The economic crisis seems to be over (in Spain): this is good for many reasons, but one kinda weird is that households are renewing their equipment, and one, me, can find old electronic apparel by the garbage containers: a real mine!
One of my recent findings is a microwave oven model Teka MW-170, made in Korea. I have not found any specific image of it in internet, and I disassembled the casing without taking all the necessary pictures, as I see now.
Model 170G, of which there is more information available and seems more recent, is similar but I have noticed at least several differences: MW-170 has one button to open the door plus another one to start the machine, and it has no turning table (the tempered glass is square!), while 170G does not have door-open and power-on buttons (just open and close the door), and it has a turning table. The documentation of the MW-170G also mentions a couple of heaters and additional switches which are not present in my model.
The specifications: 950 W nominal power, 500 W max output, working at 2450 MHz:
Here the wiring diagram included in the machine:
I have dismantled the parts and the wiring diagram is OK:
The fan is connected in parallel to the timer. Its purpose is to cool down the magnetron.
The maker is Samsung, model RP-0650TE(N) 220V 50Hz.
The casing and the impeller are in plastic. The impeller diameter is approx 90 mm. I have not checked the speed.
The motor seems of the single-phase induction shaded-pole type.
The power selector is motorized.
The manufacturer is also Korea Nakagawa Elec. Ind. Co. Ltd. Model is DF-3CV24SI rated 15A at 250 VAC.
I have not found information on this motorized switch.
The drive is a small synchronous motor. The dial lever has 5 discrete positions, and its turning changes the position of an intermediate plastic wheel along the axis of the motor shaft. The motor turns this wheel, which is in discontinuous contact with the switch through a flapping plate. The shape of the wheel, and its position as regulated by the dial, control the amount of time per cycle of the motor that the switch is open/closed. This is how the power is controlled, just by varying the amount of time the magnetron is powered.
The transformer is from Hankook Steel Works Co. Ltd. in Korea. Model is H-5220-5(55VA) 220V 50 Hz. It is labelled as "DANGER HIGH VOLTAGE", "DISCHARGE CAPACITOR BEFORE SERVICING".
The transformer has two secondary windings, but no indication of the output voltage. Internet references point to 3-4 kV.
Magnetron and HV Capacitor
The magnetron used in the oven is from Samsung, model OM55A (20), many of which can still be found in ebay and similar places.
- voltage: 4.1 kV
- standard type (30 mm of antenna, upper box 80*95 mm)
- J configuration ("vents and mounting ears opposite filament leads")
- 700-850 watt
- 35 mm hole center.
From that page:
The magnetron is the core of the machine, the generator of the microwaves that heat the stuff.
The physics behind it are moderately advanced, but without entering in details the principles can be described here.
A high voltage is provided to an anode/cathode element. The cathode, a rod bar, is also heated, and this facilitates the emission of electrodes from it. The anode is ring shaped around the cathode. A magnetic field is applied parallel to the cathode. The electrons tend to jump from the cathode due to the differential electrical voltage, and being charged particles moving in a magnetic field, forces appear on them. The result of both electric and magnetic fields is that the electrons follow spiral trajectories in their way from the cathode to the anode.
The anode has several cavities in its body (these are called "cavity magnetrons"). The interference of the moving electrons and the cavities create certain electromagnetic resonance, which is manifested as microwave radiation. This radiation is collected with an antenna, and conveyed to the oven through a "wave channel".
An aluminium turning wheel ("stirrer fan") in the wave conduit helps distributing the radiation in all directions in the oven.
The magnetron requires DC voltage, and for this reason are provided the large HV capacitor and the diode: to rectify the AC current powering the magnetron into a low ripple, high voltage direct current.
Industrial applications have other power sources, pulsed or switched, but domestic appliances follow the conventional solution with capacitor and diode.
The capacitor is from Samsung, labelled H.V. CAPACITOR SCH-212904A1, 2100 W VAC, 0.90 uF T-70, INTERNAL RESISTOR, NO PCBs.
The capacitors are fitted with internal resistors to allow the discharge when not in use. In any case, the capacitor should be discharged prior to handling it, with a suitable resistor (see here).
The diode is labelled "HVR-1X 3", "SK 8120".
If you look for some more technical information on the cavity magnetron, see here.
A good and detailed page on microwave ovens here.
And a couple of books, which I have not checked:
- Davidson, Homer L. Microwave Oven Repair, 2nd edition. Tab Books Inc., 1991.