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Low powering magnetron

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LAN_403

radiotech

Tue Jul 27 2010, 08:53PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Was the wireless down due to a buzzing sound across all frequency bands? The tube might have been arking. Earth the metal case of the magnetron, and probe the output end with a neon light on the end of a wooden stick to see if microwaves may be present. use a NE2 bulb

cedric

Wed Jul 28 2010, 08:21AM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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Posts: 143

no,no arking I am sure ,but not clearly enough power to switch any neon light ,with an efficiency of 50/100 the set up may have provide may be 50 W of hf may be less ,but the network was down until about 10 meter away,no effect on phone,just as bit of patterned snow on tv at very close range...
the magnetron was not enclose...

Proud Mary

Wed Jul 28 2010, 05:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Without a strong magnetic field to force the electrons into orbit in front of the tuned cavities - which then resonate just as a bottle gives forth a note when we blow across it - then the magnetron is a magnetron no more, but just a thermionic diode valve.

Arcing inside a magnetron could occur with significant over-voltage, or where the magnetron had gone 'soft'.

I suppose it is possible that a magnetron which is not resonating for some reason, but is simply acting as a diode, could cause RFI by aggressive square wave switching rich in harmonics, but I don't recall hearing of such a case.

An unscreened magnetron could well cause wide spread local interference by direct demodulation, induction of voltage nodes where they ought not to be, and so on.

As I always advise people with X-rays, unless you have suitable means of detecting and measuring the radiation, and confining it within a robust perimeter so that it goes only where you want it to go, and nowhere else at all, then you would be better finding some other way of passing the time.

Conundrum

Wed Jul 28 2010, 05:45PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Same idea as laser diodes- in this case there is a minimum current below which there is no oscillation.

Interestingly removing the magnets to get a bare tube would render it a little safer, as the electron paths would no longer be within the cavities and it would behave as a diode.

-A

Thu Jul 29 2010, 12:08AM

Registered Member #2372 Joined:
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Posts: 62

If the voltage is not correct the magnetron wont oscillate. The electrons spinning around in it have to be going the same speed as the wave in the cavities, if it isnt there will not be any energy transfer from the electrons to the wave. If the magnetron is run at the wrong voltage it could be producing RF in a mode that isnt efficiently extracted and it would draw current without putting out any microwaves. These devices have to run at a certain voltage for a given magnetic field for a given slow wave structure to operate properly, you cant just adjust it to whatever you want and expect to get out a different power or frequency.

Conundrum

Sat Nov 09 2013, 09:30AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Re. X-rays from a magnetron I suppose this might work if you took all the magnets and heatsink off and ran it in cold cathode mode.
It might actually be a little bit safer than overpowering a diode valve such as the 5642 (done that) as the X-rays would only emerge at a very narrow range of angles due to all that copper shielding.

Ash Small

Mon Nov 11 2013, 04:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve's answer above is the closest. I have a post on 'cyclotrons.net' which outlines how to do this:

You basically just reduce the value of the capacitor in the doubler circuit. Remove the 1uF capacitor and replace it with a 0.01 to 0.02uF capacitor. This will reduce the power from a ~1kW magnetron to ~10 to 20W. Some have also suggested using a variac as peak voltage may be higher with less current drawn from the microwave oven transformer (not sure about this, but it sounds like a good idea).

I am working on another method for inverter microwaves, using a vacuum tube to control the current to the magnetron, but there are a lot of other 'issues' to consider once you realise you can't use the standard Panasonic control IC. (Ferro-resonant mains isolation transformers to name but one requirement, if you don't want your IGBT to blow)

Conundrum

Fri Nov 15 2013, 07:15AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

I actually need to do this because my MW is too powerful even at 10%.

For my application being able to maintain a constant temperature would be handy, but reducing the capacitance sounds like it could work in combination with a variac on the primary.

Also am I right in saying that the MW electronics should still work even though the magnetron is being run at low power?

Does anyone have any suggestions for overriding that pesky 1 hour timer limitation? 16 hours is about what it needs for Y123.

Thanks!
-A

Ash Small

Sun Nov 24 2013, 10:36AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Conundrum wrote ...

Also am I right in saying that the MW electronics should still work even though the magnetron is being run at low power?

Does anyone have any suggestions for overriding that pesky 1 hour timer limitation? 16 hours is about what it needs for Y123.

Thanks!
-A

Not sure I understand this bit. just change the capacitor, add the variac, and 'bypass' the electronics (well, maybe not the door interlock,etc).

Transformer>doubler>magnetron.

(Inverter microwaves won't work without 'extensive' modification)

Proud Mary

Sun Nov 24 2013, 11:38AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What's wrong with using an ordinary waveguide attenuator?

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