# On an AC-DC adapter, what is the use of the mA number printed on it? [duplicate]

releted to this question I wonder what the use of the number of mA printed on an AC-DC adapter/converter means.

I am charging a Bluetooth headset, which came with a AC-DC adapter, over USB using a "DIY-hybrid-cable" and it is working. The original AC-DC plug (which I've cut) has 450mA printed on it and the USB standard AC-DC plug, to which my "hybrid-cable" is connected, has 1.0A printed on it.

Does the mA number has any relevance? Why? I feel I am missing important knowledge.

• A constant-voltage supply doesn't determine the current, the device does! A device that "wants" 450mA of current will draw 450mA whether the power supply can only provide 450mA, or whether it could have supplied 500mA, 1A, or 2A. The current rating of a supply is what it can deliver, not what it will force through the device. – scjorge Feb 1 '17 at 6:05
• You can test the item against those numbers; if it falls short in a test jig, it is likely to be malfunctioning. There's a LOT more than two numbers, though, to really characterize the power requirements; accuracy, overvoltage tolerance, ripple when lightly loaded... those aren't on the label. – Whit3rd Feb 1 '17 at 7:08
• A constant-voltage supply doesn't determine the current: the load, which in this case is the device, does. The current rating of a supply is what it can deliver, not what it will always force thru the load somehow. In that sense, unlike with voltage, the current rating of a power supply must be at least what the device wants but there is no harm in it being higher. A 9 volt 5 amp supply is a superset of a 9 volt 2 amp supply, for example. – scjorge Jan 7 '19 at 15:21