Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!mit-amt!mit-caf!ankleand
From: ankleand@mit-caf.MIT.EDU (Andrew Karanicolas)
Newsgroups: sci.electronics
Subject: Re: Transformer Troubles (LONG)
Message-ID: <3054@mit-caf.MIT.EDU>
Date: 26 Aug 89 00:16:00 GMT
References: <536@winnie.fit.edu>
Reply-To: ankleand@mit-caf.UUCP (Andrew Karanicolas)
Distribution: sci.electronics
Organization: Microsystems Technology Laboratories, MIT
Lines: 62

In article <536@winnie.fit.edu> acs60222@zach.UUCP ( 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.
>
>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)
>
>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.
>
>I appreciate any and all discussion about my problem...
>
>Thanks   me


If you use a single transformer, it needs to have something like a
500VA rating to drive all of the bulbs to a reasonable brightness.
If you use multiple transformers to do this, the effective rating also has
to be in the 500VA range.  This IS the 'elegant' solution, assuming you
wish to have each of the bulbs draw 20W or so.

I would imagine that a single transformer to do the job is less costly than
using many smaller transformers placed in parallel.  A difficulty with placing
tranformers in parallel is that mismatches between output voltage magnitudes
and phases will result in a secondary loop current, possibly large. If you
used two transformers and could be sure that the loop current is small enough 
that the sum of the transformer's current share (load-current/N, for N 
transformers) and the loop current is smaller than the transformer's rated
current then you have no problem.  This condition will be harder to guarantee
for larger number of transformers placed in parallel or with larger 
uncertainty in output matching.  

Depending on how much you were quoted for the transformer you need,
it might be cheaper to use a transformer out of a battery charger.  Battery
chargers are commonly available and the cost of the whole charger could be less
than the cost of buying just the transformer alone.  The transformer in a 
50A charger would easily illuminate your bulbs.  I would skip the rectifiers
since they are not needed in your application. You could use a car battery 
as well, but that's not too practical if you plan to use these lights often.

I think the safest bet would be to get a hold of a UL rated transformer for
the job at hand so you won't have to worry about overheating and possible fire;
depending on how the cost of transformers go, this may be the cheapest way
to go and you would not have to incur the cost of rewiring.

Be sure to use heavy guauge wiring at the input to this lighting system.


Andy Karanicolas
MIT Microsystems Laboratory

ankleand@caf.mit.edu