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#46773 - 01/02/05 03:48 PM Low voltage x-former problem?
Northbayec Offline

Registered: 12/16/04
Posts: 26
Loc: San Rafael, CA U.S.A
Hello everyone and happy New Year!

I had a problem with a some site lighting at a residence a couple days ago where the owner said he could not get half of his walkway lights to work. It is a 12 volt system and when I check them out 3 of them are working but the other 7 are not. I figure there is a bad splice so I re-splice a 3-way connection after the 3 light (the farthest one from the x-former that worked) and when I turn them on I get 12 v at the connection for the next light but when I hook that light up the votage drops down to zero at that connection but the first three lights are still working. There were ten 25watt fixtures on a 250 watt x-former so I am assuming that I need to replace the x-former but I don't see an explaination on the voltage differences on the same system. Anyone out there have any input on this.

Thank you

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#46774 - 01/02/05 05:57 PM Re: Low voltage x-former problem?
dereckbc Offline

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
John, the transformer is not the problem. You still have a continuity problem between the 3rd and 4th light. You are seeing high resistance, so you see 12 volts with no load but 0 when load is applied. Sounds like it is time to dig things up and find the bad connection.

#46775 - 01/02/05 07:03 PM Re: Low voltage x-former problem?
Wirenuttt Offline

Registered: 11/10/02
Posts: 267
Loc: Massachusetts
Also some of these LV lighting systems transformers are designed to run no lower than 50% and no higher than 80% of computed light load.

#46776 - 01/02/05 10:54 PM Re: Low voltage x-former problem?
Physis Offline

Registered: 12/28/04
Posts: 85
Loc: Burlingame, Ca. USA
I did an installation where the owner called me back a couple of monthes later because half of one of the branches were not lighting (I think he thought I didn't do it right). I went for the wire after the last working light and snipped out the piece where the gardener's shovel got a little too close. He wanted that to show to the gardener.
Sam, San Francisco Bay Area

#46777 - 01/03/05 12:55 AM Re: Low voltage x-former problem?
Trumpy Offline


Registered: 07/05/02
Posts: 8532
Loc: SI,New Zealand
What you have is a Dead-Short in the 4th fitting.
It sounds to me like these lights are in Parallel, not Series.
The short is preventing any current flow to the other fittings further down the track.
If the guy keeps running the system like that he will need a new transformer.

{Message edited to add last bit}

[This message has been edited by Trumpy (edited 01-03-2005).]
Let's face it, these days if you're not young, you're old - Red Green grin

#46778 - 01/03/05 06:00 AM Re: Low voltage x-former problem?
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
What you have is a Dead-Short in the 4th fitting.

How do you figure that? If there was a dead short the other lights would go out.

I go with Dereck's answer: There is a bad (high-resistance) connection somewhere. When you just put the meter at the 4th lamp terminals it's drawing very little current, so you see the 12 volts. As soon as you connect the low-resistance of the lamp, the high-resistance of the bad connection becomes the dominant part of the circuit, so the voltage at the lamp drops to zero (or very nearly so).


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