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9 volt Battery usb charger?
Hello im building a usb charger and need clarification if using the following parts i can make a decent usb charger:
330ohm 1/2 Watt 1% Metal Film Resistors
5.1V 1N4733 1 Watt Zener Diode
9V battery
9V battery connector
Female Usb
Would this work?
thanks for reading
Well, you can connect the resistor and zener in series across the battery, and if you get the polarities right, you will get 5.1 volts across the zener, which can be used in a USB charger.
But check a few things. The current through the zener = (9–5.1)/330 = 11.8 mA, which is the most you can draw from the charger, and that is a pretty small amount. The number will go down as the battery ages. USB bus specifications says you can pull as much as 500 mA, so you are at a small fraction of the max, and won't charge most batteries.
Can you up the current? The 1 watt limit on the zener means the max current is 1/5.1 = 196 mA, a lot better. That means you should change the resistor to R = 3.9/0.196 = 20 ohms. The resistor needs a power rating of 16/20, close to 1 watt, so you need a 1 watt or higher rated 20 ohm resistor.
Not that at 196 mA, you will get only an hour out of the battery so include a switch that disconnects the battery. Overall, a zener regulator is not very efficient, wastes a lot of power.
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