question about combining USB power and other voltage power

Hello and thanks always.
I would like to ask a question related with what the title says.

In the past I have built systems which use two voltages. Say 12V and 5V. For that I used either two power sources, or one power source that gives both voltages. Of course the ground is joined.

I have also built systems of 5V only which are USB powered, meaning I dont use any power source, just the USB gives the 5V that my microprocessor needs.

My question is, what happens if I want to built a system in which the 5V part is given by the USB but the 12V by a power source. I am building this right now but, is there no problem that the grounds are joined?? I dont want to burn anything ....

There should not be a problem PROVIDED THAT YOU MAKE THE 12 VOLT SUPPLY FLOATING. (Floating means that no point on the secondary side of the transformer is connected to earth or any power source that has a path to earth.) This means that you need to make sure that the transformer that you use is double insulated (With the primary and secondary on different parts of the former.) and preferably has an inter winding screen.

Les.

You should also make sure that the circuit won't be damaged if only one power supply is there, and that there is no way that the 5 V supply could be accidentally powered from the 12 V supply.