Quote:
Originally Posted by Glinda
Ah. Now this makes perfect sense to me. I was sure there was a house wiring component to this mystery.
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No house wiring is involved in this mystery (if AC electric lights do not flicker or change intensity when major appliances power cycle). Posted were electrical fables invented by many who forgot to first learn how electricity works.
If you had a surge, then a microwave is damaged - no longer works. And some of other less robust appliances (ie GFCIs, clocks, central air controller, door bell) are damaged.
Not one reason is given to suspect house wiring or a surge. So those wild speculations are binned immediately. Honest replies also say why with numbers.
We even saw this once in a factory where a toxic gas (phosphine) was vented into an area of construction workers. Fortunately a bird fell dead before those construction workers got too close. That is when I was brought in.
Sometimes power does not fully turn off or on. Intermittent and quick AC offs and ons cause DC voltages to vary to intermediate voltages. (What glatt calls a power flicker.) Computers that run on 5 volts get disoriented when that voltage is below 4.8 and does not fall to zero. Then the computer can start babbling; executing code that makes no sense.
In our case, a babbling computer powered open every valve venting a large tank of toxic gases. In your case, it simply instructed one relay to close and stay closed.
Your PC is not a real time computer. So it does not need this major human safety device. That computer inside a microwave is real time. So a watchdog timer literally creates a heartbeat. If the real time computer does not issue the proper heartbeat every few milliseconds or seconds, then a watchdog timer cuts off power to everything.
But back then, despite even telling this to some people's face, some so called designers would ignore the warning and not implement that watchdog. Or would write code in a manner that even a babbling computer would still act as if functioning properly.
In short, your microwave either does not have or did not properly implement a watchdog timer. So quick AC power interruptions caused a babbling computer chip to power on the microwave with nothing to time it (power it) off.
Can we say that with certainty? No. Do you have symptoms to justify that event? Yes. Did anyone support their conclusions with the always required reasons why and perspective? Obviously not. So this is your only possible answer that has any credibility.
Back then, watchdog timers were typically separate chips provided by companies such as Dallas Semiconductor. Today, that essential human safety function is standard in all single chip computers.
And yes, this many decade old (that well understood concept) is clearly something new to you and others. So understand it will require at least three (or more) rereads. And maybe only after asking a few questions.