Microwave ovens use technologically advanced cooking methodologies, expediting food preparation in comparison to regular or convection ovens. The secret behind the success of the modern-day microwave is in the magnetron used to increase voltage and cooking temperatures simultaneously, cooking food from the inside out, as opposed to the outside in. This technology comes in especially handy with a common microwaveable item: popcorn.
Predominately, microwave ovens share construction components. The majority of the interior is comprised of plastic or fibreglass, with a small metal plate on the side or top of the oven. The metal plate makes the microwave magic happen, acting as the conduit for increased voltage, thus speeding cooking times. This device, commonly referred to as the "magnetron," is also the catalyst for sparks during cooking, under the right set of circumstances.
Microwave ovens spark when metal is present in the oven, increasing the voltage of the magnetron exponentially, resulting in a discharge. If a microwave is sparking when preparing popcorn, it is likely that either a piece of metal is in the microwave, or there is a metal component on or in the bag. Yet sparks are not relegated to existing metal alone; even spilt food or grease can cause a magnetron discharge. If a microwave begins sparking, discontinue use and investigate the source of the discharge. If you fail to do this, your microwave could burn out completely, requiring replacement.
If metal, grease or spilt food is not the culprit, sparks during popcorn preparation could be a result of a faulty stirrer element, causing unequal heat distribution. This causes food to burn consistently and also produces sparks when cooking. If the stirrer is not broken, the high-voltage diode might be to blame. If the high-voltage diode is shorted or opened, test the component is with an ohmmeter and, if found to be faulty, replace it. However, if you are not a licensed appliance repair person or technician, this might be outside your level of expertise, and calling a repair person is prudent.
No microwave lasts forever. According to MrAppliance.com, microwaves have an average lifespan of eight years if properly maintained. However, they could require replacement in as few as five years. If your microwave is over eight years old, it is probably more cost-effective to replace it than repair it. Since microwaves are relatively inexpensive appliances, replacing them is usually not detrimental to a household budget.