Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!bridge2!gpz
From: gpz@bridge2.ESD.3Com.COM (G. Paul Ziemba)
Newsgroups: sci.electronics
Subject: Re: Transformer Troubles
Message-ID: <811@bridge2.ESD.3Com.COM>
Date: 20 Jul 89 02:13:19 GMT
References: <536@winnie.fit.edu>
Distribution: sci.electronics
Organization: 3Com Corp., Mt. View, CA
Lines: 71


acs60222@zach.fit.edu ( ENRIQUEZ) writes:

>I have encountered a problem in which a expensive solution has been
>recommened.  I have a 120v/12v transformer driving about 25 12v, 20
>watt halogen bulbs (I am not sure of the output current of the trans-
>former).  The problem is that after the 12th light is installed,
>the system begins to dim, and by the 20th bulb, almost no light is
>present.

Let's see, that's about 42 amps at 12v you want. Are you sure that
the house wiring can handle that amount of current? For that amount
of current, I'd use at least 8 gauge copper wire. Anything smaller
is probably inviting disaster!


>The solution (given by an electrician) is to buy an large, expensive
>transformer which will supply more current.  Simple enough, I say, but
>I began to think that there might be a more elegent solution (i.e. 
>cheaper)

A reasonable solution. You can get 50 amps at 12v transformers from
surplus places (granted, you might have to mail-order them. The rest
of the world is not filled with electronics surplus places like the
santa clara valley). 

>Since this is for a pre-wired house, not a lot of rearranging of bulbs,
>wires, etc can be done.  Being a *lowly* undergrad in CP and having some
>slight EE experiance, I began to think about putting another, smaller
>transformer is parallel with the other to increse the driving current.
>However, I doubt this simple idea can work and would enjoy hearing why
>it would not.

Well, that could be dangerous if the two transformers are not identical.
Let's say one transformer has a slightly higher output voltage than the
other. You would burn up a lot of energy just between the two transformers
even with no load. If the voltages were V1 and V2, and the impedances
of the secondary windings were Z1 and Z2, you would have |V1-V2|/(Z1+Z2)
amps roaring around just between the two transformers.

It could work if the transformers were identical; that way you would
minimize the power lost "internally" to heating.


Another possibility would be to partition your circuit into two
halves. It probably would involve some hunting around the various
fixtures, disconnecting the wires on each side and finding out
where to break the circuit. Then you could buy a 25 amp transformer
and use it on the other half. 

(I am assuming a topology like this, initially:)

             break here
                  V
   -------------------------------
   |   | | | | | | | | | | | | | |
   T   O O O O O O O O O O O O O O
   |   | | | | | | | | | | | | | |
   -------------------------------
                  ^



Good luck!

 ~!paul
-- 
----

Paul Ziemba  ...!pyramid!zapi!gpz  gpz@bridge2.3com.com
		(415)940-7671 (w)