After 14 years, my microwave oven has stopped working. So I took it apart to see how it worked.
I've always wondered what powered a Microwave Oven. (I know, microwaves) but I was curious about the various components.
I come by my nick-name "Curious George" honestly. I often take old appliances and gadgets apart, just because...
What's inside a microwave oven?
This old microwave oven has served me well but one day expired and stopped working.
So I took it apart to see what it was made of.
It does not take a genius to figure out microwaves are involved but I wanted to see the guts and how the microwaves were generated and how things were controlled.
First What is a Microwave?
Microwaves are a kind of electromagnetic radiation. It's a relatively low energy form of radio waves. Electromagnetic radiation is characterized by wavelength and frequency. The higher the frequency the smaller the wavelength and vice versa. The higher the frequency the more energy the radiation carries and the more dangerous it is. Radio and microwaves are at the lower frequency end of the spectrum and can be readily shielded, while gamma rays and x rays are high energy radiation and require a great deal of protection (such as lead aprons for x rays) in order to be used safely.
I wrote a page on "What is EMR Electro Magnetic Radiation" while trying to figure out how carbon fibre can shield some EMR. Here is a link to my Page on Electromagnetic Radiation. It's a good intro.
When some kinds of radiation reach a susceptible material such as water, the molecules of this material vibrate at a faster rate and this translates into heat being produced. As humans filled with a lot of water, we can feel this in infrared radiation and mild microwave. In fact heat lamps and electric heaters do just this, produce infrared radiation that makes us feel warmer.
Opening up the Microwave
Most of the screws on the microwave were regular phillips screws, annoying when I'm used the much easier Robertson square heads, but not difficult to use.
There were a few which were a star shaped with a little peg in the centre. I found out they are called Security Torx, or pin-in Torx. I don't have a driver for these but since the screw was so high, it was easy to grab the edge with a pair of pliers and get them off.
WARNING, there is a reason they put security screws here. Concentrated Microwaves can hurt you. There are high voltages if you tamper with the mechanisms. My microwave was unplugged and would never be powered. Don't go opening plugged in microwaves, working or not. ALSO I saw several large capacitors on the various circuit boards. These can hold a charge even if the microwave oven is unplugged and give you a nasty dangerous shock.
The metal casing acts as shielding to keep the microwaves from escaping. It's part of the safety design of the oven. The case acts as a Faraday cage.
The perforated metal mesh that is in the front door window also shields the user from microwaves. It allows visible light through but stops the larger microwaves from passing through the small holes.
I wrestled the stainless steel enclosure off and finally got to see what was inside.
Getting the casing off any machine is always a thrill. Whether it's an old watch, a sander, or a microwave oven, it's always great fun.
Here we are looking at the side of the microwave. On the left is the front of the microwave, with the door and the controls, on the very right is a fan in a plastic enclosure and what is probably the power supply above it, green and yellow wires connects the power supply to a small thermostat connected to the back casing. In the middle is the actual microwave generating assembly.
On the underside of the microwave is a small synchronous motor. This little motor is used to rotate the glass cooking platform in a very precise way. It has a built in gear reducer.
A synchronous ac motor rotates at precisely calibrated speed linked exactly to the current frequency.
The precise rotation speed is useful to regulate the even exposure of the food to the microwaves.
The main purpose of a precise motor is to move the food around and avoid a situation where standing waves would cause hot spots.Good old Wikipedia's article on synchronous motors.
The power cord come in from the back into a small area that has a circuit board and a few windings and components. My guess is that it's a power supply. There is a fuse, and some large capacitors.
A small thermostat is directly connected to this power supply. It backs onto the metal enclosure, presumably a safety device to prevent the outside of the microwave getting too hot and causing burns or a fire.
Backing onto the power supply is a fan. I removed the fan assembly by cutting some plastic clips. They were too tight to un-click easily. The fan draws air from the outside and points it towards the centre, cooling the microwave generator area. It does not vent the inside of the microwave oven area. It looks like there is a transformer that reduces power to a small motor connected to the plastic fan.
The control panel
The touchspad and knob, control the operation of the microwave oven. There is quite a large chip on the screen circuit board. The touchpad wiring is on the green card.
The round component that looks like a small stand up mirror is a buzzer.
The writing is printed on a thin plastic sheet glued onto the green circuit board. It is visible on the last picture, on the bottom left of the image.
The magnetron is the component that creates the microwaves
By passing a strong electric current in a tube built with various cavities, surrounded by strong magnetic field, microwaves are created. These get guided towards the inside of the oven to act on the food we place there.
For a somewhat better explanation have a look at Explain that stuff website That is a readable explanation somewhat simplified.
If you look at the picture, you can see at the top a tube that goes through several metal plates. The tube is where the microwaves are created, the metal plates are heat exchangers that cool the apparatus.
The photo also shows the 2 strong magnets above and below the heat exchange plates. The metal rod at the very top guides the microwave to the cooking chamber.Wikipedia also has a page on magnetrons.
What's an inverter?
An inverter, is a device that takes direct current power and turns it into an alternating current. In the case of a microwave, a 110 VAC sine wave comes in from the plug, gets rectified to direct current, and gets inverted back into whatever frequency and waveform required.
In regular microwaves the microwave are either being generated or are turned off. In an inverter oven the microwaves are more consistent and supposedly the food cooks more evenly and efficiently.
I've had both kind, inverter and regular microwave ovens and have not seen much difference.
The component in the photos is the inverter. There are heavy wires (now removed) that go to the magnetron from it. I can see various coils a few hefty capacitors and a heat exchanger.
There were warnings of high voltage on the plastic housing. I tend to be shy around capacitors so I did not dismantle it any further, but there is not much to see anyway.
Early styling of microwave ovens
When I was at Industrial Design School, microwave ovens were discussed. The reason they were often sold with a fake wood casing was a very deliberate choice. The concept of cooking with "radiation" was quite scary and the designers wanted the ovens to look as familiar as they could make them. For this reason they were styled to look like the televisions of their time.
email me if you find mistakes, I'll fix them and we'll all benefit: Christine