I am building a machine that uses a microwave oven. I need a simple and cheap way to reliably determine 1) that the magnetron is operating, and
2) that the oven cavity is not empty.
My first attempt was a diode RF sensor on a board mounted outside the cavity with a short probe projecting inside through a hole. The food item being heated moves relative to the probe and causes the response to increase and decrease as it acts on the E field. When I see the changing response I know the magnetron is working and the oven is not empty. This works fine if the food is by itself; but, the food rests on ceramic which moves. The ceramic material influences the E field to a large degree and makes it impossible to detect the food item. I can't eliminate the ceramic or change to another material. It seems I need an entirely different approach.
If I put two RF probes in the waveguide spaced one-quarter wavelength apart, I think the phase relationship between the two probes will change when the oven is empty.
I want to avoid the expense of adding a secondary waveguide in order to make a directional coupler.
Does anyone have an idea for a simple cheap way to detect that the oven is empty.
Maybe narrow band down to detect for the EXACT food frequency. I am an amatuer in RF, ask an expert about how to filter for the exact frequency at which food excits and re-radiates.
Another way to detect the presence of food is to have a micro-switch operated by the weight of the food on the carrousell. The carrousell slips up and down along the driving shaft. => switch opens and closes. An opto interuupter may do the same.
yet another way, use an optical transmitter - receiver and notice the beam being broken by the food.
The response of the cavity should also change when it is not empty. There's an article floating around about how to measure the dielectric constant of an unknown material by placing it inside a cavity filter and measure the pre and post response. You might
I would think that the process might apply to what you are doing. I will try to find the docment in my libary and email it to you... Or you can google it.
Given the two things he wants to do, I do believe the approach has merit. You can check for undercurrent on either the magnetron, or the power supply feeding it. That should solve problem #1
Note: Variability shouldn't really matter. They are either on, or they are off (...or they are broken!)
As for problem #2 - You're right, maybe he needs better accuracy?
Considering the Cavity empty / occupied, again this can be derived from RF response of the cavity, which might be able to be derived from the Magnetron current(?). Or, as already suggested, some mechanical or optical switch means. I think this is the trickier part..? But the cavity (should have) fixed and unchanging mechanical dimensions so it's RF response should be fairly determinable over time, even over temperature - at least when compared to shoving something in the cavity. (That sounded dirty! HA!)
I still think measureing magnetron efficiency when loaded vs.unloaded is the key.
It has been a long time since I fooled with ovens. TV transmitters now, that's another story. (And trust me, when those fail, they usually do so with a loud crack, or a goopy glycol mess, or both, -- and never before 3AM!!.)
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I just had another weird thought, if the cavity can be made airtight,
I wonder if you could pump air into it and measure the volume. That
would tell you if the cavity were occupied, within limits of course.
Might be difficult to set-up initially, but if the project would
benefit from having a known atmosphere (pressure, Nitrogen content,
etc..), that might make it worth the effort?
And I think you\'d want to put dual detectors a half-wave apart.?
I have to think about that one some more......
One more thought! - Why not just sit the whole contraption on a scale
and weigh it before and after placing something in the cavity?? Maybe
with one that has a digital output or something? You can even get
pretty accurate scales these days, assuming the items being placed are
small.
I love low-tech sometimes!! Good luck -mpm
Just an idea, but the actual power consumed by the oven will probably go up when it contains food or other heatable substances. You would need to measure true power, as the VA may not change, but the phase angle might (if you measure the AC input). The DC magnetron current should also change, but it might be hard to measure.
I have run and maintained several TV transmitters. One Gates 250 W on Ch 8, One RCA TTU-25B, at 25 KW, and a Comark/Thales 130 KW visual, including changing the coolant on the EEV Klystrons and flushing the system with Tyglos. I made bypass adapters to keep the Tyglos out of the Klystrons, as well. The only on air failure of a transmitter I've seen was when the local PBS station reluctantly let me and an army buddy take the nickel tour. Neeley was having trouble not laughing in the guy's face, because the place was a nightmare. Just as he was bragging about how reliable his transmitter was, a bypass capacitor on a 4CX250 driver exploded, taking the socket with it. He turned to me and snarled, "You damn jinx!" My buddy tore into him, telling him that he had watched as I rebuilt a AFRTS TV station with a couple hand tools and almost no spare parts. ;-)
See, the old engineers knew what they were doing by being off the air for four hours every night. You can't do PM while you're on the air, but I did make some repairs to that Gates transmitter while on the air. I was standing on the plate transformer, adjusting the drive coupling with a fiberglass rod while the other engineer took the readings. ALl the interlocks were in place, but there were none on the end cap.
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Service to my country? Been there, Done that, and I've got my DD214 to
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Member of DAV #85.
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Central Florida
You could talk all night about this stuff and I would listen to every word. One of the few things these days that still fascinate me. Got any good links to high power transmitters, tv stuff etc?
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Harris Broadcast had a lot of data on their new solid state TV and radio transmitters, but I can't find it anymore. It is such a small market that everyone is afraid to let out too much info. They don't make TV transmitters the size of that old RCA gear. It took 4' * 36' of floor space for the nine interconnected 4' *4' cabinets. The ONLY semiconductor was in the Bird watt meter at the drier stage. The TTU-25 series used water cooled power tetrodes, designed for that transmitter. They had a pair of 1.5 V 1000A filaments that had to be balanced to .01 V differential. The "adjustable resistors" were long copper buss bars that had studs embedded in them to stretch the copper to raise the resistance. The tubes had 7000VDC on the plates, so there was 7000 VDC across the water in the cooling system. Water purity was very critical, because if it becasme conductive, electrolysis would build up oxyegen and hydrogen to the pint it would blow the cooling system apart.
formatting link
has pictures of some radio broadcast transmitters, including the experimental 500 KW AM transmitter WLW used on 700 KHz.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
Oh yes they do! I'll have to send you some photos of the TV-68 NEC transmitter that used to be in Orange City before it was moved. I think the beam supply cabinet was the size of your RCA!!
Actually, it was a few feet short of 36. But it was still a beast!
LED and photodiode aimed just above the platform. If there's food on it, it'll break the light beam. You should be able to drill two tiny holes in the walls to keep the components outside the oven cavity.
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The Ch 68 transmitter that they never got to work right? That was given to a college, when they traded channel allocations for Ch 18, and they replaced the NEC with another brand. I was there when they were trying to get the bias right on the finals, which should have been done before it was shipped.
The exciter was a TTU-1 transmitter, in three cabinets. The 25 KW Visual and 12.5 KW Aural amplifiers, controls and power supplies filled six more cabinets. Actually, I forgot that there were two, two foot wide cabinets for the the sections of reinforced rubber hose used in the cooling system, so it was 40 feet long. That job was around 1990, and I've forgot a few details. Since this was in the Florida panhandle, I set the cabinets up on 6" * 6" landscape timbers just in case the area flooded during a hurricane. They were less than a mile from the gulf, and a couple feet above sea level:
Try moving a full size wave solder machine sometime. It took two to lift it and get it into the building.
It took three 28 foot U-haul trucks because of the weight. One load was less than half full, and I was less than 25 pounds from it being overweight.
None when I moved it. It was all 14 gauge stranded rubber covered, with a fabric jacket. It was so old that I had to replace the entire inter cabinet harness, one wire at a time. Just moving the old harness made you sick to the pit of your stomach, knowing that more rubber had turned to dust, and the chance of another exciter going up in flames, like the original.
The only local electrical supply house had three colors is stock, so the new harness was Brown, blue and white, and made in expandable mesh tubing. It took three full days to make, label and install that harness.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
The same. I guess it's still in service for another year or so? I would not want to be guy converting that to DTV, I can assure you!!
But as for things shipping from the factory "correct", well I can recall (2) cases where that just didn't happen:
The first was WCIX TV-6 after hurricane Andrew (1992). 1750' tower comes crashing down, hits the building, water in the building, floods the transmitter. (Not that could get it out of the crushed building anyway..) You get the idea.
By pure luck, we were just about to ship a 40kW Larcan solid state to Charlotte for the DTV tests. It was fully tested and ready to go, and was already tuned to CH-6! (What amazing luck that the Charlotte DTV test were taking place on Ch-6!). So we offered it for sale, and eventually CBS took it off our hands.
But when it arrived, one of the blowers was installed backwards (possibly by the local engineers, though nobody can say for sure...), and for the next year or so, they "mysteriously" kept blowing finals in the PA drawers. It was on low-power on a temporary tower for a couple years while the replacement tower was being erected (which we also provided!), and this got fixed / noticed when they added the extra PA cabinets. But this one you can almost forgive, because they had a transmitter in a week vs. being off the air for ...??? (You know, there aren't too many high power CH-6's laying around..)
The next one I won't give call signs or antenna manufacturer names (to protect the "guilty" - hey, we all make mistakes...)
Eastern United States. Multiple Combined FM's (all Class-C's) all operating from a common panel antenna. One night, antenna goes up in flames! Everyone off the air. Everyone pissed. And of course, it's well past midnight! Swept the line - major problem at top elbow complex. Mobilize crew. Bring elbow complex to ground.
Guess what was Inside one of the elbows? A pack of washers!! (Yes! That's right boys and girls. These were the very same washers that were supposed to tighten up the flanges so water wouldn't get into the line in the first place.) I guess the original tower crew couldn't figure that out?!
Needless to say, it was a little hard to hide what went wrong when all the station engineers looking on...... And of course, it pretty much resolved the nagging question of why the line had so much trouble holding pressure! In all my travels, I had never seen a miter-reinforced 6" copper elbow get that bad. (I think it almost melted!, it was definitely warped.) That's a lot of RF.
Ah, those were the days. Did I mention I don't really miss 'em?
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