Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!uunet!portal!cup.portal.com!dbell
From: dbell@cup.portal.com (David J Bell)
Newsgroups: sci.electronics
Subject: Re: 100V Japanese products on 120V US lines
Message-ID: <25539@cup.portal.com>
Date: 1 Jan 90 07:06:29 GMT
References: <3215@uceng.UC.EDU> <1201@ariel.unm.edu>
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Organization: The Portal System (TM)
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Duke McMullan responded with a neat little bootstrap transformer circuit, and
eventually warned us:

              . . .                Wire appropriately; don't exceed the current
rating of the secondary (primary current won't be a problem).

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.

If the equipment you want to power doesn't give a current draw, calculate it
from the power spec: Equipment_power / 100 VAC = Equipment_current, which
must be significantly less than the transformer's primary current rating...

Good luck!     Dave   dbell@cup.portal.com