Magnetron

[pinank]

Guys what r the best ways to control the output of a magnetron? Output would be the energy received as microwaves.

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[thejamesclan]

variac ??

[pinank]

variac ??

u mean varying the voltage applied to the magnetron?

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[mathias]

What exactly are you going to do with the microwaves coming out of this magnetron?

Perhaps you can regulate the output power as they do with X-ray tubes: vary the heater current to control the electron emission and thus the power.

[hedgewallace]

I'm not sure whether you would be able to change/regulate the output of a magnetron.
A microwave controls the output level from a magnetron by adjusting its it on and off times.
High will have the magnetron going all the time.
Medium might have the magnetron going for 10 sec and off for 5 sec.
Low might have the magnetron going for 5 sec and off for 10 sec.

[SeanB]

You can vary the power over a small range by controlling current, either by dropping the voltage or by varying heater current. The problem is you will have a very low range of control, from full power to a lower level before the magnetron stops oscillating. The newer inverter style ones AFAIK vary power by lowering input power by dropping voltage, and get the lower power positions by doing duty cycle variation.

[Fraser]

Suggest you do some reading on Radar systems as that may assist you. The Decca Radars that I worked on had a Magnetron rated at only around 600W CW yet they were operating at 10kW peak. The Pulse Repetition Frequency (PRF) permitted such use. The average power then remained within safe operating limits. If you are looking to modulate a magnetron as part of a transmitter system, that would need a different approach.

It would help to understand what you are intending to do with the magnetron.

Aurora

[Stray Electron]

Magnetrons are usually powered by capacitor bank charged to a fixed voltage and the circuit is completed by firing a hydrogen gas Thyratron. The easiest way to control the output power by changing the capacitance of the capacitor bank or changing the charge voltage.

[NiHaoMike]

Assuming you're trying to get better control for a microwave oven, power the filament from a source independent of the HV supply and do phase angle control of the HV supply.

[cellularmitosis]

Standard safety nag about playing with magnetrons.  FYI the back of your eyeballs have no pain nerves, so in theory you'd be blind before you even knew you were in trouble if you fired that thing up while pointing it at your face.

[max666]

Standard safety nag about playing with magnetrons.  FYI the back of your eyeballs have no pain nerves, so in theory you'd be blind before you even knew you were in trouble if you fired that thing up while pointing it at your face.

I think the absorption length, for 2,4 GHz microwaves, is in the order of the length of the eyeball, so most of the energy should be absorbed right throughout the eyeball.
Hey but maybe after the proteins in your cornea and lens turn cloudy, perhaps we could even get an exploding eyeball ... wouldn't that be cool!   

[pinank]

My project is about heating a liquid. The only problem being that the incoming liquid would be of different temperature like once it may be around 10°c and the next time it may be 30°c. So I need to design a control system which would heat up the liquid up to 50°c. So I want to control the output of the magnetron such that as soon as the liquid heats up it stops heating..

Also I don't want to get blind while doing this. So peoples please suggest me protection measures I need to take. Thank you


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[VK5RC]

Pulse width control seems like the way to go,  a Faraday cage is needed for safety,  easiest is to use an old microwave oven!

[PA0PBZ]

My project is about heating a liquid. The only problem being that the incoming liquid would be of different temperature like once it may be around 10°c and the next time it may be 30°c. So I need to design a control system which would heat up the liquid up to 50°c. So I want to control the output of the magnetron such that as soon as the liquid heats up it stops heating..

I'm probably missing something here but it looks as simple as stop heating when the liquid is 50°C?
The only control you need for that is on and off.

[Fraser]

I am going to sound mean here but this appears to be a classic case of over engineering a solution to a simple problem. I may be wrong and please feel free to correct me, but the use of a sophisticated heating tool such as a Magnetron water heater seems very over the top when for years manufacturers have been providing exactly what you are aiming to achieve using simple high efficiency micro boilers.....you have likely used one many times..... the humble electric shower    Cold water comes in at various temperatures depending upon the time of year (in UK anyway), it passes through a safety pressure monitor to ensure that the supply pressure is adequate, it then passes into a micro-boiler that can be rated at very high wattage provided over temperature and water flow monitoring is in place. 10kW heaters are common in such devices ! Temperature is controlled either by water flow through the boiler or/and thermostatic temperature control of the boiler. It would be relatively simple to design a closed loop temperature control for the heating element drive.

You may have very good reasons for wanting to use a Magnetron for heating water but please consider the KISS principle here. Magnetrons are nasty bits of kit if not handled correctly. The HT supply WILL kill you if you get across the 10kV capacitor and the RF output WILL blind you if you are exposed to its full RF energy for any length of time. I love Magnetrons as I worked on them in RADAR systems and they offer a great way to heat food in a containment situation BUT as an alternative to a nice safe and simple water heating element....nah doesn't sound a great plan .... sorry.  To use Magnetrons you need to understand high Voltage safety, RF theory of Microwave energy production and RF theory of microwave transmission combined with low SWR containment. If you don't have experience with the use of microwave energy, at best you will destroy the Magnetron very quickly indeed, and at worst you will be rendered permanently blind with fried Retinas. If this is a commercial idea, it dies the moment you consider parts and maintenance costs versus a conventional boiler. The Chinese are even selling tankless water heaters for showers !

http://www.cn-haiot.com/products/7-000W-Made-In-China-Tankless-Electric-Shower-Heater-Siver-Black-1303699.html

Correction...the term 'tankless' is misleading (it still contains a micro-boiler)....it just means that it is an 'on-demand' electric water heater and not a stored hot water tank system. Language differences !

just my 5 Cents and I know its not what you want to hear but sometimes you need to step back and ask yourself whether you are doing something logical and in its simplest form, or just over-engineering the solution for the fun and challenge. The latter case can make little sense in terms of the required result.

[Fraser]

Looks like someone already had a similar idea to you 

http://www.faraday.ru/mtgsum.pdf

http://www.faraday.ru/magnetron1.pdf

Looks like its development was going to be expensive !


And more !

http://www.gpcservices.com/bouarfa_mahi/category/microwave-water-heater

http://www.faqs.org/patents/app/20080197127

http://www.google.co.uk/patents/US6064047

http://www.steriwave.com/Steriwave_Web_12-08-12/News/Steriwave%20Plc%20Microwave%20Hot%20Water%20Heater.pdf


Interesting that I have yet to see this so called super efficient technology in the marketplace 

Heating of water using a standard heating element is actually pretty efficient (around 95 % IIRC), so how a high loss item like a Magnetron and its associated driver can compete with that I am not sure.


This page provides plenty of comment on such designs.... as one person states, Magnetrons can heat small volumes of water quickly, but are very slow to heat large volumes of water. They are also only around 65% efficient, and so far inferior to direct water heating using a resistive heating element.

http://physics.stackexchange.com/questions/22158/microwaves-vs-gas-or-electric-coil-heating-of-a-water-boiler-in-a-typical-househ

[max666]

Pinank, you didn't fully explain why you are choosing a magnetron, for heating the liquid, so I’m not going to bash you for your choice.
There certainly are situations where a magnetron could be a good choice, like for example if you must not get the liquid in contact with anything other than it's container (thinning about contamination for example). Or have the ability to heat something quickly throughout without stirring it.
But in your case you need to measure the temperature, so I don't know how you're going to that. Maybe some contact heating approach would work too and be a lot simpler?
Also you just said you have an "incoming liquid", how exactly is the liquid coming in? Pipe? Canister? Squirting across the room?

[DmitryL]

Also I don't want to get blind while doing this. So peoples please suggest me protection measures I need to take. Thank you

The best protective measure for this level of dealing with magnetrons: Don't do it.

If you fry your regular balls, it is acceptable - you'll get your Darwin award and everythig will be fine.
If you'll have your eyeballs boiled, you will become blind, which is a burden to a society.

BTW, I read that 2.4GHz rediation may quite happily reflect from metallic surfaces, so that you don't necessary need to look directlyu into the waveguide to blind yourself

[pinank]

Hey people.. I choose magnetron because the liquid flowing through the tube will react with microwave radiations and hence heat up. So I well have to use a temperature control close loop system for it.

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[pinank]

Also, I need to control the energy supplied to the liquid by microwave as the liquid would heat up and evaporate..

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[max666]

Ok, assuming there isn't a language barrier here; those aren't good enough reasons for me to use a magnetron.

I concur with Aurora, I would use or hack or build one of these:


It's more efficient, faster in heating the liquid, easier to be controlled with a PID controller, allows more precise control over the temperature, a lot less complicated and you avoid having to deal with safety concerns about microwaves.

[NiHaoMike]

Exactly what sort of liquid are you working with? Resistive heating is nice and simple, but maybe it's not workable for some reason.

[gardner]

If it *had* to be microwaves, I think I would hack a standard microwave oven to run a fluid line in and out, and just have a coil of tygon or silicone tubing in the main compartment.

[mhzghz]

You can control the frequency (somewhat) and output power from a standard 2.4 GHz microwave oven magnetron by controlling the current.  You can use old horizontal sweep tubes (6JE6/6LQ6/6MJ6/etc.) in a constant-current bypass configuration.  You need to turn the original microwave "pulsed" HV power supply into a stable 4 kV power supply though.

Here is a good article from 73 magazine:

http://blockyourid.com/~gbpprorg/mil/ads/ATV_Transmitter_from_a_Microwave_Oven.pdf

[pinank]

You can control the frequency (somewhat) and output power from a standard 2.4 GHz microwave oven magnetron by controlling the current.  You can use old horizontal sweep tubes (6JE6/6LQ6/6MJ6/etc.) in a constant-current bypass configuration.  You need to turn the original microwave "pulsed" HV power supply into a stable 4 kV power supply though.

Here is a good article from 73 magazine:

http://blockyourid.com/~gbpprorg/mil/ads/ATV_Transmitter_from_a_Microwave_Oven.pdf

The link doesn't open...


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