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Ultraviolet Microwave (name subject to change)

Convert Microwave into UVC Chamber.

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We’ve had to wear masks to both prevent the spread of COVID-19 and to block airborne virus from getting into us. What was to be disposable turned out not to be and even the medical community have had to reuse them. If the mask is doing its job, then it has virus on it. Use of disinfectants isn’t recommended for mask or electronics. UVC exposure was found to make the virus ineffective and one of the very few alternatives that would be a safe way to reuse our masks.

So I went to Amazon to look for disinfecting chambers and what they had for looked sketchy. I asked my friend Keith Hargrove to take a look too. Neither of us could find much that we could trust to do a good job. We talked about building our own. We found a few DIY projects but they looked like a recipe for trouble. A UVC lamp that is capable of destroying the virus poses a health risk if you are exposed to it. Keith came up with  a great idea of retrofitting a Microwave. The Microwave has a radiation sealed chamber,  switched interlocking door, safe area for AC power, timer, light, a little turntable and power cord.

I used a UVC germicidal tube designed for this kind of application and it has a long track record.  It is in a small enclosure so it is within close proximity of the UVC tube and we made an instrument that would measure the intensity.

When you are re-purposing appliances for what they weren’t intended for, there are  unforeseen circumstance.
 For example, it would be convenient to use the power to the magnetron transformer – it has a beefy relay and is switched on when the door is closed and on a timer. The problem is microwave power is controlled by cycling this relay using a long duty cycle. That is if the power level is set 50% it may cycle power every 10 seconds. I found that, independent of power level, it would do an 80% on 20% off duty cycle after 8 minutes, probably to keep the magnetron and electronic from overheating. This kind of duty cycle will shorten the life of the UV bulb. The alternative is to use the relay that controls, the turntable and fan. This relay stays on while the magnetron power cycles but the interlocks and timer still controls them.

I went with a 6in 9W compact fluorescent UVC disinfectant tube, GCF9DS. This is a folded tube that mates with the G23 two prong socket. This is old school fluorescent with integrated starter and a 9W magnetic ballast. Definitely not recommended for new designs but all the parts are still available. Since it has an integrated starter, you cannot use solid state or “electronic” ballast for these tubes and it will flicker when it starts up. You could probably scrounge a microwave although a simple new one can be gotten fairly cheap. Advantage of a new one is it won’t smell like old food. (the ozone generated from the UV tube may kill the smell). I got a cheap retro-looking microwave, the “RetroWave Nostalgia”. The chamber length is 11 inches and the tube will fit.  

  • 1 × GCF9DS 6in 9W UVC Germicidal Tube
  • 1 × 9W Magnetic Ballast
  • 1 × Satco 90-1542 G23 Vertical Tube Socket
  • 4 × #6 x 1/2in sheet metal screws Electronic Components / Misc. Electronic Components
  • 4 × AMP 42490-3 Quick Connect tab connector male 0.187in

View all 12 components

  • It's only skin deep

    Bob Alkire08/01/2020 at 20:17 0 comments

    Brian Cockfield wrote an article about this hack - thank you!

    Some of the comments left were very useful and I appreciate them.

    I had focused on what the CDC recommended, https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontamination-reuse-respirators.html and the use UVGI. This was not universally accepted by WHO or CDC organization in other countries.

    I set up a simple experiment with my UVC lamp, a UVC sensor with amplifier and oscilloscope.  It has about 40dbV dynamic range to measure over.

    I put a transparent glass slide over the UVC sensor and exposed it to UVC, it effectively blocked the UVC light. Both the N95 and KN95 mask blocked the UVC light.  The cheap safeway mask did show UVC at -30dbV, a little got through.

    If the mask is designed for filtration of smallest particles and the if the material is not transparent to UVC,  the dense forest of filtering fibers isn't gong to allow UVC to pass through. It cannot sanitize what it cannot reach.

    There is practical information on this in other applications when using UVC.  Air-filtration and Water-filtration units that utilize UVC have prefilters whose function is to block the larger particles, The larger particles need to be removed as they will block UVC from reaching pathogens.

    Again, thanks for the useful comments.


  • UVC vs Rubber, rubber loses

    Bob Alkire07/25/2020 at 18:55 0 comments

    So I found one con to using UVC for N95 mask.

    UVC will cause rapid deterioration of rubber.

    So one of my N95 masks had it's rubber strap snap when putting it on. The rubber had visibly changed and become brittle.

    My other mask from the same pack but also not subject to the UV were still pliable.

    Of course, this was after about 10 times in the UV Microwave and a few of the times, I didn't have the vent fan running.

    At first I thought it was generating ozone, but going back to the datasheet,  this tube is not supposed to create ozone. I did some research and UVC will cause photochemical change in plastics and rubber. (see FAQ at https://iuva.org/).

View all 2 project logs

  • 1
    Tools and Skills Needed

    Phillips screwdriver, #6 nut driver, portable drill with drill set, two step tab connector crimper (Molex 63811-1000 or equivalent), needle nose pliers, wire cutters and wire stripper, scissors, and masking tape. You may need the Tamperproof Torx.

    This is a simple modification; but does require working with line voltages. It needs basic electrician skills akin to changing a light fixture or AC socket safely. No soldering is involved. I used quick connect tab connectors which are crimped and insulated with heat shrink tubing. You need to know how to use the two step crimp tool and how to apply heat shrink tubing. If you are using a different microwave, then you need to be able to reverse engineer the wiring. Wiring this up incorrectly can lead to all sorts of badness such as electrical shock, fire, acrid smoke, property damage and personal injury. It is always a good practice to double check the wiring when done and before applying power.

  • 2
    Tear Down

    Microwave can generate lethal voltages, and as a reminder, please make sure it isn’t plugged in when you start working on it.

    Remove the U shaped metal shroud over the top and sides of the microwave. These are held in by a few metal screws and one of them may be a security screw. A security screw or Tamperproof  Torx screws have  a metal post in the center. They are there to keep you from trying to open this up as it does contain a death ray. Of course, about every hardware store carries a cheap set of Torx tamperproof screwdriver set so you can tamper with this screw. 

    All the electronics is on the right side behind the controls. You want to disable the magnetron and make room for the ballast. Behind the controls is the control PC board with a relay which switches power on and off to a large power transformer (above photo inside ellipse) and a smaller relay for fan and turntable. The secondary of the transformer goes to a high voltage diode and oil capacitor which then feeds the magnetron. These are all connected together using 0.187in wide tab connectors.

    Disconnect the female tab connectors from the transformer and the magnetron. Unscrew the transformer  and oil capacitor from the case and remove them. This will give a lot of free space to put a small ballast in. The connectors were very tight and took a lot of force with good size needle-nose pliers to pull them off.

    Leave the lamp, turntable and fan alone. The fan primary function is to cool the magnetron and electronics; however it does circulate air through the chamber. Since the UVC lamp will generate ozone, you will get hit with an intense ozone smell when you open the door. Not only is ozone not healthy in large doses, it smells bad. The fan will help ventilate and dilute the ozone produced. This needs to be used in a well-ventilated area.


  • 3
    Installing the UV Light System

    There was a convenient and well-done schematic on top of the chamber inside the microwave. Unfortunately it doesn’t actually represent the way the microwave was wired up. I had to reverse engineer a bit of it to figure out how to wire it. If you use a different microwave, reverse engineer it to figure out how to hook this up.

    I put the light socket on the side with the electronics close to the top of the chamber. Put masking tape down first to protect the paint and the tape makes it easier to mark and drill. I used the socket as a template for the four metal screws (7/64in) and also marked the center of the socket for a larger hole to pass the wires through (3/8in). The metal is very thin and easy to drill but still recommend doing small pilot holes first and gradually increase drill size for a cleaner job. Remove the masking tape and deburr the holes, especially the larger hole where the wires will pass through. I put a coat of enamel paint on the large hole to help smooth the surface.  

    The G23 socket has a pair of push in connector holes for each contact. Since this is a two wire socket, this is a shorting pair and only one wire on each contact is required as shown in the picture. The wires are about 1.5ft long and will be trimmed once installed. The socket came with information for the type of wire (18AWG solid) and how much to strip it. I added a short length of heat shrink tubing over the individual wires and a larger heat shrink over both wires to help protect the wire passing through the hole.  Once installed, a bit of silicone goo can also help keep the case from cutting into the wires. Use four #6 ½ inch screws to hold the socket.

    The choke or magnetic ballast is a current limiting inductor for a preheated fluorescent tube.. The ballast can mount in the same area as the transformer you removed. Reuse one of the mounting holes for the transformer and drill the second for loose fit #6 machine screw. (#25 drill 0.1495in).  Use 6-32 hardware to attach the ballast to the case. Finish up by adding Loctite or lock washers on these screws so they don’t back off.

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Discussions

Deneteus wrote 07/25/2020 at 03:08 point

Make sure you are aware of what UV wavelengths and times actually work. Currently there are companies going around pushing UV-C as the next messiah.
https://iuva.org/iuva-covid-19-faq

Some medical disinfection lamps have to be really close to kill bacteria. If you searched through the testing from the FDA you will see some examples of what they do to actually test the device is working as required.

Here's an example where people used UVLEDS to kill coronavirus and they left out the number of leds.
https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/ultravioletled-maker-demonstrates-30second-coronavirus-kill

  Are you sure? yes | no

Bob Alkire wrote 07/25/2020 at 18:03 point

You bring up an important issue. There wasn't much information when I did this and it is very much a moving target. At the time, I had information that said 254nm UV would inactivate the virus. There was no mention of exposure intensity.. The testing was on similar SARS and MERS viruses and TBD on COVID-19. I've read the use of UV for disinfecting and killing bacteria and viruses has been around for about 100 years.

I saw this paper: https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontamination-reuse-respirators.html It has been the first I've seen of testing with some intensity levels.

I also read the FDA has said little about UV for disinfecting since they don't test devices. Interesting they have done some testing.

I went with a old-style UVC germicidal lamp made by a reputable manufacture. It emits UVC 254nm. A real UV tube does not use standard glass, it needs to be formulated to pass UVC. I doubted the UV LEDs would be powerful enough to actually do anything. I am not sure the one's being sold are nothing more than cheap blue LEDS. Probably would not have built this thing, if I could actually buy something that would come close to working.

I think I said this was a desperation hack as it's based on pretty slim and evolving information.  I think publishing it in hackaday was the right thing.. This provides a forum to discuss the pros and cons of this technology..

  Are you sure? yes | no

Deneteus wrote 07/25/2020 at 19:21 point

I have been using the crystal AC UV lamps to erase chips for several years now so I had already done alot of research into the Crystal UV lamps. The crystal tubes put off more UV-C than led by far and they also generate alot of wasted heat which is why people wanted LED's instead.

I am also a member of the COVID19 Opensource Medical Supplies FB group so back in late March and February we had a few people that were interested in using UV making posts as well some guys posting about it in the FB groups for LED and UV.  In the group there are doctors and engineers as well as Covid-19 working group and frontline workers so they have been vetting any ideas that come through that may be useful as well as providing info that isn't common knowledge in the industry. You might want to join the group as there is alot more testing and feedback going on. 

The FCC and the FDA have done testing on a couple companies UV devices. It was a Korean company that had applied for a patent on a UVC room cleaning solution. If you look around there are data sheets that cover what hardware was used for testing and you can see pictures of the equipment they used for testing. Their testing setups are written in detail in those documents. People rarely check the patents to see what has already been done and there is alot of information in there.

  Are you sure? yes | no

Bob Alkire wrote 07/21/2020 at 00:58 point

Not really.

The door has two pieces of glass of unknown origin with a metal RF screen in between. When I looked into it, some glass will pass UVC (such as what is used for the glass UVC tube, quartz for example will pass UVC.) Other glass formulations block it it. For example, green glass or laminated glass were shown to block it. see  "The role of glass as a barrier against the transmission of ultraviolet", by Ida Duarte, Anita Rotter, Andrey Malvestiti & Mariana Silva. Not to detailed as to what materials actually do block it nor why some block it and others don't.

What I did was to measure it.

We made an instrument that measures UVA, UVB and UVC light. While the instrument inside the chamber had saturated the UVC intensity, I had negligible to nil readings outside the microwave next to the glass. So I would say, experimentally, it didn't pass UVC.

If I didn't have that instrument, I would err on caution and cover it with aluminum foil, acrylic plastic which blocks UV and/or paint it.

I thought of dayglo UV reactive paint but that is only reactive to UVA. It didn't react to UVC.

  Are you sure? yes | no

Ewald wrote 07/25/2020 at 07:49 point

is there a description of your UV measurement device somewhere?

  Are you sure? yes | no

Bob Alkire wrote 07/25/2020 at 18:14 point

Not yet. It had three sensors for UVC, UVB and UVA (Genicom). These went to a transimpedance amplifier and filter. We used a ATSAMD10 to read the sensors and drive an LED bargraph. The bar graph was quick and cheap but very lacking in measuring the intensity,. So we are working on v2 with a small transflective LCD display. We are also working on a calibration and unit test. I'd like to put this on Hackaday and possibly Tindie when we are done.

  Are you sure? yes | no

Dan Maloney wrote 07/20/2020 at 20:23 point

I kind of like this idea! Did you do anything special to make the window on the front door filter out the UV-C?

  Are you sure? yes | no

Bob Alkire wrote 07/25/2020 at 18:37 point

I did nothing special. I covered it initially just to be on the safe side. Since we had a way of testing for UVC, Inside the chamber, it saturated my meter. I had made it too sensitive. Outside the window with the unit on, I  measured nil to negligible on the outside.   Not too surprising, Wikipedia (Ultraviolet) states, "Common soda–lime glass such as window glass is partially transparent to UVA but is opaque to shorter wavelengths, passing about 90% of the light above 350 nm, but blocking over 90% of the light below 300 nm."

  Are you sure? yes | no

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