Archive for August 7th, 2011

The Microwave Oven High Voltage Circuit—How AC Becomes DC

Sunday, August 7th, 2011
     My mom was a female do-it-yourselfer.  Toaster on the blink?  Garbage disposal grind to a halt?  She’d take them apart and start investigating why.  Putting safety first, she always pulled the plug on electrical appliances before working on them.  Little did she know that this safety precaution would not be enough in the case of a microwave oven.  Let’s see how even an unplugged microwave can prove to be a lethal weapon and, yes, we’re going to have to get technical.

     Last week we talked about the magnetron and how it needs thousands of volts to operate.  To get this high of a voltage out of a 120 volt wall outlet–the voltage that most kitchen outlets provide–the microwave oven is equipped with electrical circuitry containing three important components:  a transformer, a diode, and a capacitor, and just like the third rail of an electric railway system these items are to be avoided.  If you decide to take your microwave oven apart and you come into contact with high voltage that is still present, you run the risk of injury or even death.  But how can high voltage be present when it’s unplugged?  Read on.

     First we need to understand how the 120 volts emitting from your wall outlet becomes the 4000 volts required to power a microwave’s magnetron.  This change takes place thanks to a near magical act performed by AC, or alternating current.  In the case of our microwave components, specifically its diode and capacitor, AC is made to effectively mimic the power of DC, or direct current, the type of current a magnetron needs.  This transformation is made possible through the storage of electrical energy within the microwave’s capacitor.

     Next week we’ll examine in detail how this transformation from AC to DC current takes place, as seen through a device called an oscilloscope.