Usb power

  ashleycardwell94 23:11 08 Jan 11

I need to convert USb ( 5v ) to 3v. I need to run an LED light set.

I have a bunch resistors but i need to know, what sort of resistor do i need and do i need a relay. can it be done without a relay?

  robin_x 00:30 09 Jan 11

See these simple guides.

click here

click here

  robin_x 00:33 09 Jan 11

Any old type of resistor is fine

  Bris 17:30 09 Jan 11


Excellent links.


Do you need a relay? No, unless the light set exceeds the 500ma maximum of a USB port and then that assumes you will use the USB supply to operate the relay and thus require a seperate power supply and circuit for the light set.

The fact that you are talking 3V suggests that the LEDs are connected in parallel, can you confirm this, also how many are there and are they all the same colour?

The maximum I would run from a USB port would be 20.

As you are intending to use USB as a power source, personally I would connect up the light set to a powered hub and avoid using a port on the PC.

Have you considered using a mains adaptor instead of USB?

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