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I am basically a software engineer but have some interest in electronics so this is probably a very basic question.

I understand theoretically a battery or power source can provide as much current as needed by the circuit. However, I see in battery charger they say 5v and 500 mA. I don't understand what this is. Isn't it the current will be drawn as needed by the circuit? I understand if too much current is drawn it might burn something.... but having a current specified on charger (or a power source) is what I don't understand. Same is the thing in case of computer's USB (I heard it cannot supply more than 500mA. what if a circuit tries to draw more than 500mA?).

Hope I was able to explain my question well.



marked as duplicate by PeterJ, Daniel Grillo, Asmyldof, nidhin, Dave Tweed Jan 30 '16 at 2:56

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The voltage rating is the nominal fixed output voltage. The current rating is the maximum current that the supply can reliably give while still keeping the rated voltage.

Depending on the charger/wall supply, going over the rated current could do a few things including:

  1. Output voltage sag and increased ripple voltage as the regulator is taxed too hard.
  2. Overheating/damage as components inside are taken above their rating.
  3. Automatic shutdown if the charger has overcurrent protection.
  4. In the case of USB, the port can be shutoff until the overcurrent condition is fixed.

As a rule of thumb, you must have the correct voltage charger and at least the amount of current rating that the device that's being powered will draw.

5V is a fixed value. Can't be more or less than you need. 500mA is a max value. Just needs to be greater than the current you need.

  • \$\begingroup\$ USB is a bit of an exception since USB devices actually "ask" the computer or charger how much current they can draw, and can thus automatically limit their current draw to the largest permissible current. This is why your phone/tablet will usually charge faster when using a charger with a higher current rating. \$\endgroup\$ – jms Jan 26 '16 at 16:38
  • \$\begingroup\$ They can ask, if both sides are properly engineered. Barring that, the actual bus voltage can be measured and, if it dips too far below 5V, the port can be shut off. The AM335x does this, for example. But, you're right, the USB case is more complicated and that's not even mentioning new USB 3 devices with higher power/voltage capabilities. \$\endgroup\$ – scld Jan 26 '16 at 18:14

There is a difference between a battery charger, and a charger power supply. Unfortunately many people don't make this distinction, leading to confusion over what the specs mean.

A battery charger limits current going into the battery, and its current rating will be matched to the battery it was designed to charge. Depending on the battery chemistry, voltage may also be limited, eg. Lithium Polymer is 4.2V/cell. However the label usually shows the nominal voltage, eg. 3.7V for a 1 cell Lipo, 7.2V for a 6 cell Nicad.

Any 'charger' that is rated at 5V and has a USB socket or cable is actually a power supply, designed to emulate the 5V output of a USB port. The charging circuit is built into the device being charged. Standard USB can put out 500mA, but a device may ask for higher current if the port can do it. USB Power supplies designed to work with such devices have resistors connected to the data pins of the USB socket, to fool the device into thinking that they are a high current USB port.


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