Old microwave ovens (like the one from 1988) were electrically quite simple; a relay applied AC power to a step-up transformer, and the secondary output was grounded (one side) and connected to a diode (other side). The diode allowed a low-capacitance HV capacitor to charge and the magnetron to produce RF output. It was a simple half-wave rectifier circuit.
When the door safety switch open, the relay would drop out (anywhere along the AC sinewave), and the magnetron would continue to produce RF until the capacitor voltage dropped below the minimum magnetron "excitation" voltage. I can't recall magnetron current nor the capacitor size, but the decay was "pretty darn fast." I wonder if the capacitor was large enough to sustain RF output through the negative half of the AC power? So, although electrical shut-down was quick, you have to take slack in the door safety switch mechanism into consideration. Best if the switch was linked to a safety latch, but not so good if the switch was sensing door position. New microwave ovens, more computer than appliance, and with a switching power supply, might have a larger HV capacitor, and thus longer decay time when being shut down. People who have built their own ham gear or have built lab equipment such as pulse generators, know the hazards of HV and interlock systems, but when your whole experience is built around 5 & 3.3 VDC equipment, RF door leakage may come as a big surprise. I think EMC engineers would be aware of this, but the issue is much more under the domain of safety. If nothing else, an EMC engineer should flag this to the safety guy for proper evaluation. Interestingly, many news sources quote the oven emission as "1.4 GHz" which is very unlikely for a consumer oven. They didn't mention if the telescope was being used in the 2.4 GHz band (again unlikely), so I wonder if the telescope was being overloaded by the out-of-band 2.4 GHz oven fundamental, if the oven harmonics were to blame or if the telescope data-processing equipment was actually the victim. One story at least admitted that researchers were suspicious of the signals, as they only were a problem during "working hours." Ed Price WB6WSN Chula Vista, CA USA From: Ken Wyatt [mailto:[email protected]] Sent: Friday, May 15, 2015 8:55 AM To: [email protected] Subject: Re: [PSES] Interference Caused by Microwave Oven Hi All, I recently wrote an article on wireless interference, with microwave ovens being one source of that interference. Fortunately, the use cycle of microwave ovens is relatively short, so any interference is short-term. However, any leakage is certainly an issue for those standing nearby - maybe listening to popcorn, etc.? http://www.edn.com/electronics-blogs/the-emc-blog/4438663/Wi-Fi-network-inte rference--analysis--and-optimization In this article, I show a real-time spectral plot of microwave oven leakage at 12 inch distance from the oven. The peaks were around -20 dBm using a short "rubber duck" antenna on the Tek RSA306 analyzer. The leakage limit is 1 mW/cm^2 with a test load (water) and 5 mW/cm^2 without a test load (measured at 5 cm from the oven surface). I measured mine just now at 0.1 mW/cm^2 loaded and 0.5 mW/cm^2 unloaded. The Australian Government did a study on the subject in 2004 and includes some interesting background on the door interlock designs: http://www.arpansa.gov.au/pubs/emr/microwave.pdf Health Canada also did a survey of several new and used microwave ovens in the Ottawa area: http://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/microwave_ovens-micro_ondes/i ndex-eng.php Cheers, Ken _______________________ Kenneth Wyatt Wyatt Technical Services, Inc. 56 Aspen Dr. Woodland Park, CO Phone: (719) 310-5418 Email Me! <mailto:[email protected]> | Web Site <http://www.emc-seminars.com> | Blog <http://design-4-emc.com/> The EMC Blog (EDN) <http://www.edn.com/blog/The-EMC-Blog> Subscribe to Newsletter <http://www.emc-seminars.com/Newsletter/Newsletter.html> Connect with me on LinkedIn <http://www.linkedin.com/in/kennethwyatt> On May 15, 2015, at 9:00 AM, Ken Javor <[email protected]> wrote: My microwave oven used correctly interferes with wi-fi and Bluetooth, but with blue tooth if I bring the devices close together I can overpower the incidental oven radiation, right next to the oven. I can't do that with wi-fi at home, because of the fixed location of the wi-fi base station at the other end of the house from the oven. Obviously I can bring the wi-fi device closer to the base station and at the same time farther from the oven and make the problem go away, but that is hardly surprising. But the point of the news item was that it seemed the interference was tied to the oven door opening, which is problematic, at least to me. I don't buy the delay thing someone posited - I'm thinking that it is a switch activating a relay interrupting power to the magnetron, and that happens before any pressure on the oven door gasket has been released. Ken Javor Phone: (256) 650-5261 From: Michael Derby <[email protected]> Reply-To: Michael Derby <[email protected]> Date: Fri, 15 May 2015 08:02:00 +0100 To: <[email protected]> Subject: Re: [PSES] Interference Caused by Microwave Oven This isn't a particularly helpful contribution, but when I use my microwave (used correctly, with the door closed), my Bluetooth speaker completely stops working and any WiFi activity grinds to a halt. I try to keep the kids away from the microwave when it's in use, as much as I can. Michael. -----Original Message----- From: John Woodgate [mailto:[email protected]] Sent: 14 May 2015 23:42 To: [email protected] Subject: Re: [PSES] Interference Caused by Microwave Oven In message <[email protected]>, dated Thu, 14 May 2015, Doug Powell <[email protected]> writes: My question is this. If the RF generated inside the oven does not stop prior to the opening of the RF seal on the door, which testing is responsible to identify this problem, EMC or the Safety testing Not 'either/or': emission with the door open violates CISPR 11/EN 55011, it's also a violation of the Section of IEC/EN 60335-2 (too late here to look it up) and potentially a violation of the regulatory EMF exposure limits as measured according to IEC 62233. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk When I turn my back on the sun, it's to look for a rainbow John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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