MOT wire hookup

[guest]

My friend and i are each building demo fusors. we took apart an old microwave, for the MOT. we were trying to figure out how to heeok it up i.e. which wires in the output are combined for a flow of current. we tried to figure this out by plugging it in, and checking for an arch, but non was present. we think that it may have something to do with the ground. i have provided a picture of the 3 output wires, labeled A, B, and C. any help provided on this would be much obliged.
thankyou,
John Poleto

[Adam Szendrey]

Hi there,

A and B are for heating the magnetron filament , a low voltage output.
Wire C is the HV connection (2 kV RMS), the other end of the secondary is connected to the iron core which is grounded, meaning that you'll have the high potential between the core and wire C. Be careful with MOTs they can provide a current of 500 mA which at 2 kV can be quite lethal.

Adam

[apollo]

Does this mean that if I were to touch the case (John and I are using identical power supplies and cases) which is metal, but the case is grounded to the third prong of the outlet, would I get shocked (I would be grounded too). Also, can I still use the positive as ground, or are there too many electrical things on ground? I need HV positive to go to the base of the bell jar, but this is already on ground (I'm pretty sure I can but just making sure).
Thanks,
Liam Bowen

[Adam Szendrey]

Liam,

Let's clear things up a bit. IF the core of the MOT is grounded you will not get shocked if you touch it. Anything that is grounded is NOT a shock hazard, since current will choose the easier way (smaller resistance), and that is the ground connection not you (in most cases).
A MOT has an AC output by itself. It needs a rectifier to give you DC, which in it's simplest form is a single HV diode, connected in the proper direction to give you either a positive or a negative supply.
It is wise to take out the diode and capacitor of the oven with the MOT when disassembling it so you have a rectified supply (it's a negative supply if i'm correct).
The outer grid of the Fusor is grounded (in most cases), and the inner grid is connected to the negative of the rectifier, that's how the grounded outer grid "becomes positive" (relative to the negative inner grid).

Adam

[Carl Willis]

Hi Liam, John,

Experimenting with the microwave-oven power circuit is recommended only for people well-experienced in electricity. Everyone is encouraged to honestly judge their level of preparation before diving into a circuit like this, keeping in mind that an MOT will stop your heart with ease. So Liam, John...just don't hurry into this project and try to make arcs until you're 100% sure what's going on. Sorry if I sound condescending, but the safety reminder should preface any instructions on how to hook that tranny up.

The MOT has three windings: the primary (at the bottom in the photo); the filament secondary (a few turns of heavy wire with leads marked A, B), and the high-voltage secondary (outer turn connected to lead C, inner turn riveted to the transformer core right above the HV secondary in the picture.

The core MUST be electrically wired to the grounding prong on the plug that you use to plug the power supply in. Without doing this, you risk the possiblity of bringing the core to a high potential through your efforts to get an arc out of the MOT. If the core is allowed to "float," it's just easier to ruin the transformer or be electrocuted if the rest of your supply wiring is not perfect.

As you mentioned, the microwave oven--like a fusor--uses a negative HVDC with respect to ground. The diode and capacitor serve the function of converting the AC from the transformer in to DC while simultaneously raising the peak voltage to around 4 kV.

I presume you have no future intention of powering a magnetron, so just forget about leads A and B. Cut them off so they don't get in the way.

When the transformer is on, arcs can be made between lead C and the transformer case or any other grounded object, including you of course.

Could you post a diagram of your power supply? That would make it easier to "debug" it or offer other suggestions.

Thanks!
Carl

[guest]

so when i hook up my power supply, it should be rectified? as in the transformer case hooked up to the diode, and the capacitor to the wire, C, and then the diode to ground, and the capacitor to the inner fusor grid, and the fusor case to ground?

[Richard Hester]

The supply should be rectified. It doesn't necessarily need to be filtered.

[guest]

ok, but would the set up that i just said work?

[Richard Hester]

No, indeed not - you want HV secondary to diode cathode (stripe), diode anode to limiting resistor, resistor to inner grid, outer grid/ chamber wall to transformer mounting bracket. This applies a negative half-wave rectifed voltage to the fusor inner grid. You can leave the capacitor safely out of things. If you want it in there, connect it from the diode anode to transformer frame. The transformer core/mounting bracket should be hard-wired to earth ground. This scenario has the transformer half-wave rectified, but this is the only thing you can do as the other side of the secondary is hard-wired (riveted, even) to the transformer core. You might be better off with a 15kV neon transformer. Neon transformers have a center-tapped secondary, with the secondary grounded to the transformer chassis. With two microwave diodes, you can full-wave rectify the transformer output. They are current limited, but this shouldn't be a really big deal with a demo fusor. That's all a neon sign or microwave transformer are good for, anyway. Look up half-wave, full wave center tapped, and full wave bridge rectiifers on Google so that you can see what I'm talking about.

[guest]

for a 2Kv MOT, and a MOT diode, what would size would the limiting resistor be?

[Richard Hester]

Try 10k,100-200W. The high wattage is so that the voltage gets spread out along the resistor body to reduce the chance of breakdown. You can probably find a suitable resistor at C and H Sales, Surplus Sales of Nebraska, Fair Radio, or Ebay, To find the URLs of the surplus companies I mentioned, use Google or look in the Links directory for the links I published a year or two ago.

[DaveC]

One alternative to a single high wattage resistor is a string of lower wattage resistors. This will further spread out the voltage drop. But at "only" 2-3kv , breakdown issues are not too troublesome.

With 2 kV AC input and a single diode, you will get half wave rectified output, with a peak voltage of 2820 volts and an effective (heating value) of about 1kV, (the RMS output for a full cycle) Since current I = V/R, then to keep the current to a reasonable level such as 100 mA , you need a resistance of 10K ohms as was suggested above. The power level this resistor has to handle is P = V*V/R or (1000 x1000)/10,000= 100 watts. !! If you string together 10 1K ohm resistors of 10 watt rating... you will have the value needed. Otherwise you will need to scrounge the surplus places for the high power, high current resistor.

Proceed with care. You are asking good and sensible questions.

Dave Cooper

[guest]

i've been looking at power resistors on the surplus sales of nebraska site, and a 10K ohm, 200watt resistor is $45! is a limiting resistor absolutely neccisary? (it probably is, i'm just trying to save money :-/)

[Richard Hester]

Look elsewhere - $45 is a ripoff. The limiting resistor keeps things from vaporizing if you \\r fusor happens to arc. C and H is a lot more reasonable for thier power resistors. Try Ebay, too, as you can never tell what (and when) something will pop up.

[guest]

i found a place that sells 3k ohm 150watt resitors. would u be able to use 3-4 of them in line to make the approximate resistance of 10k ohm?

[guest]

wait a minute......what would that chance of an arc be if the 2 grids are 5" apart, and in vacuum? furthermore, the arc that my mot makes is about 2mm wide, so the probability of an arc is about a zillion to one.
sorry if i sound critical, maybe i don't see where you're coming from.

[henryhallam]

The usual danger isn't an arc inside the fusor itself (although that can occur more readily at low pressures that aren't hard vacuum) but more the fact that your feedthrough can arc over if it is damp, cracked etc or just a wire gets too near..

[guest]

ok, now i understand, but would that configuration of resistors work? if so, would i hook them up in series or parallel?

[Richard Hester]

Series resistors would work just fine.

[Adam Szendrey]

John,

I seriosuly advise you, to educate yourself on the basics of electricity and electronics. People on this forum help you to build a fusor, yes.
But in the future you might want to upgrade it, change the power supply, build some measurement electronics, etc..
I'm almost sure you'll need electricity/electronics knowledge in the (near) future.
The other point is the safety issue. HV stuff can be very dangerous to you and others if you don't know what you are operating or how.
Where do you live?
If in the U.S. i'm sure some guys here can give you advise on what books to read (titles, authors). I live in Hungary so i cannot really help you on that one.

Adam

[guest]

i'm sorry,
i know that my questions seem a bit vague, if not completely vague, but i am not always sure of my knowledge and have a bad habbit of always needing to verify my logic and reasoning, as i do not want to get killed, seeing as this project does require lethal voltage. i must admit however, that i could use more knowledge on this topic, and as i learn more and more about this project, it is as if i am uncovering an electronics iceberg-simple looking on the top, but extremely complicated as you investigate farther. people may have misunderstood me on this, but i am not planning on doing much of anything having to do with so much as plugging in any electronics,(i must admit that my attempt to find the high voltage wires in the MOT were fairly dumb.) however, i am trying to be as safe as possible with this project, and am making headway on putting together safety devices, such as 17Kv linesmen gloves, triple redundancy on the grounding, and many other things. i am sorry to have mislead you all into thinking that i do not know anything on this topic, but you are right in saying that i could learn more. i live in the US, and any reccomendations on reading that i could do would be much appriciated.
thankyou all for being very helpfull, and caring about my wellbeing,
John Poleto

[Adam Szendrey]

John,

You don't have to be sorry. I for one lack some self-confidence and ask many, even trivial questions! This is not a bad habbit. It's rather good! Too much self-confidence can lead to nasty things...
You are right about the iceberg of electricity/electronics...i'm sure not even the most ingenious physicists know how deep it goes...
I've learned a lot since i'm part of this team, believe me!
Though i have a small advantage, as i've been educated in an electronics secondary school, and that i've always been interested in physics so i had some pre-knowledge.
But to build and know (building it is not too hard (though quite hard without a decent money background) but Really understanding it is another thing!) a fusor one must have a quite deep understanding of particle physics, electrostatics, vacuum technology, various fields of nuclear physics etc...
All this accumulates as time goes by.
A fusor looks simple...but it has a much more complicated "soul".
Ofcourse if one has several thousand dollars to buy professional vacuum gear, power supply, everything to the last bolt, building it is not an issue...
But learning is in the procedure of bulding with al the failures and successes (as others have said on this forum before).

Adam