Auto smelting with an electric furnace is a major convenience and one of my favorite circuits to set up. It frees you up to do other things, and is a lot simpler than you might suspect.

The natural question is why does this work without any filters? Based on how these systems are suppose to function, it's a little surprising this works. But in this circuit's loop each entity can only take and hold the resources that you want them to. You can't put smelted resources or any other items into the furnace, so only ore gets moved into it without even touching the filters.

You can actually expand this to include regular furnaces and even an oil refinery, though you'll need to manually start them up or use an igniter system.