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Is it safe, or maybe I should I say "logical", to take an AC adaptor and splice the wires, reconnect them and solder together with a USB cable to power a device?

I had an old gameboy advance sp laying around and had an idea (sometimes my ideas are good, but most of the time they are not so good), well I cut the ac adaptor wire about half way and then took a usb cable, cut it and then soldered the wires together. Not knowing which wires should be connected to the corresponding wire, I took a guess and matched most of the color coded wires together. There were some that didn't match up so I just guessed and prayed.

The original plan was to be able to use it as a transfer cable, but like I said sometimes my ideas aren't so good. It turned out, it actually charged the gameboy. To my surprise, nothing bad happened, it charged the gameboy just fine, no smoke came out from anything, no sparks either. But that brings me back to my original question; is it safe? Would the watts or amperage be an issue with the original charger being 100V-120V?

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    \$\begingroup\$ There should be a small type plate on the power supply with specifications. (eg. like the 100-120V, what else is printed on there?) \$\endgroup\$ – jippie Mar 29 '13 at 19:31
  • \$\begingroup\$ USB is 5v. if your ac adapter is 5v you can do it but be careful with the wiring \$\endgroup\$ – user23914 May 15 '13 at 1:46
  • \$\begingroup\$ which cables to which cavles did you join?? \$\endgroup\$ – user25248 Jun 15 '13 at 18:58
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It can be safe, and it can be logical, but based on your description, it is neither. By "AC adapter" I am assuming you are talking about a regulated wall pack (wall wart, as some people say)...

Every device has different power requirements: voltage level and current input. It is becoming easier to mix things now that so many devices charge with USB like cell phones, tablets, etc. USB is 5V DC, and can have a wide range of currents. First, you have to make sure that the AC adapter has a 5V output, and not some other value like 6V or 12V DC. Next, you have to see what the current output from the adapter is. It may well be higher than your intended device was designed to handle. It also might not supply enough current to power the device. This information should be printed on a small sticker on the side of the AC adapter. The power requirements for your device should be in the operator's manual.

On top of that, a USB cable has at least 4 wires in it. A power cable typically only has 2, unless the AC adapter has additional sensor wires in it. Yes, typically a red wire is V+, and a black, white, or uncoated wire will be ground. But this is not always the case! You would really need to test each wire with a voltmeter to determine what the correct wires are.

A good piece of electronic equipment should have protection from over current, over voltage, and reverse voltage, but not everything does. If you apply the power backward or use too high of a voltage/current, you could easily damage the circuitry or battery you are trying to charge. Also, not everything has an internal battery charger circuit, some of them rely on the circuit held within the AC adapter designed for that device. If you connect it to something else, you could damage the battery or cause it to overheat and explode.

Lastly, I don't understand how you plan to use an AC adapter as part of a "transfer cable." That doesn't make any sense.

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With respect to @Kurt E Clothier's answer above, there are some clarifications I feel that need to be made.

You can't "overcurrent" a device. It just won't use the excess. If your AC adapter has higher current rating it won't destroy your device.

You can certainly exceed the device voltage which is why his AC adapter should be 5V rated.

USB is 500mA max but an unenumerated device can only draw 100mA. When you plug a device that is meant to be powered through USB to your computer it can only draw 100mA which is why most USB power stealing designs (i.e. ones that don't enumerate) are designed with that limitation in mind.

USB charger cables only have power and ground connected. You can leave the rest of the pins unconnected and a device that is powered through USB would still work.

As long as your wallwart puts out 5V at 100mA (which is minimum for USB) it should work.

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    \$\begingroup\$ This should have been addressed as a comment or edit to my answer, not a separate answer that doesn't offer anything new. I never said that the wall adapter WOULD over-current the device; however, that is a possibility. Some chargers are constant current, hence, they output a specific current which could damage a connected device. The OP obviously doesn't know much of anything about electronics. That is why I was skeptical of the logic behind what he was trying to do. He says "I took a guess and matched most of the color coded wires together" which is never a good idea. \$\endgroup\$ – Kurt E. Clothier May 15 '13 at 3:49
  • \$\begingroup\$ It wouldn't let me post it as a comment because it was too long. I wasn't aware I could "edit" your post. I don't see the reason for downvoting. He talked about an AC adapter not a battery charger so I don't see how constant current would apply. \$\endgroup\$ – EEToronto May 15 '13 at 3:57
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    \$\begingroup\$ The AC adapter for a portable device is a battery charger, they just aren't always designed very effectively or efficiently. In this particular instance, you are right - there is probably little chance of over current to be a possibility. However, in general, I would recommend against taking that chance unless you know what you are doing. \$\endgroup\$ – Kurt E. Clothier May 15 '13 at 4:25
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A USB wire has two main functions generally: power and data transfer. The usb data standard is not similar to the 90's standards. The power supply for your gaming device has only one function: power. You will not be able to transfer data to a device designed to power devices as well as to send and receive their data. Further, the gaming device is not meant for external data transmission.

In theory, however, you should be able to power the device in the aforementioned manner by connecting the red and black usb wires with the appropriate ones on the adapter. However, the usb may provide insufficient power voltage and amperage. Another worthwhile mention is that this action may not always be safe and it will usually not work. However, if you do your research, it might be fruitful.

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