On Tue, 2 Dec 2003 14:01:11 -0800, "Rod McInnis"
wrote:


"Tom Best" wrote in message
. com...

I though the low voltage yard lights were transformed to 12 volt DC
not 12 volt AC. Is that the issue?

The light bulb would not care if it got 12 VDC or 12VAC.

I'm going to go out on a limb here. Assuming the voltage is in the 12
Volt range and the bulbs are of reasonable quality, there might be a
difference between AC & DC regarding these bulbs.

A 12 Volt bulb (automotive/RV type) running on AC is being turned on
and off 120 times per second (60 Hertz). Although way too fast for
general themometers, it's also experiencing a temperature change 120
times per second. By design, the bulb may be designed to withstand the
vibration associated with auto/RV applications, but not the super
rapid temperature changes.

I've noticed a similar scenario with garden lights. They use the exact
same type automotive bulbs used in some tail lights, yet last only a
fraction of the (relative) time. To save money, the transformer output
is AC, eliminating the bridge rectifier and smoothing capacitor.

I would guess that were the same string of lights hooked up to a
"smooth-ish" 12 Volts of DC, as experienced in a car charging system,
they would outlast the 12 Volts AC system every time. A liberal
sprinkling of diodes, chokes & capacitors will go a long way to
increasing the life of your bulbs.

FWIW, Tungsten filament bulbs are not the most efficient means of
lighting an area. They produce a great deal of wasted heat.

Regards.




It might help if you provided more detail on the "DC Converter".

Why are you using a DC converter instead of a simple AC transformer?

You said it was made for this so one would assume the voltage is correct,
but you should measure it anyway. Have the lights been on 24 hours a day, or
just a few hours each evening?

Light bulbs are rated in number of hours they can be expected to last. Some
light bulbs will last thousands of hours. Others, like a projector bulb for
example, may last only tens of hours.

Heat, shock and vibration are reasons for premature bulb failure. Some
bulbs are not rated to operate in an "upside down" position, as it will
cause the base to overheat. Make sure that the bulb is not being used in an
inappropriate application.

Perhaps if you told us exactly what bulbs you were using it would help. You
said they were in a screw base, are these the 12volt bulbs that are often
used in RVs or boats?

Rod McInnis