Weird Microwave problem - trips breaker when you shut the door!

[Sam Goldwasser]

Since virtually all microwaves now seem to be run by electronics, I
wonder why they don't just program the CPU to detect interlock failures
and go into an error condition and refuse to operate, instead of
shorting the AC line? That would sound a lot safer to me. Then again,
I'm not an expert, so maybe I'm missing something.

Have you ever heard of software screwing up? Nah. That arrangement
of 3 interlock switches is much more fail-safe than anything depending
on 0s and 1s.

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[NSM]

Have you ever heard of software screwing up? Nah. That arrangement
of 3 interlock switches is much more fail-safe than anything depending
on 0s and 1s.

A Canadian company made a programmable radiation treatment machine. If you
entered the wrong setting, then backspaced and corrected it, the patient got
the maximum the machine could deliver. Killed some people before they found
the bug.

N

 

[Sam Goldwasser]

A Canadian company made a programmable radiation treatment machine. If you
entered the wrong setting, then backspaced and corrected it, the patient got
the maximum the machine could deliver. Killed some people before they found
the bug.

Yes, that was a classic. I always love how some people think software can
take care of everything. There are some cases where a simple switch is far
more reliable.

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[JustMe]

Since virtually all microwaves now seem to be run by electronics, I
wonder why they don't just program the CPU to detect interlock failures
and go into an error condition and refuse to operate, instead of
shorting the AC line? That would sound a lot safer to me. Then again,
I'm not an expert, so maybe I'm missing something.

When electronic control boards were still on only half of the microwave
ovens sold, it was a common-place to suggest to customers on the phone that
if they were to unplug the oven from the wall outlet for a minute or two,
the (software) fault they were describing would probably correct itself.
Many times it did!

 

[NSM]

When electronic control boards were still on only half of the microwave
ovens sold, it was a common-place to suggest to customers on the phone that
if they were to unplug the oven from the wall outlet for a minute or two,
the (software) fault they were describing would probably correct itself.
Many times it did!

When ovens all had mechanical timers I got many a panic call from someone
who couldn't figure out why their oven had stopped working but the hotplates
were fine. I got quite skilled at leading them through the reset procedure
over the phone - saved a drive.

N

 

[James Sweet]

Since virtually all microwaves now seem to be run by electronics, I
wonder why they don't just program the CPU to detect interlock failures
and go into an error condition and refuse to operate, instead of
shorting the AC line? That would sound a lot safer to me. Then again,
I'm not an expert, so maybe I'm missing something.

Then you're relying on all the electronics to be functional as well as the
software to be bug free, in real life there's just way too many chances for
that to not work. They've been doing it the way they do for 30 years now and
I've not heard of a single safety issue coming up in relation to that.

 

[Asimov]

"James Sweet" bravely wrote to "All" (01 Jul 05 06:30:35)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you
shut the door!"

JS> From: "James Sweet" <[email protected]>
JS> Xref: aeinews sci.electronics.repair:52068

Since virtually all microwaves now seem to be run by electronics, I
wonder why they don't just program the CPU to detect interlock failures
and go into an error condition and refuse to operate, instead of
shorting the AC line? That would sound a lot safer to me. Then again,
I'm not an expert, so maybe I'm missing something.

JS> Then you're relying on all the electronics to be functional as well as
JS> the software to be bug free, in real life there's just way too many
JS> chances for that to not work. They've been doing it the way they do for
JS> 30 years now and I've not heard of a single safety issue coming up in
JS> relation to that.

Software isn't as unreliable as you infer since there are planes which
operate "fly by wire" over our heads at this very instant. Hundreds of
millions of passengers per year are trusting their lives to a line of
code translated to 1's and 0's. These FBW planes even use joysticks!
Like it or not, the world is now a giant video game... PONG rulz!

A*s*i*m*o*v

.... Digital circuits are made from analog parts.

 

[Sam Goldwasser]

"James Sweet" bravely wrote to "All" (01 Jul 05 06:30:35)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you
shut the door!"

JS> From: "James Sweet" <[email protected]>
JS> Xref: aeinews sci.electronics.repair:52068





JS> Then you're relying on all the electronics to be functional as well as
JS> the software to be bug free, in real life there's just way too many
JS> chances for that to not work. They've been doing it the way they do for
JS> 30 years now and I've not heard of a single safety issue coming up in
JS> relation to that.

Software isn't as unreliable as you infer since there are planes which
operate "fly by wire" over our heads at this very instant. Hundreds of
millions of passengers per year are trusting their lives to a line of
code translated to 1's and 0's. These FBW planes even use joysticks!
Like it or not, the world is now a giant video game... PONG rulz!

OK, next time I want to spend $20,000,000 for a microwave oven I'll
keep that in mind.

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[NSM]

Software isn't as unreliable as you infer since there are planes which
operate "fly by wire" over our heads at this very instant. Hundreds of
millions of passengers per year are trusting their lives to a line of
code translated to 1's and 0's. These FBW planes even use joysticks!
Like it or not, the world is now a giant video game... PONG rulz!

Some ground worker duct taped over the instrument holes on a plane and
killed everyone on board.

N

 

[James Sweet]

Software isn't as unreliable as you infer since there are planes which
operate "fly by wire" over our heads at this very instant. Hundreds of
millions of passengers per year are trusting their lives to a line of
code translated to 1's and 0's. These FBW planes even use joysticks!
Like it or not, the world is now a giant video game... PONG rulz!

The process of designing, writing and most importantly, testing the software
used in an aircraft are completely different from that for the firmware in a
microwave oven. There's just no comparison.

 

[Asimov]

"Sam Goldwasser" bravely wrote to "All" (01 Jul 05 14:35:24)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you shut the door!"

SG> From: Sam Goldwasser <[email protected]>
SG> Xref: aeinews sci.electronics.repair:52109

"James Sweet" bravely wrote to "All" (01 Jul 05 06:30:35)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you > shut the door!"

JS> From: "James Sweet" <[email protected]>
JS> Xref: aeinews sci.electronics.repair:52068





JS> Then you're relying on all the electronics to be functional as well as
JS> the software to be bug free, in real life there's just way too many
JS> chances for that to not work. They've been doing it the way they do for
JS> 30 years now and I've not heard of a single safety issue coming up in
JS> relation to that.

Software isn't as unreliable as you infer since there are planes which
operate "fly by wire" over our heads at this very instant. Hundreds of
millions of passengers per year are trusting their lives to a line of
code translated to 1's and 0's. These FBW planes even use joysticks!
Like it or not, the world is now a giant video game... PONG rulz!

SG> OK, next time I want to spend $20,000,000 for a microwave oven I'll
SG> keep that in mind.


Oh, be serious! The door interlock mechanism is an inherited safety
device from the era when microwave ovens were controlled by what was
basically a washing machine timer switch. The door could be easily
monitored by an optical sensor. If the beam doesn't line up the
microcontroller doesn't power the magnetron. That's about 1 line of
code and no way would it cost $20,000,000 to do that!

A*s*i*m*o*v

.... Puddy-tat's not so bwave in Gwanny's microwave!

 

[Asimov]

"James Sweet" bravely wrote to "All" (01 Jul 05 23:59:06)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you
shut the door!"

JS> From: "James Sweet" <[email protected]>
JS> Xref: aeinews sci.electronics.repair:52141

JS> The process of designing, writing and most importantly, testing the
JS> software used in an aircraft are completely different from that for the
JS> firmware in a microwave oven. There's just no comparison.


I feel you are exagerating somewhat. True there is no comparison
between the two but the principles are the same. Both use positive
goal oriented programming where error checking and input validation is
often an after thought. The truth is that "shit" happens and both have
no way to handle "it" when "it" does. Hell, even with human pilots at
the controls they still managed to crash them without help of software.

A*s*i*m*o*v

.... 'Keep the smoke inside.' -- 1st Rule of Electronics.

 

[Sam Goldwasser]

"Sam Goldwasser" bravely wrote to "All" (01 Jul 05 14:35:24)
--- on the heady topic of "Re: Weird Microwave problem - trips breaker when you shut the door!"

SG> From: Sam Goldwasser <[email protected]>
SG> Xref: aeinews sci.electronics.repair:52109



SG> OK, next time I want to spend $20,000,000 for a microwave oven I'll
SG> keep that in mind.


Oh, be serious! The door interlock mechanism is an inherited safety
device from the era when microwave ovens were controlled by what was
basically a washing machine timer switch. The door could be easily
monitored by an optical sensor. If the beam doesn't line up the
microcontroller doesn't power the magnetron. That's about 1 line of
code and no way would it cost $20,000,000 to do that!

And what happens if the software has a bug is gets kicked into a
funny state by a power glitch? Hypothetical product recall notice
for the HiTeck Internet-Ready MicroNuke 1:
"We have determined that a particular key combination will allow the
oven to run with the door open. If you've already experienced this,
We sincerely hope you haven't put your head inside to try to figure
out what was going on. Owners are requested to return the
ovens for a software upgrade or to cut off the power cords and throw
them away. The software upgrade may also be downloaded from our Web
site. We believe this will solve the problem but of course, bugs
are to software like flies are to honey."

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[NSM]

device from the era when microwave ovens were controlled by what was
basically a washing machine timer switch. The door could be easily
monitored by an optical sensor. If the beam doesn't line up the
microcontroller doesn't power the magnetron. That's about 1 line of
code and no way would it cost $20,000,000 to do that!

Would you like to rely on this line of code when your 4 year old gets up the
heat her breakfast? Buggered if I would. Even elevators with 100+ years of
debugging kill people every year. I don't want my kid's eyesight ruined
because some slave in a Chinese factory coughed at the wrong moment.

N

 

[James Sweet]

And what happens if the software has a bug is gets kicked into a
funny state by a power glitch? Hypothetical product recall notice
for the HiTeck Internet-Ready MicroNuke 1:
"We have determined that a particular key combination will allow the
oven to run with the door open. If you've already experienced this,
We sincerely hope you haven't put your head inside to try to figure
out what was going on. Owners are requested to return the
ovens for a software upgrade or to cut off the power cords and throw
them away. The software upgrade may also be downloaded from our Web
site. We believe this will solve the problem but of course, bugs
are to software like flies are to honey."

The power glitch issue is the most realistic problem, as a user of
microcontrollers in my own projects, I've had to track down some really
obscure bugs where a program would get in a state that it should never be
able to get into, a random bit gets flipped somehow in a register and
suddenly the program is doing very strange things and it's difficult to
pinpoint why it got there.

In the end, nothing beats the dependability of a time proven mechanical
interlock. You need the microswitches anyway, may as well have them work
directly with the power rather than interface to it through other
components. As someone else mentioned, the triacs that control power to the
magnetron can and do fail, usually shorted which would disable even the most
stable software control. The present design is cheap, effective and
reliable, there's just no good reason to change it.

 

[Travis Evans]

Then you're relying on all the electronics to be functional as well as
the software to be bug free, in real life there's just way too many
chances for that to not work. They've been doing it the way they do
for 30 years now and I've not heard of a single safety issue coming up
in relation to that.

You all have some good points. I do agree that mechanical safety
devices should be used as well. For instance, microwaves should (if
they don't already) physically disconnect the magnetron from the
circuit if the door is open so that even if the microprocessor tries to
turn it on, it can't.

The reason I brought that up was that it seemed strange to protect
against microwave exposure by using something that seemed equally
dangerous: a short circuit. Also, I remembered reading somewhere
(probably either here or on the alt.home.repair group) that aging
circuit breakers might fail to trip when they should. That may not be
true, but I couldn't help wondering what would happen if for some
reason the breaker didn't trip. So I assume that all microwaves also
have internal fuses which should at least blow in that situation.

 

[Sam Goldwasser]

You all have some good points. I do agree that mechanical safety
devices should be used as well. For instance, microwaves should (if
they don't already) physically disconnect the magnetron from the
circuit if the door is open so that even if the microprocessor tries to
turn it on, it can't.

The reason I brought that up was that it seemed strange to protect
against microwave exposure by using something that seemed equally
dangerous: a short circuit. Also, I remembered reading somewhere
(probably either here or on the alt.home.repair group) that aging
circuit breakers might fail to trip when they should. That may not be
true, but I couldn't help wondering what would happen if for some
reason the breaker didn't trip. So I assume that all microwaves also
have internal fuses which should at least blow in that situation.

Yes.

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[Paul]

Note that the Therac25 was developed from an earlier machine which had
mechanical interlocks and when you pressed the same combination of back
spaces did blow a fuse, possibly 100's of people where over exposed from
what I remember and several fatally or damaged for life.
Do you remember in the 80's or 90's when people where installing there own
mobile phones in cars and where being warned it could mess up the ABS and
engine management causing accidents.
Then there was the F*rd Probe car which had a problem with the ECU which
meant interference from the ignition system could cause it to accelerate out
of control, a man was killed when his car flew off an off ramp at over a 120
miles and hour into trees.
As for the $20,000,000 planes don't they have more than one computer for all
the main systems with the software written by different companies, and the
computers keep a check on each other to make sure they are both working.
Paul

 

[[email protected]]

I think that was covered on the very first post. Perhaps there is
nothing wrong with the microwave!

By the way, anybody else wire up a Heathkit microwave oven way back
then??
The year was 1971 I think. I was saving big bucks for buying that $400
kit!!
I still think a simple rotating dial is the ultimate
control for simplicity and speed.

greg

 

[GregS]

I think that was covered on the very first post. Perhaps there is
nothing wrong with the microwave!

By the way, anybody else wire up a Heathkit microwave oven way back
then??
The year was 1971 I think. I was saving big bucks for buying that $400
kit!!
I still think a simple rotating dial is the ultimate
control for simplicity and speed.

greg

This was quoting a bit about, trying the microwave on a different breaker.

greg