Instructables

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.

iceng3 years ago
The following may be useful information;

USB 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.
jelly.turf (author)  iceng3 years ago
would a usb 2.0 be able to light up 3 leds?
Sure, but not in series.

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.
A USB port CAN supply up to 500mA, BUT ONLY if its negotiated the connection. with the computer.
But! To answer your actual question which I realized I didn't really do, you can measure voltage on anything by using a multimeter. With connectors like USB look up the pinout online and then apply the probes to the positive and negative pins. There are plenty of tutorials online at Instructables and elsewhere on how to use a multimeter. You should learn to use one if you want to do anything with electronics. The meter is a vital core element of the electronics workbench. You can get a decent one for $15-20. Get one with a continuity tester, as I've noticed that some don't seem to actually come with this arguably vital feature.
As mentioned by the others, yes: USB ports provide an average of 5 volts. If you need more you may be able to get ten volts out of two ports connected together, but if you use LEDs for your lamp you ought to be able to work with 5 volts.
caarntedd3 years ago
I think the voltage is 5 volts. The power output will depend on how much current is drawn by the device you plug into it. I don't think it is very much as the ports only supply milliamps of current.Somebody that knows more about this will probably correct my answer for you shortly.