"Over-voltage" lamps question

[mikedr]

I installed a lighted address plaque. The manufacturer shipped me an 18V (AC) power supply (which my multimeter says is more like 21-22), two 14V LED bulbs, and two 14V incandescent (possibly halogen) bulbs, all of which are "wedge" -- .27A bulbs (pretty standard). The plaque takes two bulbs.

The first night, I used the LED bulbs. One was dead by morning.

The second night, I used the incandescent bulbs. Both were dead by morning.

What could the problem be? Bad bulbs? Or, what I'm thinking, is that the bulbs are being "over-voltages" too much, if you will.

Any thoughts? I'm trying to figure out if I should get a new AC adapter (probably 12V, so I can use standard landscape light bulbs), or just new bulbs.

[mrhappy]

I am going to guess that the power supply is unregulated. As such output of the PSU should drop when there is a load on it and be higher when there is no load on it. The amount of the drop will be dependent on the current, it may be that the LED lights at least draw somewhat less current than the filament bulbs so the drop is less so the LED bulbs are being driven at a higher voltage and failing.

You could measure the voltage with the bulbs attached, see what the drop is across the two - if it is ~18V and they are 12V bulbs then they may very well slowly burn out over the course of some hours rather than instantly fail because it is not drastically above it's rating.

Could throw an additional load (another bulb/resistor) into the circuit to bring down the output or just get a 12V regulated supply and be done with it.