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Microwave Power

Discussion in 'General Technical Questions and Answers' started by WA9SVD, Jan 9, 2018.

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  1. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Wow, great history!

    In 1957 I was five or six years old and we sure didn't have any microwave ovens. I think the first microwave I ever had was in 1974 when I bought my first house, and it didn't "come with one," I had to buy it.

    Today they're throwaways. My last one that died never failed electrically (Panasonic) but the door latch broke and surely wasn't worth repairing, so it's living happily in parts (brought it to a recycling center for appliances) in various landfills probably. Its replacement cost $179 at Costco. These are now Baofengs.

    Tom, how did Thermador (or anybody) make a wall oven/microwave like the huge one we have as a built-in? It can go to 550F using the electric heating elements, the interior walls are easy to clean (it has a self-cleaning function that gets much hotter than for cooking and just burns off anything on the interior surfaces turning it to powder you can just wipe away with a sponge), but is also a microwave in the same chamber. How did they pull that off?
     
  2. K8ERV

    K8ERV Ham Member QRZ Page

    I have been out of the field (retired) for 30 years and have not kept up with any recent developments.
    I don't know anything about the Thermador. We did try to develop a combo microwave/convection oven but the
    project was not hot (pun) and did not get far.

    I marvel at the low cost of ovens today. I have seen magnetrons being made and evacuated. Must be costly.
    We paid TI $75 for the first touch control panels, now you can buy the whole oven for about $50.

    Here they practically give ovens away at yard sales. The last one I bot cost $3!! Or to be publically correct $2.99.
    I got a full size high power oven with a very minor mechanical problem at a sale for free. Hard to beat the price.

    TOM K8ERV Montrose Colo
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    At our office we have about eight microwave ovens for use by employees, in the lunchroom. There are so many because 20-30 people at a time might be taking a break. And of course, they're also cheap!

    One of them "broke" at least a year ago in that it will continue to operate (full power) even with the door open. There must be an interlock that just isn't working. So, everyone makes sure it is OFF before opening the door.

    But I put a sign on the door: CHERNOBYL

    Some people got it.
     
  4. KW4TI

    KW4TI Ham Member QRZ Page

    Of course (this is very dangerous, so don't try this at home kids!), microwave oven transformers, diodes, and capacitors are useful for making other high voltage sources as well. Here is a Tesla coil I have built using two microwave oven transformers with a voltage doubler:



    73,
    Dan
    KW4TI
     
  5. K8ERV

    K8ERV Ham Member QRZ Page

    That is very strange. By law there must be at least two independently operated (no single mechanical failure can disable the system) and a "monitor" or "crowbar" switch that operates oppositely than the door switches and disables the oven if both of the door interlock switches fail to turn off the current. The crowbar usually shorts the power AFTER the door switches operate so put a short on the line, blowing the internal fuse. Hard to believe all three switches failed.

    These switches must operate properly for 100,000 door cycles.

    You should contact the manufacturer, he must correct the situation free. Unless the law has changed.

    I, and many others, have put our hands inside an operating oven with no harm. They just get warm in about 5 seconds and you withdraw them.

    I have heard that Glen put his head inside an operating oven, which explains a lot.

    TOM K8ERV Montrose Colo
     
  6. K8ERV

    K8ERV Ham Member QRZ Page

    Dan, that was really neat! You are sure smarter than I am.

    But that's not too difficult---

    TOM K8ERV Montrose Colo
     
  7. KW4TI

    KW4TI Ham Member QRZ Page

    It's dangerous, but its common to build Tesla coils using two microwave oven transformers (rather than the neon sign transformers, which are becoming less available) to provide the primary voltage, with the grounds of the transformers connected together, so that together these produce 4200 VAC. Then using the two microwave oven capacitors and diodes, you can get about 10-12 kV peak voltage to charge the Tesla coil primary capacitor. The hard part is that back EMF from the Tesla coil primary into the power supply when the spark gap arcs tends to destroy the microwave oven diodes, so you can use doorknob caps or a "Terry filter" to absorb these RF spikes. I use four microwave oven diodes in series rather than two potted in epoxy to prevent flashover.

    The microwave oven transformers are quite dangerous as they usually can generate well over 250 mA @ 2100 VAC which can easily kill someone, so the high voltage must be treated with extreme care. Also, I have bleeder resistors across the capacitors to make sure that for some reason if the internal resistors don't discharge, the capacitors don't remained charged.

    It's a comparatively cheap way to get high voltage because you can often find old microwave ovens at thrift or junk stores, and usually it is not the transformer, capacitor, or diode that is the problem, and so these can be recovered. I have wondered if these transformers might provide enough power for a tube amplifier.

    73,
    Dan
    KW4TI

     
  8. K8ERV

    K8ERV Ham Member QRZ Page

    Sometimes you may find ovens in a scrap yard. Sold by the pound.

    TOM K8ERV Montrose Colo
     
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Tom
    I am a volunteer at the Antique Wireless Association in East Bloomfield, NY. One of the projects I am starting to work on is a magnatron display for the military collection. The idea started when I came across magnatrons from WW2 that were brought in from a member's collection many years ago. I have one book on the radar written by Ridenhour who worked at MIT's Rad Lab. Do you gave any suggestions for reference material that covers the history of magnetrons that could be useful for us? Thanks! 73, Pete
     
  10. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I found a few uses for the oven transformers. The latest is a circuit breaker tester. I needed a simple way to generate high ac current to test a 50 A breaker (for our electric furnace). I removed the HV winding and passed one short length of heavy gauge copper wire thru the core where the HV was and connected this across the CB to be tested. The primary is connected thru a Variac and I clamped an ammeter on the heavy one turn secondary. I can easily pass 100 A plus thru the CB if needed. The test was a success in that I reproduced the very loud buzzing that was emanating from the breaker with about 40 A flowing thru it. I wound up replacing the two two-pole 50A breakers that were over 30 years old. (the furnace needs 19.2 kW at 240 V!). The terminals of the breakers were overheating as well. Lesson learned:) 73, Pete
     

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