Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!agate!apple!usc!csun!fedeva!premise!mirror!frog!john
From: john@frog.UUCP (John Woods)
Newsgroups: sci.electronics
Subject: Re: Transformer Troubles
Message-ID: <1660@frog.UUCP>
Date: 21 Jul 89 22:28:00 GMT
References: <536@winnie.fit.edu>
Organization: Misanthropes-R-Us
Lines: 51

In article <536@winnie.fit.edu>, acs60222@zach.fit.edu ( ENRIQUEZ) writes:
> I have encountered a problem in which a expensive solution has been
> recommened.  [ A transformer is not powerful enough to drive 25 halogen
> light bulbs. Proposed solution: a large, expensive transformer which will
> supply more current. ]

Paralleling transformers works quite well.  It helps if you match the
transformers so they put out exactly the same voltage, but there are
ways to balance them if they aren't quite close.

First, you need to get the polarity right.  A quick way to determine the
correct polarity is this:  Wire the primaries in parallel.  Wire the
secondaries in series, with a resistor (~ 1K) also in series.  Plug it it.
Measure the AC voltage across the resistor.  If it is double the transformer
output voltage, reverse the secondaries; if it is zero, you're all set --
remove the resistor and parallel the secondaries; if it is small but non-zero,
the transformers aren't well matched.  A scheme for matching transformers can
be found in The Radio Amateur's Handbook (by the ARRL), involving using a
center-tapped secondary of another transformer (which must be able to handle
the current but need not have any particular voltage rating, in particular can
be quite small).  I don't remember it exactly, but I believe it is

	     T1	----------------------------
		|                          | T3
	     |||S			|||S
	     |||S			|||S------------o   voltage out
	     |||S			|||S
	     |||S		           |
		|    T2	--------------------
		|	|
		|    |||S
		|    |||S
		|    |||S
		|    |||S
		|	|
		----------------------------------------o  voltage out

T1 and T2 are your power transformers, T3 is any low-voltage transformer
capable of handling the current of either T1 or T2, plus some small margin.
(Primaries are not shown; T3's primary is left unconnected.  If you feel
really adventurous, you could probably unwind a junk transformer and wind
a T3 yourself)

You should try to find a Handbook to check this yourself; I will post a
correction if I find that I am horribly wrong on my memory here.

-- 
John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101
...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu
    People...How you gonna FIGURE 'em?
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