I always thought that a transformer in most cases is 2 totally separate windings only connected by magnetism. I've been messing with some old UPS and noticed that no load draw a few A with a load it can be 15-25A. if the circuits are separate it would seem to me that input amperage would stay the same and output would always put out say 8amps. I thought this was true for computer PSUs too, but from what iv read and heard that isn't true because use of drives, cd, HDD, etc. adds to input draw.
why does this work this way when 2 totally isolated circuits?

 

The input and output of most transfromers are isolated galvanically with separate windings on the same core with energy being transmitted by the magnetic field from the input to the output. For an ideal transformer the energy is transferred with 100% efficiency so naturally the input current will directly vary with the output current. The relative values will vary by the relative turns ratio and voltage output between the windings. For conservation of energy V x I of the input winding equals V x I of the output winding.

 

Conservation of power.

The input power to the transformer primary is equal to the output power from the transformer plus a small amount for core losses, copper losses etc.

Therefore if the secondary current changes, the primary current also changes.

And autotransformers do not have a totally isolated primary and secondary.

 

ok that is kind of what I was thinking made since on auto transformer. figured related to conservation of energy, but just seems odd how 2 totally isolated circuits connected by 2 coils and a piece of iron can have this type effect.
this leads to another question I've been working with some old UPS measure input current with no load and load. I noticed just turning on inverter uses 1-2A but plug in something draws like say 4 A 125V AC it will max out my 25 A @12V DC analog Simpson meter. that seems kind of extreme to take that much input to get that little output. but may be because going ac to dc is a lot harder than AC-AC or AC-DC. or my meter could just be wrong.