There are already many designs of this kind, but I have found either very simple or very complicated. Here is now a mix.

Instructions:

In this picture and the thubnail ma actually sees everything: You build a kind of ring of rails with any number of driven rails (doesn't change much the melting speed). You also replace one corner with another driven rail. On top of it you put the box, in which you want to fry/melt everything. From the crate you put a comparator, in front of it a block and under the block a redstonedust. This should now activate the powered rail if anything is in the crate. On top of the rail you have to put a lorry with a funnel. If you like, you can also transport the output with an item hoist to the top, but I think this is too complicated and costly for this stove.