If you don't have an existing doorbell but would like to avoid the need to charge the battery of your Ring Video Doorbell (2020 release), you can directly connect your Ring Video Doorbell to a low voltage 8-24V AC transformer.
The Ring Video Doorbell can only be connected to an AC transformer.
A resistor is not required.
Do NOT install a diode.
Installation and wiring must be performed by a qualified and licensed electrician.
You can also use a Plug-In Adapter (2nd Generation) to keep the battery charged.