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I'm a bit new to this stuff so please excuse my elementary question. A gas generator rated at 700 watts and 120V puts out about 5.8 amps. Is it possible to step down the voltage to 12V with a power supply and make available 58 (DC) amps to draw?? Or is the excess voltage simply lost? Thanks.

Added:

The Generator has an AC output of 120V 1000W max (rated @ 8.3A).

Usage will be charging high capacity lithium Polymer batteries.
Basically I would like to have 20A available to charge.

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It doesn't have to be a switcher. With 120V AC you could use an AC transformer to get 12V AC at (somewhat less than) 58 amps, then a rectifier, reservoir capacitor and (if necessary for your application) voltage regulator to get 12V DC. (I'm simplifying a bit: unregulated, 12V AC would give closer to 15V DC)

A PC power supply will be easier to find at 20A+ ratings, but you may find e.g. a car battery charger that would do the job.

My main worry would be that the AC from a small generator might be too poorly conditioned/regulated for a PC power supply and cause it to fail. I might be overcautious here, but the transformer/rectifier solution is likely to be more robust. You don't say what the load is; knowing that may help improve the answers.

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  • \$\begingroup\$ Actually, switchers have a far wider input tolerance than linear power supplies, though that doesn't mean a given pair of generator & power supply is workable or safe. \$\endgroup\$ – Chris Stratton Sep 7 '13 at 19:40
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You don't say if the "generator has DC output or AC - I assume it is probably AC - which technically makes it an alternator.

If this is effectively "AC mains" then many mains to 12V power supplies rated at the required power output would work OK.

Re "... I really only need 20 amps..." -> A modern PC power supply with at least 20A capability at 12V would probably work - you MAY have to provide some load on 5V (but less liley on newer supplies) and there may be a minimum load top maintain regulation.

20A x 12v = 240W. Check specs of various PC supplies and see what is available.

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  • \$\begingroup\$ Here are some specifics. The Generator has an AC output of 120V 1000W max. rated @ (8.3A). Usage will be charging high capacity lithium Polymer batteries. Basically I would like to have 20A available to charge. So it appears that I simply need a power supply and I'm good to go. \$\endgroup\$ – Q Q Sep 7 '13 at 15:30

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