From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, February 04, 2011 2:47 PM To: Bill Owsley Cc: IEEE EMC forum Subject: Re: How much power to pop corn this fast?
More interesting questions are: - Is it feasible to hack a microwave oven apart to get a powerful signal source for this demonstration? It’s been a long time since I did so, but sure, you can use a consumer 2.4 GHz oven as a microwave source. As Bill described, you can just defeat the interlock switch and open the door. A bit more serious of a hack is to eliminate the cooking chamber and fabricate additional waveguide onto the existing waveguide stub from the magnetron, so you can now better direct that microwave power, perhaps into a pyramidal horn antenna. The magnetron is an amazingly tough design, tolerating a huge number of cycles of off to full power and withstanding all sorts of terrible VSWR in its original application. (FYI, various levels of cooking strengths are achieved by duty cycle modulation; off for 10 seconds, then full on for 3 seconds, then off again, etc.) Remember that you won’t get nice CW out of the magnetron, as the power supply is not well filtered with decent energy storage capacitors. Rather, the DC applied to the magnetron is off near the AC supply zero crossings, so you get a waveform a bit like dimmers apply to a light bulb, pulse and AM combined. - If you could pick a signal frequency for the test, would you pick the usual 2.4GHz band used for microwave ovens, or something different? It all has to do with skin depth in the load (food) that you want to heat. If you ever tried to cook a thick block of meat, you would see the skin depth problem immediately. Commercial microwave ovens operate down in the 960 MHz ISM band, so they get three times more skin depth; OTOH, they must be physically larger. True, on popcorn, the skin depth needed isn’t much (popcorn does its thing due to boiling of the water inside the kernel, creating a steam overpressure that ruptures the thin corn skin), so you could use much higher frequencies. OTOH, 2.4 GHz is a “sweet spot”, with water molecules twitching happily here, and microwave hardware is of a reasonable size. I remember hearing anecdotes about engineers (or was it Navy personnel?) cooking hot dogs in front of microwave antennas for fun. Side lobes and reflections make this a very risky thing, even if you have a very long stick. Pat Lawler EMC Engineer SL Power Electronics Corp. Ed Price [email protected] <blocked::mailto:[email protected]> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
