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Multi-tap MOT HV Source

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Firefox

Fri Sept 19 2008, 03:10AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I just got my hands on a nice 1kW oven that was mostly working, but had some problems with even cooking of the food inside. It liked to burn spots of food and leave others cold (especially evident with popcorn), so I am assuming (not sure if I am correct) that there was a problem with the magnetron. That said, I would like to try my hand at turning the microwave into a high voltage PSU, since all of the major wiring has been done for me, courtesy of Sharp.

That said, I'd like some input as to what I am planning to do, especially on the issues I've run into in the planning of it. I plan to hook the MOT up to a 6 stage, bipolar, full wave CW multiplier, and bring out taps for 2kVAC, and +- 6, 12, and 18kVDC.
The first thing I was planning to do was punch out the shunts and wind a few more turns of primary onto the core to bring the transformer away from saturation, which brings down the current draw and wasted power if I understand correctly. I have some 10kV 1200mA x-ray transformer diodes to use in the multiplier, and I plan on getting some MOCs in the future as well.

I plan on inductively balasting the transformer to 9A input so the short circuit current in the AC output will be less than 500mA.

My big problem is current limiting on the outputs of the multiplier for things like arc-drawing. According to the power calculation RI^2, I'm going to need 60kohms of resistance for every 6000 volts of output above or below, totaling out to 500W on each output. Is this whole thing an bad idea, or is there something I'm overlooking?

My other problem is understanding how the MO works. The case is grounded and the transformer is connected to the case, so does that mean that there is only halfwave output at 3kV peak from the transformer? If so, would it be possible to isolate the transformer from the case to get full wave output? Or can I connect one side of the multiplier to the case as one terminal?

Myke

Fri Sept 19 2008, 04:48AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Firefox wrote ...

My other problem is understanding how the MO works. The case is grounded and the transformer is connected to the case, so does that mean that there is only halfwave output at 3kV peak from the transformer? If so, would it be possible to isolate the transformer from the case to get full wave output? Or can I connect one side of the multiplier to the case as one terminal?

I think that full wave rectification is still possible even though one side is grounded. I guess you could think of the ground as a HV terminal. You can disconnect the HV winding from the core but still be sure to ground the core so that it doesn't float at a high voltage.

Firefox

Fri Sept 19 2008, 05:12AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

The problem is, I would like to ground the center of the CW stack to make sure that it is at ground, and I don't think I can do that if I get a -3kV peak every half cycle. However, my circuit design experience beyond simple LCR circuits is a bit shaky at best, so if someone could say something I would appriciate it alot.

Dr. Dark Current

Fri Sept 19 2008, 08:32AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The output from the transformer is "fullwave", 2kV AC, peaks ~3 and -3kV. Voltage after capacitor is halfwave doubled, peaks 0V and -6kV, average -3kV.

If you design the caps in your multiplier so that they just limit the current from the transformer to 500mA, you don't need any other ballast.
The problem is that if you feed a multiplier with e.g. 1kVA, the output will never be 1kW but much less (depending on stages, I can imagine losing >90% of the power in reactance with maybe 10 stage multiplier, but I may be off here...)

Firefox

Fri Sept 19 2008, 05:21PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I know that the multiplier will never put out the same amount of power that is put in, but I hope to get about 250W from the outputs of the +3rd and -3rd stage, for +-18kV. That would mean about 7mA of current, with a 5Meg limiter resistor.

Edit- New problems:

-I removed and discarded the magnetron (sans magnets) today, and removed the housing for the magnetron to open up the hole for cable feed through, which I plan to cover with a 6mm piece of acrylic with holes for the cables. On top of the magnetron housing was a small sensor attached to the mains input after the filter, labled 250V 15A 150C. After that is a strange round sensor that was attached to the top of the cooking cavity. Is it safe to leave these components out of the circuit and just cut and soldier the input cable together where these components were, or should I find a new home for both of them?

-Case metal is sharp. Wearing gloves from now on.

-I was going to knock the shunts and remove the heater winding so I can get a few more primary turns on the core, but before I do, what are the effects besides a slightly lower output voltage? Were my inital ideas about the effects correct?

-I was thinking about using MOCs for the multiplier for their high capacity, to achieve a higher power output. Is this a good idea, or are there better capacitors out there for this use? Also, my x-ray diodes are smaller than the microwave's diode, yet are rated for more than double the current at about the same voltage. Are they higher quality or is there somthing more to it?

Myke

Fri Sept 19 2008, 11:25PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

As far as I know, the strange round sensor is a humidity sensor. The thing that was rated for 250V 15A 150C is a thermal switch/fuse.

Firefox

Fri Sept 19 2008, 11:33PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

In that case I'll remove the humidity sensor, since I won't be boiling water in the cavity again. I'll find a new home for the thermal fuse. Can't hurt to have the extra safety measures, just in case. At least the thermal fuse will trip if I start cooking my transformer.

Edit:
Just got the microwave rewired, and put in the transformer for a quick test. Into a direct short to the case I was able to draw out the arc to about 4" before the 3 second timer I set was up. It was a nice hot yellow blue arc and had that distinctive 60Hz hum too it. Yee haw!

The lights on the circuit also dimmed a bit.

Double Edit:
Whoops, blew the fuse. I guess I need a balast on the 2kVAC output. Heh. Any oppinions on replacing the 20A 250V fuse with this 16A 240V fast trip breaker? I sure hope the final product doesn't need 1900VA (120V 16A) to work correctly.

Firefox

Tue Sept 23 2008, 03:19AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I just finished simming out the mutiplier in LTSpice, and it seems like continuous current draw will be an acceptable .4-.5A secondary side. Thats with a 5 megaohm load connecting the top and bottom stages. However, when powering up, the secondary side will be stressed to nearly two amps if the input was an ideal 3000Vpk 60Hz sinewave. Since shorting out the transformer can't draw as enough amperage to give 2A secondary side, I won't worry about it.

I'm also thinking about the fullwave multiplier and switching to a halfwave multiplier, as the negative side of the transformer needs to be grounded anyway to make the simulation work correctly. The trace also shows that at 7mA load I should be able to get a touch more than 30kV from the multiplier. From a 1kVA source, I'm happy with the 210VA output. Certainly a higher voltage than an NST

.

Now I just need to get some reasonably sized resistors to limit the current without soldiering a chain of 500 10kohm 1W resistors together.

Dr. Dark Current

Tue Sept 23 2008, 06:05AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Firefox wrote ...

-I was going to knock the shunts and remove the heater winding so I can get a few more primary turns on the core, but before I do, what are the effects besides a slightly lower output voltage? Were my inital ideas about the effects correct?

If you get out the shunts, the output voltage will actually increase, as well as the no-load input current.
Then you can add turns so you get slightly lower output than the original one, and lower no-load input too.

Firefox

Thu Oct 30 2008, 07:00PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Ok, I'm finally moving on this project again, and I have been doing a little LTSpice simulation to figure out the best configuration for a limited number of MOCs (20). I was originally going to use a full wave multiplier, but my simulations showed that a half-wave multiplier could get better power output, roughly 750VA vs the full-wave multiplier's 350VA. This was using a 400k limiting resistor, which is subject to change depending on what I end up with. LTSpice sim - Full Wave vs. Half Wave Multiplier

Also, I decided to see what adding a large inductor to the MOT output would do, because my model for the MOT uses coupling coefficient of 1. Interestingly, a 5-9 henry inductor boosted the output voltage from 18kV to 24kV, and the power output to nearly 1.5kVA. LTSpice sim - Inductor vs. No Inductor

Also, I would love critiques on how to make these models more accurate. My ability to use LTSpice is limited, and obviously my transformer model is far from ideal.

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