Kitchen radio transmitter, c. 1970s onward
A steel box inside a steel box. The inner one is the cooking cavity; beside it sits a radio transmitter that fills the cavity with microwaves at about 2.45 gigahertz. Next to it is a high-voltage supply that can give a lethal shock, and its capacitor can hold a dangerous charge long after the oven is unplugged. This is a look inside, not a guide: microwave ovens should be opened only by trained technicians.
One sheet of painted steel bent into a U that forms the top and both sides. It keeps fingers away from the high voltage and guides the cooling air; the microwaves themselves are held in by the inner cavity, not by this cover.
Made of Cold-rolled steel sheet, powder-coated or enamelled; a printed warning label.
The slots in the cover are for air, not for microwaves: the fan blows across the magnetron and out through them, while the cooking cavity inside is a separate, closed metal box.
Est. cost to the maker $2–5
Inside the window, behind the outer glass, is a steel sheet punched with small holes: light gets out so you can watch, microwaves do not. A groove around the rim, the choke, sends back the waves that try to sneak past the seal.
Made of Toughened glass, perforated steel screen, a pressed-steel frame with the choke, plastic trim and latch hooks.
The holes in the screen are a millimetre or two across, while a 2.45 GHz microwave is about 12 cm long, so to the microwaves the screen behaves almost like solid metal. Visible light, under a thousandth of a millimetre, passes straight through.
Est. cost to the maker $4–10
The keypad, display and a small computer that counts down the time. It makes no heat itself: it clicks relays that switch the high-voltage transformer, the fan, the lamp and the turntable motor.
Made of Glass-fibre circuit board, a microcontroller, relays, a small mains transformer, electrolytic capacitors, a piezo buzzer, a membrane keypad and a plastic fascia.
On most ordinary ovens a lower power setting does not turn the magnetron down. The relay switches it fully on and off in cycles of roughly ten to thirty seconds, and 50 per cent means on for roughly half of each cycle. Inverter models are the exception.
Est. cost to the maker $3–10
The door's hooks reach into this plastic latch. As the door closes they move a lever that presses three small switches in a set order, so the oven can only run with the door shut.
Made of Nylon latch body and lever, a steel spring, three snap-action microswitches with silver contacts, brass terminals.
Two of the switches cut the power when the door opens. The third, the monitor switch, is a backstop: if the others ever stuck closed, it deliberately shorts the supply so that the main fuse blows and the oven cannot run with the door open.
Est. cost to the maker $1–2
Pulls air in through the back and blows it over the magnetron's fins, then on through the cavity and out of the vents. Without it the magnetron would overheat within minutes.
Made of Plastic blade and shroud, a shaded-pole motor: laminated steel, a copper coil, copper shading rings, sintered bronze bearings.
Its motor has no capacitor and no brushes. A thick copper ring around part of each pole delays the magnetic field there, which gives just enough of a twist to start the rotor turning.
Est. cost to the maker $1–3
The heart of the oven: a vacuum tube that turns high-voltage electricity into microwaves at about 2.45 gigahertz. Electrons whirl past a ring of copper cavities in the field of two ring magnets; the small dome at its centre is the antenna that pours the waves into the waveguide.
Made of Copper anode block, a thoriated tungsten filament, ceramic insulators, two ferrite ring magnets, aluminium cooling fins, a steel yoke and filter box.
The cavity magnetron was developed in Britain in 1940 for radar. The story goes that a few years later an American engineer noticed a chocolate bar melting in his pocket near one, and the microwave oven followed. In an oven it runs at roughly 4,000 volts, so it is no bench toy.
Est. cost to the maker $6–15
Together with the diode, this capacitor doubles the transformer's voltage on every mains cycle, giving the magnetron its few thousand volts. A resistor inside the can is meant to drain it after switch-off.
Made of Metallised polypropylene film wound in oil, an aluminium can, a bleeder resistor; a silicon high-voltage diode in epoxy; a steel clamp.
This is the most dangerous part of the oven. It can stay charged to thousands of volts after the plug is pulled, and the built-in bleeder resistor can fail. Leave it to technicians: an accidental touch can kill.
Est. cost to the maker $1–4
The heaviest part, often several kilograms. It steps mains voltage up to roughly 2,000 volts on one winding and down to about 3 volts on another: a few turns of thick wire that heat the magnetron's filament.
Made of Stacked silicon-steel laminations welded into an E and an I, a primary of thick enamelled copper (aluminium in some budget ovens), a secondary of thousands of turns of fine wire, insulating paper.
Small stacks of steel, called shunts, sit between the two coils and let part of the magnetic field leak past on purpose. That limits the current, so the magnetron gets a steady supply without a separate regulator. Like the capacitor, it is lethal when powered.
Est. cost to the maker $5–12
The cooking box, a welded steel shell that reflects microwaves back and forth. The waveguide on its right is a rectangular metal pipe that carries the waves from the magnetron's antenna into the cavity.
Made of Galvanised or painted mild steel (stainless in some ovens), spot-welded; a mica sheet covering the waveguide opening inside; a lamp behind a perforated patch.
The waves bounce between the walls and settle into a standing pattern with hot spots roughly 6 cm apart, half a wavelength. That is why food rides on a turntable, or the waves are stirred by a rotating metal paddle.
Est. cost to the maker $5–12
A glass plate that rides on a ring of three little wheels and is turned by a coupler in the floor. It carries the food slowly through the hot and cold spots so it heats more evenly.
Made of Toughened soda-lime glass plate; a roller ring of polypropylene or nylon with three wheels.
The plate turns slowly, roughly five or six times a minute. The ring of wheels is not fixed to anything: it simply rolls along between the plate and the cavity floor.
Est. cost to the maker $2–5
The steel floor everything is screwed to, with the back panel, the mains wiring, the main fuse and the small motor that turns the plate.
Made of Galvanised steel base and back panel, a synchronous gear motor, a ceramic fuse, a suppression capacitor, the power cord, plastic feet.
The turntable motor is a synchronous motor: its speed is locked to the mains frequency, so the same motor turns a little faster on 60 Hz than on 50 Hz. The main fuse is usually a ceramic one filled with sand, which smothers the arc if it ever blows at full mains current.
Est. cost to the maker $3–8
Prices are rough ranges of what the manufacturer pays for each part, pieced together from published teardown and bill-of-materials estimates. They vary a lot between models, years and order sizes, and they leave out assembly, software, research, shipping and profit. The drawing shows a generic device, not any particular model.