GRJ tel me where they are wrong. I'm just repeating what they have told me because it makes sense to me. I don't agree with running them at lower AC watts will make a big difference. But not using the bridge rectifier (which I have seen catch fire on strings of LED Christmas lights) makes sense as well as not taxing the capacitor. I have many sets of these lights that have improved parts. And still when they fail its not the LED, its the bridge rectifier. Out of 26 sets that Ive replaced 24 of them were rectifiers, 2 had failed LEDs. If they arent bad out of the box, it seems they have a 2 to 4 year life span.
Jim there's a couple things to unpack here. First, you don't run things at lower watts. Engines will consume the same 'watts' of power no matter the rating of the transformer, assuming the transformer can supply at least enough to power the engine.
Next, it doesn't matter if you run on AC or DC, the bridge rectifier still gets used. It is the first part that power passes through inside the engine. (talking LC/+ here). The reason that the parts fail on cheap Christmas lights is that they are under-rated or just on the line parts. In normal engineering a little extra overhead will be worked in, and they will take heat into account. A lot of cheap products don't account for the fact that as the parts heat up, they have different characteristics than at room temperature. A diode that is rated for 1 amp at 100F might only handle 750 mA at 150F, for example. A lot of folks selling LED's don't really know much about electronics and don't think about it.
Addressing some stuff from other posts, the main filter cap in the LC engines I've taken apart is a cheap brand and that would be a problem in a high strain application. Fortunately filtering 60Hz is a pretty low strain task for it, and the part used is more than up for the task. It's also worth noting that the exact same part is likely inside the 36 and 72 watt DC wall packs, as well as any other inexpensive DC wall pack.
While I'm of the thinking that the parts in question are going to do just fine without any help, the Capacitor does actually work harder on AC, and especially on the 'chopped wave form' produced when modern transformers are run with the throttle turned down. If you're concerned about it, any modern transformer such as a cw-80, ZW-L, Z1000, or similar should be run at full throttle to cause the least strain on the LC/+ electronics.
Lastly, an LC/+ engine can not operate on less than 7VAC, and at that you are starving it, actually making that cheap capacitor work extra hard. I'm trying to think of a good analogy to what the capacitor is doing and this is the best I have. Think of it as a large water pitcher with a valve on the bottom for filling glasses. the engine needs a steady flow of water from that valve, and the rectifier is pouring water on the top. With a full 18VAC , plenty of water gets poured in in an even, slowly on and of fashion. The pitcher never runs out or even runs low. with lower voltages, the pitcher will run low and may even slow or stop pouring power into the rest of the engine. The same happens with the chopped waveform of modern transformers when turned down, A full glass of water gets dumped into the pitcher, then nothing more until the pitcher is almost empty.
Last thing. when you say ZW, everyone thinks of a postwar ZW that actually outputs a pure sine wave at all voltages, not the modern ZW-L that does not.