Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify? What if I don't know the device's specs, but am replacing a previous power supply with particular ratings?
Is it OK to go lower voltage, or should it always be higher? What about current? I don't want a 10 A supply to damage my 1 A device.
Power supplies you usually find in the market are switching power supplies, when the specification of a power supply says voltage: "x" and current: "y", means that the level of voltage "x" will always be constant (ideally) if the load current (your device's current) is lower or equal to "y", in some power supplies if the load current is lower than "y" the voltage may increase, however if the load current is bigger than "y" the voltage will always decrease.
The power your load consumes is equal to the nominal voltage times the nominal current, and the nominal power delivered by your power supply is "x" times "y".
Devices usually have specified nominal working voltage, say V_device, and allowed range of tolerance, e.g. +/- 5%. Current specified for device is usually maximum current they are expected to draw from supply, say I_device_max.
When selecting supply, its voltage should be the same as one specified for device, i.e. V_supply=V_device, and current rating for supply should be at least, but usually higher, than that of device, i.e. I_supply>= I_device_max.
Higher current rating of power supply will not damage the device with lower specified current ratings. In this case care should be taken for security fuse, if it not part of the device but relied on one at power supply side. Fuse should match the device current rating and of proper type of reaction.
Higher voltage than maximum rated one (V_device_max) can destroy the device, more it exceeds it higher probability of causing some damage.
Behavior on lower voltages than minimum rated one (V_device_min) is dependent on device, but usually will not harm the device, but it may not function properly, or it will not enter in functional mode at all.
Smarter devices usually have special circuit that blocks (RESET signal) functionality if power supply voltage is lower than V_device_min. Protection from higher voltages than V_device_max is done with electronic components (varistor, transient-voltage-suppression diode) that will cause rise of device current above the normal one with intention to prevent voltage rise, and if over-voltages last longer burn the protection fuse (if rated properly) or something else will burn, usually some PCB track.
Rated voltage is the voltage that a power supply can constantly provide to load. Rated current is the maximum current that a power supply can provide to its load. Rated power is the rated voltage times rated current. Normally, the manufacturers give you rated voltage and rated power of the power supplies. All power supplies have fuses to protect them from over currents.
Let say you have a 48VDC and 10A load which means it is 480W in terms of power. You should pick a power supply with Vrated=48VDC and Prated>=480W. If Vrated too much less than 48VDC your device might not work properly, if Vrated too much greater than 48VDC you might damage your device, and if Prated is too much less than 480W you might blow the fuses of the power supply.
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