Every two weeks for the last 2 years, a 12 volt incandescent lamp burns out.  Voltage at lamp is 10.8 VAC typically.  We called the fixture manufacturer and they said it was a transformer issue.  How can we eliminate lamp failure?

posted in: Constant Voltage Controls Power Modules Products Services Transformers


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Most people assume it is the transformer when the lamps are popping.  In past experience it is almost always a lamp issue.
People assume they have a lamp of one quality and someone subbed it out with a lower quality lamp instead.
Based on what you?ve shared so far, I would guarantee it is not the transformer and probably the lamp.

Like any mystery or scientific experiment, it is best to eliminate one variable at a time.

I would (with your own eyeballs) get the part number off All the lamps that failed and pull THAT spec sheet.
Ask the manufacturer how they rate their lamps;  
If they rate based on the point of 50% failures, then that means Half of the lamps will fail Before that rated life;

As additional FYI, 
You mentioned 10.8 volts at the lamp. Voltage below 12 volts extends this lamp life. 
A transformer is an unregulated device and will generate an output voltage that varies by the same percentage the input voltage varies. 
(+5%, -10% is the typical utility range; but verify with your local provider)
If we assume 10.8VAC was measured when the voltage was at its lowest input voltage120V x.9 = 108V(-10%), then extrapolate that data to calculate what the lamp voltage would be at +5% (126V) instead of -10%, the lamp voltage would be a maximum of 12.6V.  
Ask the manufacturer, what is the rated lamp life at 10.8 - 12.6 volts?

Chart on Page 3 of this brochure also gives you a reference
http://www.semperfipowersupply.com/kb/uploads/doc/Indoor%20Transformer%20Brochure.pdf

You may choose to put a power recorder on for a week to see if there are fluctuations; if you want to eliminate this variable. Maybe there are internal issues unrelated to the power company?

One other issue explained in the following sheet.
If you have these fixtures wired on a single wire run, you MUST check ALL the lamp voltages?.
Wiring fixtures in a daisy chain can lead to what I call the POPCORN Effect?
http://www.semperfipowersupply.com/kb/uploads/doc/ContractorHandout.pdf

Using our Power Module will eliminate all of these issues since it provides a self regulating voltage to the load.