# What kind of resistor do I need to turn 9v into 5v?

I'm making a usb charger and can't understand resistors, so can someone tell me what kind of resistor I need to make a usb charger from 9v battery?

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8 years ago
You need to use a voltage regulator. Doing that from a 9v battery is going to waste some of the battery though.

Like this.
What exactly are you trying to do? If you're trying to run a USB powered device away from a computer, using 4 x AA rechargeable batteries in series may be a better way of doing it. This would give you 4.8V which would be close enough to the 5V USB for just about any application. You could even make a very simple trickle charger for this battery pack for when you are near the mains. If you use a 9V wall wart, you would connect negative lead to negative of the battery, and the positive to the positive of the battery via a 47 ohm resistor. (Note to Seandogue - A one resistor battery charger!) This would give you a charging current of around 100mA. You could even run your equipment with this connected as the batteries will limit the voltage to 4.8V to 5V.
6 years ago
so with what your saying is that if i want to charge a Ipod touch, Blackberrry... with a 9V battery i should use a 47 ohm resistor and it should give 100mA? and my Grand pa is a electrition and he told me that i would also need to use a diode 5.1V. is that true or do i just need the 47 ohm resitor to turn the voltage down to 5.1V and 100mA?
steveastrouk8 years ago
Funny, I always do it the other way round - regulators don't like being reverse biased IME. Steve
V-Man7378 years ago
Sometimes the device you're charging has a built-in regulator, so really you might not even need a resistor (for those really hardy devices, not a wimpy iPhone or cell phone).

If you wanted to be a little safer but still only use a resistor, figure out the current needed to charge your device (for example, 500 mA). Take the current and divide it by the supplied voltage (in this example, we come up with 18). The result is the amount of resistance (in ohms) you'd need to resist 9 volts enough to drop its current to the desired current (500 mA). The internal regulation of the device (again, if it has this!) will regulate the current into its proper voltage.

If you post what device you're looking to charge, we may be able to tell if it has the proper internal regulation circuitry.
8 years ago
I ran a CD player, among other things, directly from my car battery using this method.
orksecurity8 years ago
Y'know, just a few inches over to the right is a Featured instructable for Yet Another 9 Volt USB Charger, Looks like exactly what you want. Follow the Instructable.
Nameless37 (author) 8 years ago
Obviously I know this, I would hook the 9v battery clip to the resistor thru a swich connecting to the usb extension.
orksecurity8 years ago
V=IR. The voltage drop across a resistor depends on how much current you're pulling through it... and that isn't constant as the battery charges.

As others have said, you really want a regulator chip, plus a few supporting components. Still fairly ceap.