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USB wall adapter for powering usb devices

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dmorr

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Hey There,

I have a music keyboard which can be powered via USB. I would like to buy a usb wall power adapter so I can plug it into a wall and not have to have a keyboard around. Will this work if I buy a USB wall adapter for an iphone/ipad or something like that? Because it seems to me that when devices are powered via usb they draw only as much power as they need, as opposed to a wall unit which would just give a constant stream of 5V and 500mA. So I am worried that my keyboard might not need a full 5V or 500mA, and may get damaged by using a wall adapter.

Can someone tell me whether this will work or not?

Thanks!
 

It will work.
USB supply voltage is 5Vdc . That is OK.
Only thing is that supply current of power adapter must be equal or higher than your keyboard needs.
ie. if your keyboard current is 500mA or lower ,then power adapter for 500mA is that what you need.
 

Awesome. Thank you for the response!

Just out of curiosity what would happen if the voltage was like 5.25 volts? What is the general voltage threshold before you start to risk damaging the device?

thanks!

dmorr
 

Specified USB port supply voltage range is 4.75...5.25V.

Voltage Low-power device............... 4.4 to 5.25 V
Voltage High-power device ...............4.75 to 5.25 V at upstream connector
Maximum low-power current............. 100 mA
Maximum high-power current............ 500 mA
Maximum power draw ......................2.25 W

How the device behaves outside this range is device-specific property.
 

Because it seems to me that when devices are powered via usb they draw only as much power as they need, as opposed to a wall unit which would just give a constant stream of 5V and 500mA. So I am worried that my keyboard might not need a full 5V or 500mA, and may get damaged by using a wall adapter.
This is a common misconception. A power supply, whether it's USB or something else, does not force the full rated current into the load. As with your understanding of a USB-supplied system, the load will draw only the current it's designed to draw and no more. The current rating of a power supply unit is just the maximum that should be drawn from it without overloading it.

Having said that, to complete the picture, there's one type of supply that does force a pre-determined current into a load. That type is called a current source or a constant current supply. It's a special type of power supply which is generally encountered only in niche applications. It's often an integral part of a complete system rather than a standalone unit. It definitely does not apply in your case.
 
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