death by computer

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

 

Buzz,
If I were you I would get rid of that microwave oven, not worth the risk.

Sal Brisindi
www.tuberadios.com

 

Thanks for posting that, Buzz. Safety stuff is important.

Sal makes a good point, but forgot to mention the simple adjustments to
make to the oven's software, first.

1) Unplug the power cord.
2) Snip the cord off where it exits the oven.
3) Place the oven on a tarp in the back yard.
4) Don your face shield, earplugs and gloves.
5) Carefully remove the door. (You may use an angle grinder or torch)
6) Cut the door into two pieces of approximately equal mass.
(You may use a fire axe.)
7) Gently tap the exterior in several places using a long handled five
pound maul. (Overhead swings, with as much force as you can muster.)
You may stop tapping when the cubic area occupied by the oven is reduced
to 70% of its original value.
8) Retrieve the various pieces of oven that attempt to exit the area.
If you have tapped the exterior properly, exterior parts will be found
as much as twenty feet away.
9) Toss one of the door pieces in the trash, along with the power cord.
10) Over the next few weeks, place two random pieces of oven per week into
the trash until it is all gone.

This will put the proper values into those registers and cause the
oven to stop endangering you and your visitors.


--Winston

 

While both responses are correct, the manufacturer *needs* to know that this
can happen. That is **really** bad design. It should indeed be a dumb-ass
interlock so that this can't happen. I'm appalled - what brand is it? I
don't ever want to see one in my vicinity!

Ken

 

Ken Taylor wrote:

I thought the standard way to do this was with 2 microswitches, one to cut
the power when the door was opened and the other to short circuit the
primary of the transformer under the same conditions (obviously mounted to
ensure the correct sequence of operation).....

Why the hell didn't whoever made the silly thing do what every other
microwave manufacturer has been doing for years (I know, a small signal
switch and use the same power switching device used for the duty cycle
control is cheaper!).

BTW: the correct fix for this unit surely involves thermite?

Regards, Dan.

 

Agreed, but that only solves the immediate problem. What about all the other
units out there?

Ken

 

I would claim that the majority of "broken" microwave ovens in the
US are because of failed door interlock switches.

The switches are indeed designed to fail into the "safe" position but
they fail the other way too. (especially the ones on the door latch
that trigger the light and fan.) The thing we rely on for safety is that they
don't fail simultaneously in the "dangerous" direction, but with hundreds
of millions of microwave ovens made over the years I'm sure it has happened.

I work in an industry that prides itself in vital relays that are
(supposedly) more reliable than those used to arm nuclear weapons, and
there are always freak failures if you look hard enough at a system that's
been operating for a good chunk of a century. Most of these failures
get investigated and written up by the FRA and/or DOT, and the root cause
is usually much more complex than a simple failed component or two.

Tim.

 

Why has nobody yet pointed out that the actual make and model of the
microwave have not been mentioned? That'd put the circuit configuration to
rest in about 30 seconds.

 

I pointed this out indirectly by asking for it to be posted, and I
think someone else asked too. The OP's posting address is obviously
fake, OTOH google shows a few recent legit-looking posts from that
address, posted through rcn.com.
Of course, even the most legit poster could decide to have fun by
posting a "troll"...

 

My suspicion too - even a clown in a Krusty-burger hat would feel some
indignation if their hand got warm when the microwave oven did funnies.
Still, if he posts more detail....

Ken