I was reading through a quite detailed dissertation on derating transformers in an old thread, from April. and did not readily find an answer to my problem.....

We know, that a transformer, 120v primary, and a 12 volt secondary. Unloaded, the sec. will often read as high as 18 volts, which will drop down to a given voltage, based on the load you place on it.

How far can one push a normal step-down transformer in regards to available amperage.
I'm thinking that the circular-mil size of the secondary windings should indicate a limit of what it will put out for amps ?

I need 12 rectified volts to drive a cheap chinese air-mattress pump, and basic tests with its cigarette-lighter plug, indicates an approx draw of 3-4 amps....off a car battery............ 4-amp wall-warts are darn hard to find.

I have a real nice flat-pack transformer with an unladen voltage of 24 -- my pump is not likely to draw that down far enough to avoid overvolting the pump...........I would appreciate your thoughts and input on this.
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The limit is determined by the heating of the transformer due to resistive losses in the primary and secondary windings. Since this heating is proportional to the square of the current, it goes up rapidly with output current. That's why you shouldn't exceed the current rating of the transformer otherwise it will overheat and likely short the windings.

Look on fleabay. They usually have lots of various size wall-warts for sale.

 

I would think that pump can draw more ampere than you think. Any DC power in range of 12 to 14.5 volt can be used. To be on the safe side I would have selected a power with about 8 ampere as max

 

I have a very cheap little wall wart that produces 9VDC at 100mA. It gets hot.
But without a load its voltage is 18.5VDC.

A good quality Hammond 30V/4A transformer is 30.0V at 4A. It gets warm.
But without a load its voltage rises to only 31V.

 

How far can one push a normal step-down transformer in regards to available amperage.
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You can push it to the RMS current rating the manufacturer put on the transformer secondary. Period. And the secondary RMS current is almost always higher than the DC load current.

 

I need 12 rectified volts to drive a cheap chinese air-mattress pump, and basic tests with its cigarette-lighter plug, indicates an approx draw of 3-4 amps....off a car battery............ 4-amp wall-warts are darn hard to find.

Took me ten seconds. Cost $8 including shipping.

http://www.ebay.com/itm/LCD-AC-Powe...=Laptop_Adapters_Chargers&hash=item43b2b90dbf

 

Kowabunga !! Got it covered.......Found where I had stored [ where I could find them ] some of my Kensington universals, one having the 4 amps my monster requires -- -- Thanks guys