Microwave starts when door closes

Our Samsung microwave hums and starts heating as soon as the door closes. If we enter a time and hit start the light and turn table come on and it functions as it should. However, when the time ends it continues to hum and heat. The only way to stop it is to open the door or unplug it from the wall.

Any ideas as to what is wrong with it?

One of the main relays has welded it contacts together. You either need a new relay or control board.

Samsung DE92-02329E Assy Pcb Main;mhc-3,ML1- - AppliancePartsPros.com

The main relay is the one near the bottom, left of this pic. It is larger than the rest.