I had some LED light bars for small hallway directional signs that are run 24/7 produced by a supplier who also provided the transformers. 4 of 8 units have gone dim after 4 weeks. I am having the (120 to 12v) transformers replaced with a new brand. Is there a difference in transformer brands? What makes a 12v transformer more reliable than another? Why do they fail? How can I avoid this happening again?

Most transformers are very resilient, and I have no opinion of one brand over another.

I will say, look to see what the country of origin is.

If China, then I suspect that may be one reason. Not a conspiracy nut, but I am of the opinion that prodcuts made in China will work for a while, then fail. Most of these items are so small or inexpensive, that the owners rarely go through the trouble of getting a new prodcut or credit for the failed part.

And China gets to build more crap for us.

The units may be overloaded. Need to confirm the wattage or VA (volt-amperes) rating of the transformer, and know what the load is in watts or VA of the LED lights. Transformers should be sized at least 25% larger than the load to be served.

Thanks! I'll look into the load and sizing issues.