Corrosion and poor connections are common culprits in low voltage lighting circuits.
In some cases it is easier to replace the lines than try and troubleshoot them.

Hi,

It could be corrosion or a bad connection, so check your circuits as brickeye sugessted. If things seem to be in good condition then read on for an explination.

This is a simple case of bulb burn out. Ah but how can all these bulbs burn out at once, in the space of such a short space of time?

The mechanisum is this. Low voltage landscape lighting systems use approximately 10 times the current when compared with 120v mains systems, because they run at approximately 12 volts; so to get same power, you need more current.

Because of this relatively high current, the resistance of your copper wiring becomes important. In 120V mains power systems in your house, this is not so significant because that runs on lower current.

Your low voltage landscape system is designed so that when all the bulbs are working, the current they draw down the wire causes a voltage drop according to Ohms law. V = I * R, where V = Volts, I = Current and R = Resistance. ... Don't Panic, all I am saying is that in normal operational mode, some voltage drop occurs in the wire, and the system is designed like that.

At the transformer end, the voltage may be something like 15 volts, but because of the voltage drop in the wire, the voltage at the first light bulb nearest the transformer is approximately 12 volts.

Your Halogen light bulbs in your low voltage system run best from about 10.8 volts to 11.8 volts. If they are run at a higher voltage they burn out faster. If they run at a lower voltage, they are dimmer, and tend to go black because the Halogen Cycle does not work well, and tungsten is deposited on the bulb glass.

You still with me ... wow not asleep yet ... OK here is the crunch point. At some point in time a bulb burned out. This means that less energy is used. Which means less current is drawn. Which means less voltage is dropped in the copper wiring. Which means there are higher volts on the still working light bulbs. More voltage means that the bulbs will burn out sooner. In a few days, another light bulb will blow, too high a voltage .... less current, more volts, even faster bulb burn out.

So in effect, just like a chain of fire crackers, all your bulbs get taken out over the course of a month or so, just because one of your bulbs blew. Note it is no coincidence that the ones still working are at the end of the run, furthest away from the transformer. These have the longest piece of wire, and hence the most voltage drop. I.E. the voltage for these bulbs are lower than for the other bulbs which blew.

So how to fix this....

1) Replace the bulbs in the fixtures.
2) Be vigilant. When a single bulb goes, try and replace it as soon as possible. Other wise you have this fire cracker chain effect, and all your bulbs blow in about a month or so.

Sorry this is so long. Hope this helps.

Warmest regards, Mike.

Wow, Mike! thanks for taking the time to explain this. VERY helpful.

I've seen bad connectors. Those outdoor wire nuts are the worst. Volt makes some very nice clamp style junctions complete with their own waterproof goo.

I've also seen LED fixtures just up and die. I have a string (prebuilt by the manufacturer) of three little LED pinspots in my pond. Two of the three in the string died (and oddly it was the one in the middle that was working). I just had to replace those. I took apart the string after I replaced it figuring I could salvage the individual spots, but they were deader than a doornail even applying power directly to them.