Organize and Share your Electronics the way you want. Sign-Up for a free account now. It takes only 30 seconds!

How much power does your circuit use?

A couple of days ago, I got the following email from Jacob:

[JACOB]
“I have been reading your “Getting Started with Electronics” book and have a question over problem 3.1. While I understand using I = V/R to find 0.07A which includes using 7V in the equation related to 9V battery and 2V LED, but why do we not use the 7V again in the equation P = I*V? Instead, in the answers, you use 9V.

When figuring power consumption of a circuit, do we only use the voltage being supplied and not what is being used in the circuit ie. the LED? I hope I’m not asking a stupid question.

Thanks,
Jacob

BTW: I’m enjoying your book very much so far.”

[OYVIND]
“Hi Jacob,

Great question!

First we find the current in the resistor, which is 0.07A. Since there is only one path for the current, the same amount current will also pass through the LED. That means the 0.07A is the current in the whole circuit. The voltage of the whole circuit is provided by the 9V battery, so to find the power consumption of the whole circuit you need to use 9V in the P = I*V formula.

If you use 7V in the power formula, you only get the power consumed by the resistor. If you use 2V, you get the power consumed by the LED. 9V gives you for the whole circuit.

Cheers!
Oyvind”

You can find The eBook Jacob is reading, and learn for yourself how to build electronics here:
http://www.build-electronic-circuits.com/products/ebook-2nd-edition/
Keep on Soldering!
Oyvind

Single-Sided USB Charger
A FPGA controlled RGB LED MATRIX for Incredible Effects – the Hardware
Using Nanotimers to Reduce IoT System Power Consumption by an Order of Magnitude
Replace a microwave’s beeping with the Windows XP startup sound
A DIY Laser Scanning Microscope
Reverse-engineering the surprisingly advanced ALU of the 8008 microprocessor
ESP8266 LED lighting: QuinLED v2.6 PCB
Switching LVDS Graphics in a Laptop Computer
Teleknitting: TV-based string art
DIY Bubble Machine
Start your day with Nerf target practice!
Teardown of a Peaktech 6225A

Top