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From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: 100V Japanese products on 120V US lines
Summary: it's magic
Message-ID: <1793@osc.COM>
Date: 2 Jan 90 22:25:46 GMT
References: <3215@uceng.UC.EDU> <1201@ariel.unm.edu> <25539@cup.portal.com>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Object Sciences Corp., Menlo Park, CA
Lines: 19

In article <25539@cup.portal.com> dbell@cup.portal.com (David J Bell) writes:
>Only a small quibble: the *primary* current is what you have to worry about,
>not the secondary: since all the load current flows through both transformer
>windings, they get equal current, unlike the usual case where the primary is
>low current and the (step-down) secondary is much higher. You'd want to first
>find out what the primary current rating *is* before selecting the transformer
>to use. To wit: 
>
>Power_rating = sec._voltage (24) * sec._current_rating (e.g. 3A) = 72 Watts
>Primary_current_rating = Power_rating / 120VAC, or 0.6 A in this example.

This is incorrect.  The secondary carries the output current, so, assuming we
have a 120VAC to 20VAC transformer, the primary must carry only 1/6 of that
current.  Also note that only 1/5 of the power consumed by the load actually
flows through the transformer, from the secondary to the primary.

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