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You sure about that?...

Posted by Steve O on January 9, 2017 at 20:28:44:
...using a simple single primary/single secondary transformer in a buck (or boost) config, it is the secondary current rating that counts because the secondary must pass full current drawn by the device for which it's bucking voltage. Since we know the secondary volts and amps we also know its VA rating. Neglecting losses, in this case the VA rating of the primary = VA rating of the secondary. If the the VA rating of the secondary is not exceeded it will not be for the primary either. So effectively, secondary current rating becomes the limiting factor: the primary goes along for the ride with its current reduced by the factor Vsec/Vpri , again neglecting losses.

If I get time I'll wire up something, make some measurements and report back!