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[Shop-talk] Mystery of the blown transformers

To: shop talk <shop-talk@autox.team.net>
Subject: [Shop-talk] Mystery of the blown transformers
From: Jim Stone <jandkstone99@msn.com>
Date: Sat, 1 Mar 2008 11:27:52 -0600
My parents have a gas light in front of their house that was converted years
ago to run off DC current.  The conversion seems to be through some sort of
kit.  It uses two bulbs that look to be standard 12V automotive light bulbs
and a 24V DC transformer.  It has worked fine for about 30 years, but recently
stopped working.  I checked the transformer and it had died, so I replaced it
with a standard Home Depot door bell transformer with selectable voltages of
8V, 16V or 24V.  The original transformer was a plug-in type and whoever
installed the conversion kit simply hard wired the last foot of an extension
cord into an existing junction box and plugged it in.  I removed that cord and
properly attached the new transformer to the same junction box and hooked up
the old wires to the light.

The light worked for about 10 minutes, then quit.  I checked the transformer
and it was dead.  I knew my wiring was good, so I took the transformer back
and exchanged it for another one.  That one died just as quickly.  I thought
HD may have gotten a bad batch of transformers so I returned that one and
bought an identical one at Lowes.  This time, I wired it to a plug and plugged
it in at my house.  I ran it for several hours with no problems, so I
installed it at my parents.  It blew within about 10 minutes.  The problem
definitely exists there.

Here's what I know.  The original circuit where the transformer is located is
a 20 amp circuit.  Certainly overkill for the porch light, but the original
installer probably used it because it was close to where the wire came in.
The circuit breaker has never tripped.  I put my voltmeter on the line and it
registers 120V, the same as my house.  The line is ungrounded.  I don't know
what else is on that line, but know it is nothing used frequently as I forgot
to flip the power back on and no one noticed when it was off for a week.

I can try putting a new one on a different circuit, but hate to blow a fifth
one without knowing why the first four (including the original one) have all
died.  Does anyone have any ideas or suggestions?

As always, Thanks!

Jim

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