It’s not uncommon to refer to normally closed thermostats as “resettable thermal fuses”, so it wouldn’t surprise me if some of the documentation refers to them as thermal fuses. But they are technically thermostats.
Based on the way they are used, I’m about 90 percent certain that the two identical ones should measure closed at room temperature, but the conflicting data for them that’s out there bothers me a bit. Best to measure the new one when it comes in and replace the one that differs from it.
So now that the question has been answered, how do these small devices (thermostats) work? How can one work while in an open state yet another, in the same system, work in a closed state?
I know enough about electricity to be dangerous, but how does the one that is open ever work? Even as the microwave heats up or does whatever, no current could be flowing through it. I would just sit there, no? (The 115 part)
Okay, so I spoke too soon. The microwave turns on, it goes through a cycle. Does everything except, well, actually heat anything. So step one in the checklist is the diode that goes into the capacitor.
IIRC from my electronics knowledge a diode should be open in one direction and closed in the other, correct?
That’s an “ideal diode”, which is a nice concept for teaching electronics. Unfortunately, in the real world, ideal diodes do not exist. You need a multimeter that has a diode check function. This is similar to the resistance measurement except that the meter puts out enough voltage to overcome the diode’s forward junction potential.
A better (though still definitely not 100 percent accurate) model of a diode is an ideal diode in series with a constant voltage drop and a resistor.
Poking around on google, I found that wikipedia has a page that discusses the details of different ways to model a real world diode.
Anyway, if you don’t care about any of that, the only important thing here is to check the diode with a meter that has a diode check function. Meters that have a diode check function are cheap if yours doesn’t have it, and you are really determined to get this thing working again.
Which thermostat/fuse was the bad one? Was it the one by the magnetron or the one by the transformer? Which ever one it was, take pictures of the magnetron or transformer from as many different angles as you can. I’m wondering if the reason that the thermostat blew was because the device it was protecting had a major fault of some kind.
The last microwave I tried to repair had 3-4 interlock switches to prevent the magnetron from turning on with the door ajar or open. I was debugging these switches and noticed in some cases the microwave would appear to turn on but the magnetron would not. Turns out the plastic mount for one of the switches was broken in such a way that operation was intermittent. After trying to repair the bracket I finally just went and bought a new microwave.
ETA: Tracing through the schematic I saw plenty of other interlocks for the magnetron, including fuses and thermal cutouts.
You need to be careful with the capacitors. I used to use a couple of insulated screwdrivers to short out the caps.
Thermostats have bimetals in them, so one may make when it reaches a certain temperature (for turning on a fan, say.) They can come in NO or NC states.
The most common problems with microwaves are blown fuses, bad diodes, and bad capacitors, though the last one I worked on had a bad transformer. You need a high voltage probe to test those.
I say the last one, because the guy liked the microwave and paid for a new transformer. Most people just junk them and get new ones.
I was too late to edit my previous post. I would see if the transformer is getting 120V when you test the microwave. If not, the problem is in the 120V side (fuse, themostats, door interlock). If 120V, the problem is in the high voltage side (diode, capacitor, transformer, magentron).
A little more detail on this: the diode test function is a constant current source, usually around 1 mA. The compliance voltage for the constant current source is usually somewhere between 2 V and 10 V. When you connect the meter to the diode, the meter displays the (measured) voltage across the diode when 1 mA is going through the diode.
