# How can i tell how much power a usb port supplies? do they all supply the same amount?

I'm trying to make a cool lamp that would be powered out of a computer and i don't know how to find out how much voltage comes out of a my ports.

I'm trying to make a cool lamp that would be powered out of a computer and i don't know how to find out how much voltage comes out of a my ports.

active| newest | oldestUSB 2.0 can deliver 500 ma at 4.4V - 5.25V

USB 3.0 can deliver 900 ma at 4.4V - 5.25V

A Dedicated USB Charging Port can supply a max of 1800 ma at 5.25V

Sleep-and-charge USB ports can be used

to charge electronic devices even

when the computer is switched off.

Today's LEDs light up any where from 1.8 VDC for Red up to 3.6 VDC for White up to 4.0 VDC for Blue up to 4.5 VDC for Ultraviolet.

You can see three Red LEDs in series would need 5.4 VDC that is more then a USB output can deliver.

These days I decide the super bright LED voltages are fixed on arrival at my door step, meaning that I measure their voltage at the recommended current when I get possession of them in my hand. Then for a long life these LEDs are bright enough to reduce the current by 15%.

Back to your question, each of your 3 LEDs must have its own series resistor, and the the 3 groups must be wired in parallel to the USB port .

The math is simple, Subtract the LED VDC from 5VDC that is your working Voltage, divide the current into the working Voltage and you have a value for the resistor.

First example; Red LED 1.8V less 5V gives a 3.2 working Voltage divided by

20 ma gives a 160 Ohm resistor for the Red LED to light,

Second example; Blue LED 4.0V less 5V gives 1.0 working Voltage divided by

20 ma gives a 50 Ohm resistor for the Blue LED to light,

Yes you will need to use a Real 49 Ohm 1/8 Watt and a 150 Ohm 1/8 Watt as the series resistors.

The resistor LED combination will light each led from the USB power.

Hope this helps you.

I have a circuit that lights a 12 Volt LED from only 5VDC and pulls 42mA, two transistors. it can even be made to run at 1.5 VDC and drive a 12 Volt LED circuit.