Microwave Oven Interlock Switches == Seppuku (ritual suicide) - Page 1
Forum section: electronics / repair

author: garrettm
author_rank: Frequent Contributor
timestamp: December 09, 2024, 01:25:08 am
content: I recently ran into a bit of trouble with my Fridgidaire microwave oven. The home came with it installed as a "fume hood / grease trap" over the stove. Which makes it annoyingly hard to replace since the cabinets were positioned around it when they built the home.Before I get into the microwave oven, there was a second problem that masked some of the microwave's issues: All six of the Eaton GFCI/AFCI breakers installed in this home are faulty. They will falsely trip showing a ground fault code via the diagnostic LED. Multiple breakers for various outlets, the refrigerator, the microwave and so on keep tripping. This is, apparently, a well known problem with early Eaton GFCI/AFCI breakers (easily identified by their extended length vs a "dumb" breaker). If you have one of these pieces of crap installed in your breaker panel, Eaton will replace them with new ones free of charge. The new breakers are much smaller--apparently they can now fit all the electronics into the same space as a classic "dumb" breaker.Back to the microwave. The buffoons at Frigidaire decided to let people open their microwave ovens while they are running... This, of course, requires interlock switches and a relay to shunt the magnetron transformer to dampen the output sufficiently that people don't get cataracts when opening it before the timer elapses. To do this they added three switches to the door and a "protection" relay that shunts the transformer. Then there are two more relays to power the magnetron: an inrush limiting relay that uses an NTC resistor in series and the "big boy" relay that shunts the inrush circuit to fully power the magnetron.The initial problem was that two of the interlock switches were intermittently welding together. This then caused the primary relay to weld together, which then caused the microwave oven to remain on even with the door open! Fucking nuts. Anyways, after replacing the interlock switches and eventually the primary relay (as it was damaged from the stupid interlock switches failing), the damn microwave would still occasionally trip the new breaker. After fiddling around with the door, I found out that the tolerances for the door to interlock switch contact was about a millimeter off from reliably engaging one of the lower switches... I ended up adding a strip of flexible plastic to ensure the pressure on the switch contact was enough to keep it fully engaged when the door was closed (using a good wiggle test of the handle and my DMM on continuity mode). I now believe that this tolerance issue was the primary cause of all the damaged components...The funny thing is, if one pressed "stop" instead of impulsively opening the door, the magnetron would turn off far faster, and without all the bullshit needed to keep people safe. I think people's expectation that "I should be able to open the door even while running" is idiotic and needs to be beat out of them. The parts needed to allow this add cost, make the product less reliable, and don't make them understand that they are still being exposed to microwave radiation that can cause serious harm to their eyes, especially if the interlock system fails in some unexpected way like my unit.TLDR; microwave oven interlocks should _prevent_ people from opening the door while running, _not_ let them open it.

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2024, 04:10:19 am
content: That is not a new problem in microwave ovens. The door switches are not rated to interrupt the current which is present so if the door is opened while the oven is operating, the door switches are damaged and eventually fail.Adding RC snubbers to each individual door switch could be enough to prevent them from failing.

author: garrettm
author_rank: Frequent Contributor
timestamp: December 09, 2024, 05:14:30 am
content: Quote from: David Hess on December 09, 2024, 04:10:19 amTrue, but not all microwave ovens allow the user to open them while operating. The ones I grew up with in the 90s and 2000s didn't let me do that.You bring up a good point about the RC snubbers though... Why they aren't used in the first place? Is it bad engineering or planned obsolescence?Unfortunately, even if Frigidaire used RC snubbers, poor mechanical tolerances/alignment of the interlock system can wreak havoc on the relays if the switches aren't engaged / disengaged properly. I think some sort of optical based interlock that is hard to "defeat"/trick and a TRIAC or two would have been the better design.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2024, 06:21:24 am
content: It's teenagers that press the button hard, pop open the door so fast the mech can't keep up.Try to tell them to stop doing that when they are hungry and checking on the food temp.Whirlpool will never lose pennies adding snubbers to any of their products.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2024, 06:39:48 am
content: or is the engineers not designing a sufficiently heavy switch that can take it.I noticed MW ovens are built down to the fractional cent lol. But the cheap ones will burn out the magnetron within a year before the switches goI think there is something up with the VWSR, when I see the input 1 inch waveguide launcher melt a hole in itPoorly specced out switches and such is ultra common cost optimized products. Its too easy for like non technical people to say they understand a switch and take over the decision making process because its 'simple'. I feel like they excel at this.The ones that are up in arms about price, away from any engineering, suddenly think they have a 'in' because its a switch. Thats just my experience.I suspect something funnny happens with those launchers too, like an argument about how many bends are allowed in the design (price per bend), ignoring the fact that you are replacing a precision power microwave frequency component with thin sheet metal bent in a shop (is already more then a bargain, normally it would be made from at least 3/16 inch thick machined and silver plated copper, its being replaced with sheet metal).I think microwaves are losing out to little toaster ovens though, their pretty fast and taste better and can make more complicated food with less work. I think the quality difference makes microwave lose regardless of how cheap they make them, because people are pretty discontent with soggy food when the internet is teaching so many how to cook in different ways.I noticed if I am relying on the microwave oven for some reason (other then softening butter), it means I gotta get something out of the way, so I stop relying on the microwave!It's like a symptom.

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2024, 08:54:09 am
content: Quote from: garrettm on December 09, 2024, 05:14:30 amI have never used one which locked the door.QuoteThey are just being cheap, and it is not like the switches will fail within the warranty period.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2024, 09:14:59 am
content: and what would you do if it jammed with food inside of it? That mechanism will break tooI think you are over reacting about how much exposure you can get. its not a 10 gigawatt radarPeople will get a 1 time exposure to some 'heat', close the door, and throw it away. If you set the oven for 500F and open it while hot it can give you a hot air burn. Common sense, not locks. I think you have a misconception about how strong it is. If you lean to look into a oven cooking a pizza and open it in your face, your eyes will get plenty of heat too. Its not like microwaves magically heat your eyes more then anything else, they just cool bad, so hot air can do something very similar.The thread title is sensationalism.

author: garrettm
author_rank: Frequent Contributor
timestamp: December 12, 2024, 10:46:46 am
content: Quote from: coppercone2 on December 09, 2024, 09:14:59 amThe title isn't sensational at all. It was a joke based on reality. A poorly designed interlock will literally destroy all the current carrying switches and relays as I've described.I have seen two microwaves act up that locked the food inside. It happened, and it was annoying. (IIRC, the power went out and the door remained locked until ran again and the timer elapsed.) But I'd rather food be stuck inside than my eyes being irreversibly damaged. You clearly have never heard of what happens to tissue like the eyes when exposed to significant microwave radiation. Its a serious safety concern:https://pubmed.ncbi.nlm.nih.gov/2488031/https://pubmed.ncbi.nlm.nih.gov/14628311/https://www.fda.gov/radiation-emitting-products/resources-you-radiation-emitting-products/microwave-ovens#Have_Radiation_Injuries_Resulted_from_Microwave_Ovens_"Exposure to high levels of microwaves can cause skin burns or cataracts. Less is known about what happens to people exposed to low levels of microwaves."An open microwave door with the magnetron on is not safe to be around. This is what happened while I was working on it due to welded contacts on the relay and "safety switches". I do not know why you are making such inane arguments for why this is not a problem.