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

1 Answer

Answer by mikerogerswsm

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

Your Answer

You must log in or create an account (free!) to answer a question.

Log in Create an account

Go Ad-Free. Activate your CircuitLab membership. No more ads. Save unlimited circuits. Run unlimited simulations.

Search Questions & Answers

Ask a Question

Anyone can ask a question.

Did you already search (see above) to see if a similar question has already been answered? If you can't find the answer, you may ask a question.

About This Site

CircuitLab's Q&A site is a FREE questions and answers forum for electronics and electrical engineering students, hobbyists, and professionals.

We encourage you to use our built-in schematic & simulation software to add more detail to your questions and answers.

Acceptable Questions:

  • Concept or theory questions
  • Practical engineering questions
  • “Homework” questions
  • Software/hardware intersection
  • Best practices
  • Design choices & component selection
  • Troubleshooting

Unacceptable Questions:

  • Non-English language content
  • Non-question discussion
  • Non-electronics questions
  • Vendor-specific topics
  • Pure software questions
  • CircuitLab software support

Please respect that there are both seasoned experts and total newbies here: please be nice, be constructive, and be specific!

About CircuitLab

CircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems.