DC Power Supplies

[moparx12]

Hey guys,

I have a general question about power supplies. Specifically the ones we use to power anything that runs on DC in our breweries.

With efficiency losses aside, if i have a 5A, 12V power supply, does that mean that the 120VAC powering the supply would need to provide 5A? It seems kind of silly because that would be ridiculous power loss - 5A @ 120 vs 5A @ 12VDC on the output. I guess it would make more sense that the current input would be ~.5A @ 120 to get 60W. I just want to verify.

 

[DeafSmith]

The current spec. is for the output; i.e., 5 amps at 12 volts, or 60 watts, so the input current will be much less, but more than 0.5 amps because some power will be lost as heat. If you assume the supply is, for example 70 % effecient, then the input power would be 60w/0.7, or about 86 watts and thus input current would be 86 watts/120volts = 0.715 amps.

 

[moparx12]

thanks!

 

[audger]

as a general rule, when you double the voltage, you halve the amperage required.

so 1 amp @ 120v would be 10 amps @ 12v to do the same amount of "work". (10x more voltage is 10x less amperage)

(2 amps @5v) does the same amount of 'work' as (1amp @ 10v)

this is why large transmission lines are 100,000+ volts. at that voltage, the amperage is much smaller. amperage and resistance is what causes heat to be produced, so with less amperage you lose less energy to the resistance of the wire.