On Tue, 12 Dec 2017 08:43:05 -0800, John Robertson
wrote:

When in doubt find a real answer:

When in doubt, read about what manufacturers have done about the empty
oven problem.

Some patents:
"Oven protective device"
https://patents.google.com/patent/US3281567A/

"Electronic oven protection circuit"
https://patents.google.com/patent/US3412227A/

"System for sensing the presence of a load in an oven cavity of a
microwave cooking appliance"
https://patents.google.com/patent/US6867402B1/

There are probably other patents.

Basically, there is a directional coupler VSWR (voltage standing wave
ratio) detector or other scheme for detecting if the oven is empty,
which senses the high reflected power produces by an empty oven and
shuts it down.


Learn by Destroying(tm) or this should be tested by Mythbusters:

About 15 years ago, I was drawn into a discussion about what bad
things might happen if the oven were to run empty. Opinions varied
ranging from nothing to planetary destruction. I wasn't sure but
based on my RF experience, I guessed(tm) that it would be either a
huge increase in voltage across the magnetron, or a huge increase in
current through the magnetron. I placed my bet on some kind of
arcing, but didn't offer any specific location.

Three allegedly functional junk microwave ovens were purchased from
the local thrift shop for about $20/ea. When the owners of the store
found out what we were doing, she threw in three more ovens that had
various defects which made them unsellable, but were allegedly able to
heat water.

I don't have time right now for the whole story, so I'll just
summarize. Every oven acted or failed differently. As I vaguely
recall:
- One immediately shut itself off and would not restart until I
unplugged the power cord. My guess(tm) is I tripped an overcurrent
breaker.
- One turned itself off after about 30 seconds by blowing a fuse.
- One made a noise indicating the something had blown up inside, but
continued to run. After about 1 minute, there was another noise
followed by smell of burning electronics.
- One arced over some burned food on the waveguide window. I removed
the window and tried again. This time it arced intermittently inside
the waveguide near the window for about 15 minutes. There was a hint
that something electrical was burning inside, so we turned it off.
- One had some mechanical damage to the case, which caused some arcing
outside of the cooking area. Since that meant that we might have high
levels of RF leaking from the oven, we terminated the test early.

I have photos of the ovens and list of makers and models. I'm too
lazy/busy to find them right now. Suffice to say that there were no
fires, explosions, implosions, lightning bolts, ball lightning, toxic
discharges or devastating EMP. In most cases, the fuse or breaker
tripped, which is easily replaced or reset. Someone did an autopsy on
3 of the ovens and found one shorted magnetron and two blown Hi-V
diodes. The ovens that seemed to have burning electronic were not
inspected.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558