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Making a universal USB charging port? Is it possible to reduce a DC current's AMPs without affecting its voltage? Answered

After cleaning my drawer i've noticed that alot of my chargers these days are using a usb port to charge their device,
so i came with an idea to create a universal port that will charge all my devices so i got to thinking
First i would buy a AC/DC power plug(I live in Australia) to reduce the current to 5V @ 2.5Amps,
Link : http://bit.ly/L89ez5
I was planning to strip the end port so i'd have a positive and negative wire, which would eventually lead to the + and - power wires on a female USB port.

However i came to 2 problems with this.

1. Power Amperage/Voltage
Since it supplys 2.5Amps i found out this would be too much for some of my devices, after researching on the internet(Correct me if i'm wrong) I'm led to believe that with the way DC currents work, devices will only draw the amount needed, however my concern is that some devices sometimes make use of the extra amps and allow themselves to charge faster at the expense of a shorter battery life. To combat this i thought up of using a rotary switch linked with different amounts of resistors to allow me to select how many amps the port will give.
However my first problem is that after more researching that i can't simply use normal resistors to reduce the amps without affecting the Voltage. My aim amps choices are:
500mA(iTouch 4g)
1000mA(Mobile Wifi device)
1500mA(Playstation Vita)
2000mA(Just in case)
2100mA(iPad 2)
How would i achieve this? what resistors would i need to buy/how many? I read something about some forumla but couldn't find what exactly it is.

2. The Data Pins
During my research i found out my PSVita is picky with its USB Port choice, its one of many few devices that like to be charged using a dummy port, In which i found out a Dummy port has the Data pins shorted, if they arn't shorted then the device will refuse to charge from the port. At first i thought it would be simple, i'd just have a switch that would short pins when i needed it. But then i realized what about my normal devices? What would i have to put between the the data pins to allow normal devices to charge normally.

Any and all advice would be appreciated.

This is what i had planned:
(MS Paint!)


Have you seen this Universal USB Power Supply yet? Probably not because its not showing up in the main listings like it should. But for around $8 USD you can build that simple USB power supply that is able to power/Charge almost all USB devices.

You don't need to reduce the amperage from a power source. The rated amperage is the max available. Meaning that is the most it will allow a device to pull before it either shuts down or breaks. Your device will only pull what it needs. So the more your adapter is able to offer the better. But keep in mind the max amperage your voltage regulator is able to offer. Just read through my instructable i linked to and you'll have a better idea of what you will need to do. 

Thanks! This is exactly what i was looking for, Although I think i'll wait abit when you mange to integrate a switch to switch between 1A and 2A although i really hope you consider what one of the commenters said and use the 'thingy' that bumps it up to 5A.

The only thing i'm trying to figure out is a way to integrate a switch so that if its on, it'll receive the required voltages to satisfy ipods/iphones, and when its off it'll short the data pins to satisfy other devices such as the PS Vita and some phones.

Sorry for disappearing, school got hectic with tests.

If the newer version uses a pair of 7805s (which it won't) then the switch is needed to help conserve power. These linear voltage regulators are not all that efficient.

Any device that uses USB only needs 5V. What will vary is the amount of current it would like to have for charging. The more current that is available the faster the battery will charge. In the case of the iPad 2 it will charge on only 500mA to 1A. The problem is it can't charge and be used at those low amperage since the device can pull more then that under normal usage. The new design with the proposed regulator will not need any kind of toggle switch.

Something to keep in mid is if the main supply feeding the regulator only offers 1A you won't have a full 1A available on the output side.

"In the case of the iPad 2 it will charge on only 500mA to 1A."
Actually the iPad 2 charges at 5V @ 2.1A...

"Something to keep in mid is if the main supply feeding the regulator only offers 1A you won't have a full 1A available on the output side."
Is this is trade off from the regulator thingys in the circuit?
How much will the main supply need to feed if i want an output of 2.1A max?

Yes the charger that comes with the iPad is a 5V 2.1A charge but the unit will still charge with 5V @ 500mA, it just charges very slowly. The unit is able to charge on a regular USB port which only offers up to 500mA, but you have to keep the unit off or in standby otherwise it won't charge at all.

This is from the Apple Support Page:

Charging Tips

The fastest way to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter. When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.

This project as it currently stands uses the 7805 which is only capable of up to 1A output. So it doesn't matter if you put a 4A or a 10A supply on it the regulator will only draw what the output needs, up to 1A, and no more. I haven't had a chance to look at the recommended regulator for the improved version of this. So i'm not familiar with it's specs. I just haven't had time to revisit this project and see how everything will work out. 

A very long question for a very short answer:
You don't need to "reduce a DC current's AMPs".

2- From a standard USB socket, I say that most things charge anyway regardless of the data lines.


1. Could you care to explain why i don't have to reduce the amps?

2. Are you certain? Because if i know for a fact that iphones & ipods won't charge unless a certain voltage is sent between the data pins, but i'm not sure about other normal devices.

As you said devices will only draw the amount needed.

2 With the number of USB sockets about, including e.g. cars, I assume that for charge-only most devices just use the power (data serves no purpose for charging).


Most chargers now have circuitry to check the temperature of the batter, monitor and control the charge rate and turn off the charging when the battery is full.

On nicad batteries not having this smart charge circuitry just means shortened battery life.  On some of the more exotic battery chemistry it is much more serious.  Check out this clip before you decide to build a battery charger. 

Yea, thats one of my main concerns about this which is why i don't want to play around with allowing my devices the extra amps.

I opened up 2 of the devices and 1 is a Li-ion and the other is a Li-Polymer Battery, Are these 2 dangerous?

Yes, if not charged and even discharged correctly. There are numerous youtube vid's showing them catching fire by overcharging, shorting etc. They are safe is charged by the provided charger and used for the purpose intended and unsafe when abused.