Faithful inSync reader Hugh A. asks about wall wart ratings (see WFTD archive Wall Wart):
“Can you elaborate on the mA (milliamps) rating on a wall wart? I know that milliamps matter, but will a 300mA wall wart damage a device that calls for 250mA, or vice-versa?”
This can get deep. Milliamps are thousandths of amps (see WFTD Amperage above). Any power supply has to be capable of delivering the required current (at the proper voltage) to a device it is powering. This means the mA rating must be as least as high as what the devices needs. If not, the device will load the supply down and try to pull too much current. At this point three things can happen: (1) The device will not have enough power to operate and will exhibit a variety of symptoms – none good (usually this does not cause permanent damage). (2) More likely the supply will burn up trying to supply the power needed by the device. (3) Hopefully a fuse or circuit breaker in the supply will trip and prevent (1) and (2). The bottom line: always make sure your supply can deliver the current (mA) required by the device it must power. It’s okay if it can deliver more current, that’s just extra available power that will not be used.
We’ll cover voltage requirements another time. For now suffice to say the rules are very different for voltage.