for the help and sorry about the English, it's not my first language. by EduSales October 26, 2018

 Power in a resistor is I squared R. This is equal to V squared divided by R. You are using 150ohms to dim the light, and it has less than 5.25V minus 3V across it. So the power is less than 2.25 squared divided by 150, or 34mW. It won't get hot. +1 vote by mikerogerswsm October 26, 2018

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