Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!uxc.cso.uiuc.edu!uxc.cso.uiuc.edu!m.cs.uiuc.edu!irwin
From: irwin@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: Transformer Troubles
Message-ID: <21000039@m.cs.uiuc.edu>
Date: 28 Aug 89 15:08:00 GMT
References: <536@winnie.fit.edu>
Lines: 45
Nf-ID: #R:winnie.fit.edu:536:m.cs.uiuc.edu:21000039:000:2101
Nf-From: m.cs.uiuc.edu!irwin    Aug 28 10:08:00 1989


/* Written 12:07 pm  Aug 15, 1989 by acs60222@zach.fit.edu in m.cs.uiuc.edu:sci.electronics */
/* ---------- "Transformer Troubles" ---------- */
>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
/* End of text from m.cs.uiuc.edu:sci.electronics */


Since you are using a transformer, one can assume that all of the
wiring for this was done by you, external to the house wiring.

At a rating of 20 watts, the current per bulb is slightly under
two amps. The solution is to power 5 bulbs from your present
transformer and to series the other 20 bulbs in two groups of
10. Since each bulb requires 12 volts, 10 of them in series will
consume 120 volts at slightly under 2 amps. Two groups 10 ten will do it.
This is the manner in which most low cost Christmas tree light strings
are wired.

If you wire the series string in a clean and safe fashion, I see
no problems in doing it. See to it that there is no frayed connections,
insulate all splices. Wire nuts work good for this sort of thing.

Of course, the restriction is that you will have to energize 10 bulbs
at a time and will not be able to turn them on one at a time.