I have 7 hardwired under cabinet puck lights which are 10 year old halogen type . All 7 or powered as 120 from one wall switch . Limited access to each light other than removing old puck and pulling down about 8" of the existing wire . Desire is to replace all with Similar in style, LED puck lights which are the 12 v . Question is can I go to the output from wall switch and simply install 120v to 12v transformer between there and the first puck light ? If so how to you choose the right transformer ?
What you want to do is fine but, it would all depend on how many puck lights you have and the distance between them. Or for example if the puck lights connect to other puck lights on a different cabinet. Using a typical 12 Volt electronic transformer may not be ideal because of voltage drop.
Due to how electronic transformers "chop" the sine wave, the frequency on the load side is increased by a factor 400% or more. A typical puck light transformers load frequency is 28,000 Hertz compared to a magnetic transformer at 50 or 60 Hertz.
This increase in frequency directly affects the voltage at longer runs. The higher-end electronic transformers use 24 Volts to help compensate the voltage drop.
When using electronic @ 12 Volts anything over 10' I think should be avoided, especially if multiple lights are involved.