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[on:]

I have an existing usb device that I would like to add led's to. It's not arduino based at all. It looks like its only using 100mA. So I had some questions.

What is the best way to go about adding them? Is that 100mA usage limited by the device's design, such that using more than that would damage it? Or is it set in the usb controller and that's all the computer will deliver to the device?

Basically I have alot to learn and figured this would be the best place to start asking.

[Reply #1 on:]

The standard for USB is 500 mA of supplied current; now, that isn't to say -your- computer will supply that, knowing how manufacturers like to cut corners and such...

But anyhow, assuming that standard, you could add extra LEDs to the device, provided you added them before any on-board regulator (unless that regulator had some headroom beyond 100 mA).

Your best bet would be to solder your LED supply "bus" to the 5V line on the USB port on the PCB of the device; hook your LEDs up, paying attention to giving them each the proper amount of forward current to turn them on (using ohm's law and knowing the forward current and supply voltage, you can figure out the needed resistors). Drop each one from the bus, to the resistor, then to the LED, then from the LED to ground; as long as all the forward currents (plus 100 mA for the device) doesn't exceed 450 mA (to give everything some head room) - it should work OK.

[Reply #2 on:]

I am just questioning crush's explanation.
I was under the impression that each USB device tells the computer how much power it requires and that ,and only that, is supplied to the device up to 500mA.

Each device contains its manufacturer's ID, its unique productID, ,its power requirements ,its interfaces and details of each interfaces addresses on each device, is that not so ?

here is what is returned from a typical USB enquiry

Configuration descriptor:
Total length: 0022
Num.intf: 01
Conf.value: 01
Conf.string: 00
Attr.: A0
Max.pwr: 32

This power requirement is in hex = 50mA
I think that's the max the computer will then supply