Back to Back transformers and safety ground

[dsavitsk]

I was thinking of using a 120:12VAC wall wart transformer for a project, powering the heaters from the 12V, and using a small 12:240 transformer inside the chassis for B+. Problem is that the wall wart only has two wires, so there is no connection to safety ground. This will probably be connected to a more conventionally wired source, so it may have a safety ground connection that can be piggybacked from, but I am not sure about it. Any suggestion on a safe way to do this, or is it just a bad idea?

[HollowState]

Originally posted by dsavitsk
I was thinking of using a 120:12VAC wall wart transformer for a project, powering the heaters from the 12V, and using a small 12:240 transformer inside the chassis for B+. Any suggestion on a safe way to do this, or is it just a bad idea?

I don't think it's a 'bad" idea. If you leave it encapsulated in it's plastic housing, then it's isolated from your touch and the chassis. If you open it up to use the bare transformer, then be sure the core makes a good physical ground connection to the chassis which will/should have a safety ground. I might be a little more concerned with any possible hum problems.

Victor