battery powered magnetron voltage regulation (using PWM?) - Page 1
Forum section: electronics / projects

author: pyotrgiraffe
author_rank: Newbie
timestamp: June 21, 2015, 08:23:35 am
content: i intend to power a magnetron with 200 12 volt lead acid batteries in series (for now i can't say why)the voltage of the batteries is to high for the majority of the operation of the magnetron and thus i think PWM is the best way to regulate the voltage as it causes little efficiency dropthis diagram shows how i'd like to do it (the resistor in the diagram is the magnetron) but i can't find a transistor that can handle thati can only use the capacitor that came with the magnetron which is 1 microfarad rated to 2500 volts thus i need at least 60 hertz which rules out relays as an optionthusly i need either a suitable transistor or a different way to limit the voltagealso i'm sort of a noob as far as electrical engineering and the sort so any pro tips are much appreciatedalso also in case you were wondering about the 200 lead acid batteries are 5 Ah each (12V*5Ah*200=12kWh and 12kWh/700W~17.143h)

author: iampoor
author_rank: Frequent Contributor
timestamp: June 21, 2015, 08:30:43 am
content: If your a "n00b" should you really be dealing with such lethal voltage potentials!?!?

author: SeanB
author_rank: Super Contributor
timestamp: June 21, 2015, 08:42:44 am
content: You probably will be much better off taking those 200 batteries and making a battery bank of a series parallel combination, with 25 cells per string, 8 strings in parallel, to give you a 300VDC rail. Then you can use a standard off the shelf inverter microwave control board, often available for free in a scrapped unit, or you can buy a cheap inverter microwave for the parts or just the board as a service spare part. That way there is no need for working with high power control, as it is all in the board already, you just have to connect the DC to the AC input and drive the control signals from the microcontroller. It will have the HT inverter, filament drive and protection circuitry all built in.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2015, 09:56:35 am
content: Quote from: pyotrgiraffe on June 21, 2015, 08:23:35 amMagnetrons are deeply unfriendly to novices. Worst case the microwaves can blind you within a few seconds, and even moderate exposure can cause nerve damage. A normal microwave oven in good working order has an EHT supply with enough current capability to be lethal. Finally some magnetrons contain Beryllium oxide based ceramics, which if damaged are likely to produce extremely dangerous dust.Even connecting 200 SLA batteries in series is dangerous enough in itself. The cases certainly aren't rated to withstand 2.7KV, nor are any connectors compatible with the terminals, and there is enough energy there to not only kill you but to make you a crispy critter requiring a closed coffin funeral.If you don't tell us why you want to run a magnetron off a 2.7KV battery string we cant suggest ways of getting the desired effect with a lower risk of killing or injuring yourself and/or any bystanders.

author: amyk
author_rank: Super Contributor
timestamp: June 21, 2015, 12:39:05 pm
content: Quote from: SeanB on June 21, 2015, 08:42:44 amThis. All the electronics you need can be found in a standard microwave oven.Unless you're powering avery largemagnetron...

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2015, 12:53:35 pm
content: "12kWh/700W~17.143h"A 700W load certainly isn't a very large magnetron. That's at the low end of the domestic microwave oven power range.

author: cs.dk
author_rank: Unknown
timestamp: June 21, 2015, 06:10:42 pm
content: Wow... 200 12V batteries in series - Are you serious?It will give a nice firework if you drop a spanner on the terminals though

author: Fraser
author_rank: Super Contributor
timestamp: June 21, 2015, 06:39:22 pm
content: Charging such a battery chain would prove interestingMaybe a typo and it should be 20 batteries ?A powerful inverter would provide both the correct supply rails and regulation to the required standard. Daisy chaining so many 12V batteries does sound most unusual and potentially poor tradecraft.I used to work with submarine batteries used in building UPS systems. They were scary enough and needed to be treated with great respect. A lot of stored energy to have in one location.Aurora

author: pyotrgiraffe
author_rank: Newbie
timestamp: June 21, 2015, 06:44:14 pm
content: Quote from: iampoor on June 21, 2015, 08:30:43 ami'm not a noob to safety or high voltage i just don't know my electrical engineeringQuote from: SeanB on June 21, 2015, 08:42:44 ambetween the required inverter and the protection circuitry i'd like to only use this as a last resortmy main goal is to directly power the magnetron without using the transformerQuote from: Ian.M on June 21, 2015, 09:56:35 ami'm well aware of the risks of microwave exposure and the beryllium ceramici will take proper safety precautions such as wearing metal mesh googles and putting lots of mesh between myself and the magnetronall i need to know is how to get the circuitry workingQuote from: amyk on June 21, 2015, 12:39:05 pmthat's sort of the ideaif i got a transistor rated to 3000v and 1a would that work if i used it exactly as in the diagram (provided the transistor has lots of cooling)

author: SuzyC
author_rank: Frequent Contributor
timestamp: June 21, 2015, 11:53:12 pm
content: I read about how the KGB would locate themselves near in an apartment or in a truck across from a place(i.e. embassy building) of someone and then with a focusing microwave dish, would with waveguides tie the output of several magnetrons together( from microwave ovens). Powered by lead-acid batteries, they did create a high power 'invisible ray" that could be focused onto the target nearby person/persons to cause confusion, harm, blindness or even death. In a war, there can be also a war of satellite v. satellite, and one country could focus high power microwaves to destroy land-based microwave receivers or else the military and civilian satellites of their enemy.

author: Ian.M
author_rank: Super Contributor
timestamp: June 22, 2015, 01:43:25 am
content: Its not nearly as simple as you think it is and that circuit is useless.Firstly the anode of most magnetrons has to be grounded, as it is usually integral with the wall of the vacuum chamber, so you need a large negative voltage on the cathode + a floating filament supply with 3KV isolation.Secondly, simply pulsing the magnetron with a higher voltage than it is designed for is likely to result in flashover and destruction of your high voltage transistor, so you will need a full buck converter circuit, with a large high voltage choke and a fast high current high voltage catch diode. You may be able to use the existing capacitor, but it may not have a high enough ripple current rating.Thirdly, you then have the problem of providing high current base drive to your transistor, which has its emitter sitting at -2.7KV with respect to the rest of your control systemFourthly, unless you enclose the whole assembly in a fine welded mesh Faraday cage, with all seams in full electrical contact along their whole length, significant levels of microwave leakage are likely to occur, and unless your mesh goggles are part of a full Faraday suit designed for the magnetron's operating frequency band, they may actually by chance act as reflective concentrators increasing the risk of tissue damage.

author: johansen
author_rank: Super Contributor
timestamp: June 22, 2015, 01:54:14 am
content: most household magnetrons run at about 4 kv, not 2.the output of the transformer is 2250 volts no load, with a .7 to 1.1 uf capacitor wired up as a voltage doubler.the capacitor is sized so that the power factor at the outlet is relatively high.to get the full rated power, you will need a dc supply of something in the range of 3.5 kv.the magnetron is run at a higher voltage than that, but in the range of like 50% duty cycle. so you have to drop the voltage just a bit with a smooth dc supply.as someone else mentioned the filament needs its own supply.car batteries can handle 4kv, they usually have at least 2 mm of polyethene for their case. but they wouldn't be "rated" for 4 kv.but you would need a lot of specialized equipment to switch the batteries in and out of their circuit.you could charge them from a household microwave oven transformer and capacitor and rectifier.. but that wouldn't be very efficient.

author: pyotrgiraffe
author_rank: Newbie
timestamp: June 22, 2015, 05:41:09 am
content: Quote from: Ian.M on June 22, 2015, 01:43:25 ami make a simple schematic that emphasizes the power components while ignoring the little stuff because the power components are what i'm concerned abouti'm confident i can put the proper voltages across the transistor gate and the filament heateri do intend to use a welded fine mesh cage to contain the microwaves and the googles will be tested before useQuote from: johansen on June 22, 2015, 01:54:14 ami completely forgot to look that up and was just basing my voltage estimate on what the capacitor is rated tothis actually simplifies the control thoughthe battery bank voltage will range from 2600-2000 volts during operationthus if i charge the capacitor to 1400-2000 volts correspondingly i should be able to get 4000 voltsalso would the design in the diagram work to use two lower voltage transistors (say 2 1500 volt transistors)edit: i just noticed that the battery is backwards