How do i know how many ohms are needed to drop the voltage of something from 9v to 5v?

Question

I am new to electronics and want to start with something simple, i want to use a 9v battery to power a usb device that is only 5v (if the volts of usb devices are different how do i tell?) i have a pack of 1/2 watt resistor's so how do i know what ones to use to drop the volts? there are a lot of different kinds of resistors in here, 100, 1k, 4.7k, 10k, 100k, 1 meg, and a whole bunch of others. Thanks a bunch

Answer 1

If you know the current, you could pick a resistor. R = (9-5)I.

So for example, if the current were 100 ma (0.1 amp), then the resistor would be 4/0.1 = 40 ohms.

BUT that doesn't really work, as the current drawn by most devices is not constant and varies. As it varies, the voltage will go up and down, and the device will not operate correctly.

The only way to do this correctly is to use a voltage regulator such as the 7805 which will output a constant 5 volts.

edit: there is no "standard current for a USB device", in fact that is not possible.  USB specifies the voltage (5v), and the maximum current (not sure, 500ma?), and various devices can draw anywhere between zero and that maximum.

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Answer 2

At what current? Or, what device? Ipod?