- Neil is now setup to go, need better magnetron power supply
My trip to West Kelowna was a success, I survived the 24 hours of driving (there and back) and we have a better idea of the challenge ahead.
The first thing we did was set everything up including the water cooling and magnetron and ended up with the following
On the left is the water pump and radiator, then the magnetron connected to the circulator which was connected to a section with tuning pegs finally ending in a short (metal bolted to the end). In the foreground you can see a multimeter and a high voltage probe we used to sample the magnetron power supply. We also used a microwave detector to test for stray radiation and the thermal imager to make sure the magnetron didn’t overheat.
We then hooked up the variable step attenuator, programmed it for 70dB and connected the Signalhound to the isolation port on the circulator. After getting a few sparks when the high voltage for the magnetron arced through the wire shielding, we recorded a few results.
It was soon apparent that the magnetron was jumping frequency too much to even show up on the Signalhound which takes measurements every 150mseconds. (As a reference, the HP 8753D analyzer at the university samples at 50msec, and even there it was jumping around way too much).
We then hooked up a high voltage probe to record what the magnetron input voltage looked like and I was surprised to find this:
From Neil’s blog he writes:
- The peak to peak AC voltage is about 4.4kV and is shown AC coupled. The current waveform is DC coupled and is changing directions which makes sense in an AC powered system. The peak current draw is about 12A with a RMS current of about 8A. These numbers don’t make sense though because the RMS power consumed using these numbers would be 16kW (2kV * 8A) which is quite a bit more than a 110V plug can supply. I will need to try and repeat these measurements once I go over the test setup.
I was under the incorrect assumption that domestic magnetron power supplies used some type of rectification in order to create a 4.2Kv DC cathode voltage, but apparently you can run the magnetron off AC as well. We are guessing that the reason the magnetron output jumps frequencies, is because of AC input. I plan to send Neil another newer microwave oven that uses a different kind of power supply to find out if it too drives the magnetron with AC, or a DC voltage and if it makes any difference in the magnetron output.
The conclusion of our trip is that we need to test more magnetrons and magnetron power supplies in order to find a setup that has a much steadier output. Once we have a steady output that can be measured by the Signalhound, we can then use a feedback loop to center it on the cavities resonant frequency.
Along that line, I found a paper that mentions the magnetron theory of operation which had some interesting points:
- “For most magnetrons the temperature coefficient is negative (frequency decreases as temperature increases) and is essentially constant over the operating range of the magnetron.”
- “The “automatic” synchronism between the electron spoke patter and the r.f. field polarity in a crossed field device allows a magnetron to maintain relatively stable operation over a wide range of applied input parameters. For example, a magnetron designed for an output power of 200kw peak will operate quite well at 100kw peak output by simply reducing the modulator drive level.”
- “The pushing figure of a magnetron is defined as the change in magnetron frequency due to a change in the peak cathode current.”
Here are my takeaways from the paper:
- Frequency control may be easier by controlling the magnetron cathode current with a constant temperature then controlling the magnets
Or, it may be a combination between controlling the cathode current and magnets that gives the best result.
Here is my todo list:
- Send Neil the other microwave to see what waveform that microwave uses to power the magnetron (TODO)
- Send an email to John Gerling to see if we can get the specs for the ASTeX magnetron head because his company repairs them. and John responded with “Sorry, can’t help you any further”. It turns out that John was one of the original designers for the ASTeX magnetron and Gerling spun out the ASTex company which means he considers it IP. Darn! Have to find another source…
- Get schematics and/or repair manual for the MDX-10 power supply in case we can use any of the parts. (TODO)